Cellulose Nanocrystals (CNC) Liquid Crystalline State in Suspension: An Overview

Abstract

Films made from cellulose nanocrystals (CNCs) may have iridescent structural colours (pure or in combination with other materials). Numerous fields might benefit from understanding how CNC self-assembly constructs these periodic structures. Herein, we looked at the colloidal characteristics of CNC particles as well as the development and behaviour of liquid crystals (LCs). We conducted a very brief literature analysis on the main issues related to the chiral structure creation of CNC LCs, including the origins of chirality, orientation, as well as its mechanical properties. Finally, by altering the pitch size, applications such as energy storage, humidity sensing, and photonic crystals were studied in a case-by-case manner. The manuscript, it is observed that the rational design of metamaterials built on CNCs allows for the reversible changing of colours through physical and chemical modifications by adding chemical or changing environmental factors. Examples of this alteration include the use of solvents, chemical penetration in applied fields (magnetic and electric), deflection, light, temperature change, acidity change, and molecular interaction detection. Reversible colours may be produced by altering the spacing between the particles, the filler materials, or the structural elements of the system’s refractive indices. This article briefly discusses the inner workings of CNCs, potential barriers to developing photonic structures, and several techniques and processes for achieving changeable colours.

1. Introduction

The linear chain of glucose molecules known as cellulose is held together by an acetal oxygen covalent link between the C1 of one glucose ring and the C4 of the next. The cellulose nanocrystals (CNC), created during the acid hydrolysis of cellulose, have lengths and widths of 100–300 nm and 3–10 nm, respectively. CNCs are bio-derived nanoparticles that can self-assemble in a colloidal solution into a left-handed chiral nematic (cholesteric) phase. The chemical chirality of the glucose repeating unit enables it to twist; however, the origin of chirality is still under debate [1]. Nature has created a wide range of materials and organized them so their structural colours can be seen. Since they never fade and offer no toxicity, structured colours are more eco-friendly than pigment.

The main cell walls of plants, algae, and oomycetes are structurally supported by the linear polysaccharide known as cellulose, a naturally abundant polymer [2]. It is a chemical molecule with a formula consisting of polysaccharides made up of hundreds or thousands of linked D-glucose units [3,4,5]. Nanomaterials have considerable applications in various fields, including health [6], electronics, biomaterials, and energy storage [7] and generation. Nano-sized cellulose can be found in the form of nano-fibrillated cellulose (NFC) (or interchangeably cellulose nanofibrils (CNF), CNCs, and bacterial cellulose (BC). These particles are favourable to a wide range of applications due to biocompatibility, degradability, and outstanding mechanical characteristics [8]. Other possible uses for cellulose include aerogels for tissue scaffolds, coverings(packaging goods), Pickering emulsion agents, water-filled hydrogels, and reinforcing agents in ceramic, polymer, and metal matrixes, to name a few [8]. Even though some nanoparticles, such as nanotubes (CNT) [9,10], have excellent mechanical qualities, their toxicity [11], greater manufacturing costs, and the importance of environmental tolerance are limiting their adoption within the industry. Another factor to consider is digestibility when it comes to health-directed practices; with similar agenda, biodegradability is another important advantage of CNC particles in biological applications where tuneable gradual degradation of the particles is required.

Acid hydrolysis transforms cellulose into CNCs, which are needle-shaped, rigid, negatively charged particles with a crystallinity of around 70% [8]; the majority of the mechanical delamination techniques used to manufacture CNF counters parts result in parts with decreased crystallinity and increased flexibility. Due to the fulfilment of the requirement of having one scale in nanometric scale (1–100 nm) and moving with the thermal motion of media molecule, these particles, when suspended, are Brownian colloids. The main objective of CNC research is to fully exploit the outstanding physical and chemical properties of CNC particles in many applications. CNC particles’ distinct mechanical qualities have led to their widespread application as reinforcing agents in a wide range of polymers, including polyvinyl alcohol (PVA) [12], polypropylene (PP) [13,14], polyethylene glycol [15,16] and many others, as a load-bearing element; it is noteworthy that CNC particles can also be employed on their own to make films and gels [17]. CNC particles can also function as liquid crystals or fibres due to their geometry, and their rod shape allows them to generate a variety of geometries as nanostructures, resulting in a wide array of features and functionalities. However, issues such as fine-tuning the interactions between CNC particles and polymer matrices, as well as achieving the agglomerate-free state of CNC particles in matrices, have remained challenging.

When the CNC concentration rises, the suspended undergoes an LC-to-gel transition [18,19]. Since the topology and mesostructure of the various CNC suspension states, including isotropic, biphasic, liquid crystalline, and liquid crystalline gel, differ fundamentally [20,21], they immediately affect the rheological properties of CNC solutions. According to the literature study, the formation of liquid crystals affects rheology by causing the viscosity maximum to appear in the plot of viscosity versus concentration curve [22]. This is because liquid crystalline domains may be orientated easily [18,23,24,25], the general flow pattern of CNC solutions during shearing, as well as the effects of dosage, charge density, sonication, and temperature levels, liquid crystal formation [18,23,24,25], and viscosity, have all been examined for CNC suspensions. Some material will be emphasized under the pertinent headlines of these reports to organize and further explain these reports in the sections that follow. After addressing the application in manuscript parts regarding the liquid crystalline phase creation of CNCs and presenting a section on its processing and rheology, the foundations and techniques of liquid crystal structure tuning are also provided in a separate portion.

Researcher interest in chiral photonic crystals in biological systems has come from a range of disciplines; the wings of the Morpho butterfly [26], blue-skinned mandrill [27] and spotted parachute bird all feature structural colour [28,29] on their scales. When CNC is in its chiral condition, it may replicate nature’s periodic internal structure, which changes colour according to the viewing angle. Even though liquid crystal formation is connected to a state of suspension and gelation, details of the suspension, gelation, and colloidal behaviour have been omitted due to the current manuscript’s concision; these topics are covered in other references (refs) [8,29].

Table 1. The table provides highlights on studies containing discussion and research on chiral LCs out of CNCs.

Reference	Highlight
[37]	CNCs are bio-derived colloidal particles that can self-assemble into chiral photonic structures. Their mesophase chirality’s genesis is unknown. CNC crystallite bundle might be the missing piece in the molecular-to-colloidal hierarchical transfer.
[38]	Congo red displays induced optical activity when attached to regenerated cellulose in gel films produced by gradual precipitation from LiCl/N,N,dimethylacetamide solution. The colour generation is structural rather than the result of dye interaction with chiral centres on the cellulose chain.
[39]	The bio-renewable resource CNCs spontaneously arrange into chiral nematic LCs. They reflect light, giving them an iridescent appearance. Recent breakthroughs in photonic material development employing CNCs are described in this manuscript.
[28]	In aqueous suspension, CNCs cause anisotropic order, which results in iridescence from the fluid phase. The effects of hydrolysis duration, wood pulp species, and sonication on LC phase separation are examined.
[40]	Cellulose microcrystallites can be made from wood, cotton, or animal sources. When cellulose fibres are acid hydrolysed, they produce stable aqueous suspensions. These whiskers are arbitrarily suspended in the water and create an isotropic phase at very small doses.
[41]	CNCs are elongated nanocolloids derived from nature that generate cholesteric phases in water and apolar solvents. They are made up of bundles of crystalline microfibrils that are grouped together. The genesis of chiral interactions between CNCs is unknown.
[42]	The major alcohol in cellulose I is in a trans-glycosidic (TG)-linkage orientation, resulting in the production of twist-causing hydrogen bonds. Other crystalline forms of cellulose exhibit microfibril-scale twisting. This study indicates that it is partly attributable to the absence of secondary alcohol in the TG orientation in those forms.
[43]	The helical self-assembly of cholesteric LCs is a strong, naturally formed assembly pattern. Attempts to emulate these extraordinary materials frequently result in films having a non-uniform mosaic-like quality. The researcher’s results reconcile contradictory facts and pave the way for biomimetic artificial materials.

provides an overview of varied yet focused on the main subject of chiral liquid crystalline phase formation and application formed on CNCs. Subjects include its chiral structure, left-handedness [1] and right-handedness [30], and methods of recognizing them using molecular dynamic simulations and microscopy [1], self-assembly, rheology [31,32,33,34,35,36], using rheology to recognize states, the interaction of liquid crystalline state under magnetic and electric field [31] and its subsequent orientation, the effect of confinement on liquid crystalline, and finally its application and blend with other chemical additives [12]. This table is an introductory representation of research focused on CNC with distinct agendas.

Another contender for CNC, CNF can also form liquid crystalline phases; however, it is more challenging; even though at static conditions, CNF normally does not show birefringence in flow, it does like CNC [44]. Cellulose nanofibril (CNF) LC formation is more difficult due to the greater length of CNFs and their intrinsic proclivity to produce entanglements [45]. A high aspect ratio CNF can also encourage the development of glassy regimes before the isotropic to nematic LCs transition occurs. A similar effect occurs with CNTs, which researchers shorten by treating with superacid to cause LC formation [46]. We will cover some of the principles of CNCs, LCs, and some of their applications in this review. The conclusions of this review research may provide a short guideline for the proper design of CNC LC-based products. Our earlier work provides a more thorough account of CNC chiral creation [29].

2. CNC LCs

2.1. Origin of Chirality of CNCs

Chiral mesophases are frequently produced by chiral LC molecules [37]. Asymmetry must be present in the entity attempting to generate chiral structures, and the system must not be racemic for the interaction of right- and left-handed entities to cancel out the chiral effects. A small amount of chiral dopant can produce otherwise mesophases due to the cooperative nature of LC ordering, which is typically sufficient to establish one domain handedness [47]. As chiral twist develops, the system may interact with left- and right-handed circularly polarized systems differently, suggesting that these materials function as polarization filters [48]. Moreover, interesting optical interference can be noticed if the pitch of the twist is on the order of the wavelength of the light; this interference with incident light is the reason behind the colourful appearance of photonic crystals.

The right-hand twist of cellulose has already been observed across experiment and simulation studies [49,50,51,52], although the twist can be seen in transmission electron microscopy (TEM) [49] and atomic force microscopy (AFM) [30]; it is still debatable the twist per length of fibrils. In this regard, molecular dynamic simulation can be a valuable tool to determine the twist to a CNC. Generally, the twist depends on the diameter, i.e., it is controlled by the number of constituent chains forming the CNC [50]. In reference [53], the authors showed that the twisting angle varies between 9.9 and 1.3°/nm for CNC with cross-sectional that contains two by two and six by six chains. The results of molecular dynamic simulation point to a twist around 1.4°/nm [53].

One of the main subjects that have received attention from researchers is ascertaining individual cellulosic particles’ handedness and their assembly orientation. Several experiments and methodologies have been devised to distinguish this within cellulosic domains, such as using molecular dynamics simulation [54], dyes [55] and atomic force microscopy [30]; here we have shown one of those efforts. Figure 1a–c depicts an intriguing experiment for determining the right-handedness of CNC’s chiral structure [56]. The “tactoid” rotated under the microscope allows researchers to detect the handedness of the cholesteric phase by observing the rotation-induced movement of the cholesteric bands; When the sample is rotated clockwise (right-handed); this observation is compatible with the left-handed rotation of the helix; this is the first account of observing of a single isolated tactoid made of cellulose; it appears that rotation amount and rate of rotation is not strong enough to causes disruption in the structure of the isolated tactoid. Chirality seen in ref. [56] is comparable to CNF since there is chirality inversion (right-handed individual CNF leads to left-handed tactoid), which is similar to Deoxyribonucleic acid (DNA) [57] and virus [58] but different from amyloid fibrils where left-handed particles generate right-handed cholesteric phases [59]. Moreover, temperature changes had no influence on the pitch of these tactoids within the temperature range of 10–43 degree Celsius in discordance with founding on DNA [60] and virus [61]. Therefore, changing temperature while knowing pitch is not influenced can produce an excellent system for rheological examination of the system. Moreover, a liquid with suspended glittery CNC sheets can exhibit colour as a nanofluid even at elevated temperatures.

Cholesteric liquid crystals are frequently produced by adding chiral dopants to achiral nematic hosts [37]. Twisted nematic and super twisted nematic combinations are inducible with dopant, so chiral nematic formation is not the only beneficiary of dopants. Individual cellulose Iβ crystallites are elongated and have a propensity to twist along their axis [62,63,64], a property arising from molecular chirality of glucosic repeating units [53,65,66], but it is unclear if this twist is necessary or cause of chirality of CNC based chiral nematic structures [67]. According to Parton et al. [37], CNC bundles can also operate as chiral dopants, implying a new paradigm for colloidal self-assembly akin to chiral dopants in molecular LCs (See Figure 2a–c [37]). In some sense, these structures should serve as areas where tactoids can form. As a result, chiral structures should be produced when a small quantity of well-chosen nucleation agents is added to an achiral bulk phase. It has to be investigated if CNC twist, not just in CNC suspension but also in other suspensions, might be the reason chiral structures like DNA filaments emerge [29,68]. Similarly, if nucleating agents help lower the thermodynamic barrier to the synthesis of these structures [46], the need to shorten CNF or CNC for the manufacture of chiral structures can be eliminated.

According to ref. [56], sonication, a process for creating CNC from CNF, cutting of fibrils happens not only at kinks but also at random positions inside kinks, leaving shorter rods behind, causing the overall particle length distribution to change to lower average values. The liquid crystalline transition points based on aspect ratio and fibre polydispersity anticipated by the theory of Onsager’s are undoubtedly impacted by this evolution [29].

Based on modelling efforts, as illustrated in Figure 3 for the CNC length of 41.5 nm, the equilibrated structure is a left-handed helical ribbon with the CNC directions spinning in the planes [69]. It demonstrates the model’s capacity to recreate the right-handed CNC’s inclination to develop a left-handed cholesteric phase due to its inherent twist [56]. Furthermore, the simulation results show that increasing CNC length (e.g., through sonication) can increase pitch size; this was seen in the results of ref. [70], where at the constant mass fraction in solution, CNC pitch size increased from 7 micrometres to 18 micrometres when CNC length increased from 100 to 140 nm nanometer. Other comparable molecular dynamic simulations were performed to investigate CNC’s inherent chirality and chirality that it induces in its assembled chiral form [50,53,65]. The model developed here can be paramount to explore other parameters, such as the presence of defects or the presence of agglomerated bundles on pitch size and chiral formation.

The CNC liquid crystalline allomorph assemblies were also reported to be distinct; CNC-I and CNC-II both have a right-handed twist, but CNC-II was longer and had a higher aspect ratio. Additionally, the CNC-II initial phase separates into an isotropic and lower nematic liquid crystalline phase before gradually reorganising into large-pitched chiral nematic assemblies [71].

Small angle neutron scattering (SANS) is very powerful in analysing the liquid crystalline behaviour of CNCs, authors in ref. [72] analysed carboxymethylated CNCs, and samples were shown to be arranged laterally in 2-D stacks of similarly sulfuric acid hydrolysed CNCs are showed similarly arranged microstructure; these structures were formulated to be the initial nucleus for the formation of liquid crystals; these orderings were found using SANS and dynamic light scattering (DLS) to be also reversible under semi-dilute condition. This study [72] showed that the early stage of liquid crystalline formation in CNC is probable that can detect crystal aggregate at concentration way below those reported by birefringence of polarized optical microscopy (POM). Moreover, they constructed a graph to connect the trend in apparent hydrodynamic size and viscosity to the concentration of carboxymethylated CNCs and found that both viscosity and hydrodynamic size remained relatively constant till a concentration of 10 g/L, after which both parameters increased dramatically. This increase in hydrodynamic diameter was reversible and indicated the development of small liquid crystal domains of 2D aggregates. Other studies reported in the literature show weak or no structure at the concentration range of 7.8–13.7 g/L [73,74,75]; however, a direct comparison between these results and the results explored here is difficult. Here viscosity was chosen as a method to track the development of aggregates; in ref. [76], similarly, rheology was found to be a more powerful tool in recognizing liquid crystal formation through the examination of flow curves as opposed to POM.

Moreover, it was reported that CNC attaches laterally. The lateral arrangement indicates that the surface properties of the sides of CNCs can be different. Cellulose has a crystal structure wherein polymer chains are oriented into flat structures along a particle-long direction [77]. This signifies that hydroxyl groups that are oriented parallel to the CNC plane can have two sides that are more polar than the two sides of the particle that are arranged in a perpendicular fashion [78,79]. Thus, the orientation of the polar side of two neighbouring particles in each direction leads to exposure of their hydrophilic side to the surrounding molecules of water and minimizes their free energy. Parallel stacking of CNC as the origin of chirality shown in Figure 2, therefore, can be seen as the true origin of chirality in CNC given results debuted in ref. [72].

The rheological, optical, and assembly behaviour of CNCs were shown to be significantly affected by the isotropic to liquid crystalline transition.

Table 2. Derived from the literature resources, a correlation between the volume fraction of the phase transition point and the volume fraction of the completely crystalline point was found with surfaces sulphate groups (0.89–1 wt%) and polydispersity (PDI) (standard deviation(std) around 30–50% of the average value).

Reference	$\mathbf{Aspect}\mathbf{Ratio}\left(\mathit{L}/\mathit{d}\right)$	I-N (%)	LC (%)
[18]	5	2.5	6.8
[80]	67	2.1	4.9
[81]	28	1.2	5.1
[81]	19	0.5	4.3
[21]	31	0.5	2.6
[82]	14.3	2.0	5.1
[81]	22	3.0	5.8
[83]	18	3.5	8.9
[70]	28	3.4	7.8
[70]	30	3.1	7.4
[20]	16.5	3.1	5.1
[70]	24	4.7	9.1
[70]	24	3.5	8.2

is designed to show empirical values linked to the volume fraction of transition at the beginning of phase separation and point of completion that results in the production of a single phase entirely liquid crystalline phase. I-N (%) and LC (%) are the points at which the system transition from isotropic to nematic and subsequently fully liquid crystalline. It uses data from the literature. According to Onsager’s theory, the point of transition and the point at which a completely liquid crystalline state forms scale with aspect ratio, as illustrated in equations 1 and 2.

$$C_{I-N}=\frac{3.29}{L/d}$$

(1)

$$C_{LC}=\frac{4.19}{L/d}$$

(2)

$C_{I-N}$ and $C_{LC}$ are volume fractions at transition points; L is the length of particles, and d is the diameter; based on the data points compiled here, Onsager’s hypothesis has been violated slightly. The deviation from these equations can be due to the appropriateness of this equation’s applicability to aspect ratios higher than 15 [84]. The polydispersity of CNCs used in the tests may mask the transitional point, and the resolution limitations of precision optical microscopy may also contribute to the deviation. As a result, the commencement of the liquid crystalline transition is observed at greater concentrations.

2.2. Rheology of CNC LCs

The rheology of CNC LC suspensions is extremely similar to those of rigid inclusions and LC polymers that resemble stiff rods [85,86]. For CNCs like carbon nanotubes (CNTs), the viscosity vs. concentration curves is not monotonic, but there is a point when viscosity initially reaches a maximum and then decreases as the concentration rises to a minimum value in a single nematic phase. Shear rheology and the apparent domain structure of liquid crystal states are shown in Figure 4a,b [33]. Region I display a shear thinning regime where viscosity declines with shear rate, followed by a plateau and then another shear thinning phase connected to rod orientation, in contrast, to dilute samples. Additionally, it is known that the first normal difference turns negative in area III, where the cholesteric director oscillates back and forth in its flow direction. The negative represents a vertical extension and streamlined compression. The initial normal stress differential of the polymer solution and melts is positive when they are taken from the die [87]. This die swell warps extrudate forms, lowering the physical qualities of the finished product. Integrating CNC LCs with polymer is a unique application for these materials to prevent die swell. More research is needed to fully comprehend the causes of the negative first normal stress differential and how to use it to improve CNC-based products [88,89].

Steady shear viscosity flow curves may generally be affected by changes in the ratio of isotropic to anisotropic (entering biphasic region) state in suspension or changes in liquid crystalline domain size. Sonication, for instance, can directly impact the structure and rheology of CNCs [24]. Agglomerate breakdown during sonication results in a reduction in viscosity by (i) generally increasing the aspect ratio of the CNC colloidal system, (ii) expanding the localized dense LC zones, and (iii) decreasing the effective volume fraction of the CNCs in contact with water. As the sonication or CNC aspect ratio rises, the critical viscosity—the level at which LC production begins—is pushed to a lower concentration in terms of viscosity.

Although the gradient of the viscosity vs. concentration curves in reported reports on cellulose suspensions indicates a rise in the area of biphasic coexistence, a clearly defined maximum is non-existent thus far, even though it has been seen in other nano inclusions such as graphene oxide [85]. This is due to the polydispersity of CNCs that results in the formation of a broader biphasic region than the ones theoretically predicted; thus, the decline in viscosity ensues across the biphasic region; therefore, the maximum either disappears or is confounded [33]. As opposed to this set of observations, Li et al. [90] found that when particles interact, and the double layer around them is thin, a local maximum in viscosity versus concentration develops. Therefore, maximum emergence may be more easily seen when the double layer is thin [90]. This needs further investigation in the literature.

2.3. Phase Separation

One of the distinctive properties of lyotropic rigid-rod polymers is the biphasic region in equilibrium where isotropic and liquid crystalline phases coexist. The capacity of polymeric chains to form LCs in a polydisperse environment is independent of their length; polymeric molecules having lengths less than the persistence length readily diffuse to an isotropic state, thus making their separation based on their size thermodynamically possible. As a result, fractionation depending on size, occurs in equilibrium with the isotropic state within the “Flory chimney” [46]. With a rise in concentration, the system undergoes an isotropic-to-nematic phase transition via a biphasic region, the so-called Flory chimney in the liquid-crystalline rigid polymers [91]. Rigid rod-like polymers already displayed this behaviour [92]. Similar to lyotropic (i.e., liquid crystalline phase forms as a result of a change in concentration) rigid polymers, CNC suspension above a certain concentration threshold develops a biphasic region that can separate into nematic and isotropic domains if given enough time [46]. The relaxation time to carry out phase separation depends on CNC’s aspect ratio and concentration. These relaxation times are dominated by kinetic arrest if concentration and aspect ratio is close to the glassy regime [43,93]. The separation time can be shortened if the biphasic suspension is allowed to be centrifuged [93].

Longer CNC must separate to the nematic phase at the bottom layer according to the topology of phase separation, whereas shorter CNC must float with other impurities at the top layer. Such a procedure has lately been employed to improve or purify CNCs’ average aspect ratio. As cycles of purification, whether by time passing or centrifugation, increase, the breadth of the chimney narrows, resulting in smaller length distribution of CNCs. This approach, as it has been used before [94], may result in the production of monodisperse LC, which might have profound scientific effects such as using these uniform distribution of CNCs for studying their migration on the interface for Pickering emulsion applications. Additionally, a shear fraction may be used to clean up a suspension of CNCs; as shear increases, longer CNCs flow into the bulk while shorter CNCs remain at the moving border. [76,95]. The widening of the chimney also has been observed in polymeric liquid crystals before [96,97].

2.4. Mechanical Properties

Due to the rigidity of CNCs, which hinders the release of external stresses during deformation and speeds up the emergence of cracks, pure CNC films are stiff and brittle [98]. By combining or blending CNC with other non-brittle components, such as water-soluble polymers, it is possible to create CNC-based photonic materials that can overcome this restriction [12]. To combat its brittle nature, CNC has previously been combined in the literature with surfactants [99], resins [100], and polymers [101]. The CNC’s cholesteric structure [102,103] enables it to interact with gold nanorods to induce chiroptic effects while supporting small particles without causing structural disruption. This accommodation is referred to as “templating” and will be briefly explored in relation to its application in plasmonic [103] and energy storage devices [104].

A layered, spinning microstructure known as a “bouligand structure” commonly appears in naturally occurring materials. It looks like plywood. A key element of bio nanocomposite that has not been duplicated is the architecture of pore canals (aerogels or hydrogels), which can help stiffen the material and supply components for self-healing. This potential for real estate development is still open and serves as a solid foundation to produce pure CNC goods and CNC-based composites. Additionally, polymer molecules may be held inside the openings of the chiral geometries of CNCs when polymer and CNCs are mixed. This provides an excellent opportunity to research the properties of natural polymeric in confined spaces, the cycling arrangement of CNCs and polymers in the CNC “bouligand” composites, and the adaptability of CNC-CNC spacing (pitch size). The polymers utilised may also respond to stimuli, making the “Bouligand” element sensitive to the environment; in other words, the level of confinement might be altered. Additionally, the load bearing capacity and bending moment properties of carbon-fiber reinforced composites have been studied [105], as shown by biological samples [106]. The composite loading bearing capability is also increased by the Bouligand arrangement [107]. However, no measurements have been done for the properties of CNC gel and film composites. To eliminate the ambiguity, more study is required in the area. AFM could be used to investigate the mechanical characteristics of CNCs’ chiral structures and the effect of force on pitch size.

Additionally, CNC chiral structures can increase the products’ robustness. The most common microstructural arrangement in bioactive systems with higher mechanical strength and toughness, including such bone [106] and the mantis [108], is the twisting plywood, or Bouligand, architecture. These materials display consistent qualities in all directions because of the “Bouligand” design itself, a low volume fraction of soft, energy-dissipating polymer, and shear wave filtering [109], fracturing twisting, deflection, and arrest [106]. Energy is removed at the nanoscale through delamination, matrix viscoelastic softening, and nanoparticle breaking. The process of removing heat often expands cooperatively over several length scales. The Bouligand structure lowers stress [108] by distributing the damage and filtering off shear stress at the microscale.

It has been discovered thus far that adding macromolecules or nanoparticles while the suspension is still wet leads to the development of nanomaterials with CNC seated in chiral architecture. According to the literature [101,103,110,111], the specifics that have so far been included are polyethylene glycol, PVA, glucose, and nanoparticles. This list comprises polymers that do not significantly cause CNC agglomeration, preserving their ability to keep their chirality inside nanocomposite. Energy loss during crack propagation is expected to increase if the polymer has a strong inclination toward CNC. However, this propensity leads to clustering between CNC. The infiltration of CNC chiral nanostructures that have dried up is another remedy for this problem. Both monomer solutions and small molecules have attempted this [112,113]. Infiltration permits the creation of nanocomposite with an additional concentration of up to 50%. Despite the significant possibility for agglomeration when additives are present in a moist setting, this enables the chiral nematic order to be maintained (suspension). The polymers [114,115], protein [116], and calcium chloride [117,118] were all subjected to this technique [119]. A vertically chiral-structured film can also be maintained to reduce swelling under stress.

The complexity and mechanical durability of aerogel production may be improved by chiral structure. CNCs are intriguing alternatives for the bottom-up synthesis of these structures because, when cast from liquids, they dynamically self-assemble into chiral nematic films, simplifying the scaling-up process. Materials made by the self-assembly method are the best candidates for specialised application because altering a material’s nanoscale structure permits fine-tuning of its physical features and related functionality. According to one study, chiral component orientation increased the specific strength and toughness of CNC aerogels by up to 137 and 60%, respectively [120]. According to the author’s research, chirality can result in chiral-nematically structured aerogels with precise meso- and microstructures. The amount of liquid absorbed by the aerogel can be observed by the colour it reflects [68] since the final aerogels exhibit a significant link between the mesopore% and selected light reflection (iridescence) caused by mechanical load. The scientists also pointed out that the mechanical performance of pore compression under load is markedly enhanced by chiral-nematic ordering. A recent review thoroughly summarises the bottom-up assembly process used to create CNC- or CNF-based aerogels, which are used as adsorbents [68].

However, tactoids can be employed as architecturally coloured pigments in melt processing rather than introducing additional CNCs. CNC can also be combined with water-soluble materials utilising solution processing methods or with water-insoluble polymers using melt processing. Liquid crystalline thermosets may also be produced using CNCs, a structure of initially non-mesogenic epoxy monomers, and a solution processing technique. These materials are cured at high temperatures to generate liquid crystalline phases [121]. Another strategy to embed CNC within polymers and simultaneously enhance their mechanical properties [12] is to incorporate water-soluble polymers, such as PVA, into CNC films. By doing this, polymer molecules improve stress transport throughout the network. Therefore, it is possible to incorporate chiral optical films into coating applications that use PVA [122,123] or epoxy resin. Due to the lyotropic mesogen of rod-like negative CNCs, which is easily impacted by external influences such as hydrogen bonding interactions with PVA, some polymers may cause CNC to lose their LC property in the composites. However, the use of extra chemicals may help to solve this issue. For instance, in reference [124], sodium sulphate was employed as a coagulant during the wet spinning of PVA-CNC fibres to produce nematic representations of CNC outcomes.

After drying, CNC films are likewise essentially irregular. CNC films are not uniform due to two factors. The first is that due to the film’s randomly dispersed helix orientations and various pitch length values, the dried film’s helix axes are oriented at an angle to the surface. After drying, the film forms many tactoids with a cholesteric and well-organised structure that may be evaluated by scanning electron microscopy (SEM). The length and helix axis orientations of these tactoids change throughout time (drying speed affects pitch size). When these tactoids are gradually combined and deposited, they can form a stacked chiral nematic phase structure; as a result, the structure is not homogeneous. Additionally, after drying, the tactoid would be squeezed vertically to add extra helix axes. The second factor [125] is macroscopic non-homogeneity responsible for the coffee ring effects. As a result, the nanoparticles are merely forced toward the ring’s edge. The “coffee ring” effects that result from the creation of chiral tactoids with an unequal distribution give the film its many hues. Ring-shaped deposits form because of capillary migration from the interior to the exterior, which transports suspended particles to the droplet’s edge and deposits them in a ring that forms at the border due to higher evaporation at the periphery than at the centre. Since the border is thicker than the centre, the corresponding location has different optical characteristics. The development makes the film’s optical qualities inconsistent.

Postprocessing is required to correct the structure for applications since the mechanical properties of coatings are determined by the uniform distribution of CNC tactoids over the films. The merging of CNC tactoids is usually subpar during quick water evaporation, leaving just portions of the CNC film in the LC structure as tactoids in the final CNC films, which commonly have uneven LC formation in addition to having poor mechanical properties. Additionally, water evaporates more quickly towards the petri dish’s edge than in the centre due to the “coffee ring” effect, transferring mass from the edge to the centre. As a result, reinforcement in solution casting is uneven [126]. The helix will not become uniform even if nearby tactoids with different helix orientations mix in the CNCs water suspension. A defect of this size would take a long time to fix into a consistent helix shape [126]. However, continuing water evaporation quickly transforms the system into a glassy state and prohibits further helix or director rearrangement; therefore, this is unsustainable [127]. When tactoids fail to fuse completely, flaws result, making the mechanically weak CNC pure films further weaker. The tactoids rearrange and join to produce enormous tactoids during the phase separation, and higher concentration rises over time as water evaporates, comparable to the Ostwald ripening process of emulsions [126]. This, as we will briefly discuss later, takes a lot of time; therefore, the process as it is now cannot be repeated to make goods at an industrial scale.

Additionally, at the last step of the film drying process [128], a vertical fracture parallel to the bottom edge of the capillaries was developed for CNC pure films drying CNCs in a capillary. According to reports, those fissures appeared as a result of a morphological mismatch between twisted structures and flat rectangular walls. The origin was supposed to be a stiffness imbalance between adsorbed CNC to walls; these fissures are proof of nano-crystals parallel alignment to the capillary bottom. Additionally, drying LC suspension results in additional types of defects because of the non-uniform deposition of materials at the edge of the droplets during water evaporation, which is caused by the “stick-slip” motion of the interface and Marangoni [126,129,130].

Rapid evaporation typically causes structural alterations to the film and an increase in pitch size. It is essential to monitor and adjust these variables since they all impact how quickly things dry, including humidity, airspeed, casting area, heat, and other variables. Other techniques, including employing vacuum-aided film production, are needed to produce homogeneous films at reasonable time scales since the necessity for delayed drying restricts the scalability of films. LC films are naturally uneven when manufactured; however, the film thickness may be controlled. The simplest approach applies mechanical force using vacuum-assisted self-assembly to produce a film with homogeneous and controlled optoelectronic characteristics. Deheer et al. [131] employed this technique. Chen and associates [132] conducted additional research on this method in 2014. High ratings for the films’ orientation and consistency of structure have been given. The spiral directions’ random distribution created brightly coloured films.

Mechanical crushing has been found to produce water-resistant flakes [133] from CNCs with architecture colouring by utilising desulfation and the natural brittleness of CNC films; as a result, CNC chiral tactoids may be employed as structurally coloured pigments [133]. In some applications, these pigment-free colourants should keep glittering inside a polar liquid; nevertheless, if they are not handled, the flakes may re-dispersed in water or other polar mediums. The re-dispersion of CNC films after immersion in water can be avoided by desulfation of the dry material using acidic desulfation [134], solvolytic desulfation [134], and alkaline desulfation [135,136].

One way to make films more flexible while creating an edible composite is by mixing them with safe chemicals like glucose. Kose et al. [113] and Mu and Gray [125] added glucose to CNC films created in 2019 to increase flexibility and change the pitch. The research by Kose et al. [113] is highly intriguing since it shows how the relationship between pitch and colour creation may be visually represented by stretching elastomeric composites and producing beautiful colours. These modern developments and cutting-edge utilisation methods can enhance the mechanical properties of CNC-inserted films and composites. Chemical treatment is one of the alternatives. The durability and heat endurance of vacuum-filtered CNC iridescent sheets can be considerably improved by a simple chemical process (alkali), as Nan et al. [137] show. Better mechanical properties could be attributed to increased load transfer among “chiral tactoids” because of the amalgamation of CNC nanoparticles with other compounds during processing and the subsequent fusion of ordered layers. The author proposed that the significant increase in CNC-based films’ mechanical characteristics was caused by the unstructured film produced by alkali treatment.

By altering the polymeric matrix around the liquid crystal or the nature of the charges on them, more liquid crystalline production is also feasible. In ref. [138], it was demonstrated how to produce carboxylated CNCs from cellulose by hydrolysis and esterification with the help of oxalic acid. When synthetic CNCs were combined with PEG, composite films that displayed spectacular colour snowflake patterns were formed that differed in characteristics from conventional CNCs. Through the addition of cations to the 1.27% CNC solution, the hydrogels of the treated CNCs also generated birefringence gels. Oxalic acid coupled to CNCs included 0.3–0.54 mmol/g, according to measurements.

2.5. Orientation

The LC’s arrangement and direction are equally crucial and merit careful examination. Both small-angle x-ray and small-angle neutron scattering have been used to examine the structural properties of the ordered chiral nematic CNC phase. Findings from SANS demonstrate that in magnetically or shear-oriented chiral nematic suspension, CNC rods are more densely packed parallel to the chiral nematic axis than perpendicular to it [73,139]. Orts et al. also demonstrated that rod spacing decreases when CNC number density and electrolyte concentration increase [73,139].

Up until now, attempts to change CNC director direction have solely used external fields such as extension, shearing, and the use of magnetic or electric forces, all of which have had varying degrees of success. Fibres with diamagnetic dispersion also align in static magnetic fields. Carbon fibres [140,141], carbon nanotubes [142], polyethene fibres [143], cellulose fibres, and other materials have been successfully used in the literature to demonstrate magnetic alignment. Studies using these similar rods’ liquid crystalline fibres oriented magnetically may readily be expanded to CNCs. The chiral nematic axis aligns parallel to the static magnetic field because cellulose fibres are diamagnetic, and several studies on the chiral nematic phases produced by cellulose fibre suspensions have been reported [144,145,146]. By inducing an induced magnetic field in the opposite direction, which provides a repelling force, a magnetic field repels diamagnetic materials.

The application of a magnetic field is the sole strategy that has been discovered so far to change the orientation of the helix. Although using microfluidics to create such structures has been discussed in the literature, this method has not been used to explain improving or changing orientation, particularly in relation to CNCs in their LC form. Due to the CNCs’ negative diamagnetic anisotropy, a rod aligned perpendicular to the magnetic field, this approach is ideally suited to function and orient CNCs. As the result of their negative magnetization inhomogeneities, CNCs attempt to equal parallel to an external magnetic field. At low magnetic fields (${\mu }_0 H~ 0.5T)$, the cholesteric phases may be aligned empirically [147]. Since this rod of a cholesteric CNC suspension is all perpendicular to the helix axis, the helix will be evenly aligned in the field direction when put in a suitably strong magnetic field. Revol et al. [31] and Kimura et al. [146]; experimental data affirmed this; The helical axis eventually organized parallel to the magnetic field’s direction (CNC in the perpendicular direction), and the magnetic field’s recurrent rotation caused the crystal to unwind as the suspension changed from LC to an orthotropic condition. The liquid crystalline phases were dissolved and relaxed upon the removal of the magnetic field. In addition to displaying its magnetic qualities, the mere responsiveness of LC to magnetic fields may be further exploited by, for instance, enhancing its responsiveness by adorning it with iron oxide or other magnetic particles to make switching for electronic purposes quicker [148].

The current literature review states that magnetic fields have not been investigated for their effect on tactoids fusing. Furthermore, by observing how tactoid deposition spreads across the solution, tactoid deposition may be more homogenous. Look into strategies to increase or decrease tactoid nucleation as it may be organized and examined similarly to polymer crystallization. In research conducted by Qu and Zussman, the local crystal structure (LC) was disturbed using a magnetic field, but the isotropic to nematic or nematic to chiral nematic transitions were not moved [149]. Tactoids’ orientation can be changed by depletion pressures in combination with magnetic fields.

According to Kimura et al. [146], magnetic field alignment occurs at 1 T in 5 h. However, there are still unanswered questions regarding the effects of sodium chloride, depletion force, and concentration on the rate of orientation and the potential effects they may have on accelerating or slowing the rate of orientation. To make particles more sensitive to magnetic fields and faster at aligning, diamagnetic particles, such as Fe₂O₃ [150], can also be sprayed onto the particles. In 2016, magnetically aligned CNC-Fe₃O₄ were reported to copy plywood structures by combining individual layers of unidirectional CNCs. To achieve orientation, Poly (L-lactic acid) (PLLA)/CNC-Fe₃O₄ suspensions in chloroform were left to become dry in a magnetic field with a strength of 60 mT at different orientation parallel and perpendicular to the surface [148]. However, one research [151] asserts that viscosity affects chiral direction. This study examined this in a 0.7 T magnetic field while using a fluid that changed from water to n-methyl formamide (NMF).

In comparison, the suspensions with a higher viscosity, exhibited almost little temporal change after the initial field hit, therefore, highlighting the role of surrounding viscosity on ordering. These findings represent a major step in extending the application areas of large-scale cellulose-based composites with anisotropic characteristics by simplifying the manufacture of globally ordered CNCs in a range of polymers in the presence of a magnetic field. Viscosity’s impact on CNC order Compared to CNC-water, CNC-NMF has a greater viscosity. Rotating in a magnetic gets more challenging as a result. Therefore, CNC tactoids may be used as microrheological probes, with the orientation rate correlated with the fluid’s viscosity. This probing, similar to the fluorescence recovery after the photobleaching probe [152], can reveal details about the pore structure and level of surrounding confinement. The rate of rod orientation in different suspensions can also be related to tactoid changes. Magnetic flux was also used to produce well-aligned nematic liquid crystalline and colloidal crystalline samples of the essentially monodisperse tobacco mosaic virus (TMV) [153].

Studies show that the pitch of CNC-based films decreases as the magnetic field strength increases [154]. The film’s pitch might be changed using this technique, which would also increase the film’s homogeneity. To produce highly distorted low concentrations of CNC distributed in cyclohexane, Habibi et al. [155] employed an AC field. The results showed that rod-shaped particles turned [155] degrees parallel to the direction of the electric field rather than the magnetic field direction. In the case of tunicate CNCs, there was no alignment in the samples exposed to AC electric field lower than 2000 V/cm for all frequencies applied. A better orientation was achieved at starting field strength of 2500 V/cm. Perfect orientation was observed for frequencies ranging from 10⁴ and 10⁶ Hz.

According to De France et al. [145], cooperative ordering favours field direction. To orient samples containing 1.65 and 4.13 wt percent CNC, a target recently attained for tunicates-derived CNC [156], magnetic fields of 0.56–1.2 T were insufficient. In exposure to the magnetic field, the helix was untangled, which led to an increase in pitch size (redshift) [154,157]. The method works well with suspensions that have low viscosities and low cholesteric concentration sizes. Films with consistently aligned helices may also be produced by starting with a low CNC concentration and slowly emptying the solvent under a constant magnetic field. Due to CNC’s positive dielectric anisotropy, the helix may be unwound by an electric field. In films made by drying CNC suspension in the presence of electric fields with frequencies ranging from moderate to high, Habibi et al. [155] achieved homogenous CNC alignment.

De France et al. [145] examine the impact of CNC concentration (1.65–8.25 wt%) and weak magnetic fields (0–1.2 T) on the kinetics and level of CNC ordering. For CNC suspensions above C* (4.8 wt%) in a 1.2 T magnetic field, partial alignment occurred in less than 2 min, proceeded by slower cooperative ordering to reach almost perfect alignment in less than 200 min. Magnetic alignment was not seen in suspensions below C*. Frka-Petesic et al. [156] describe magnetic birefringence measurements of dilute aqueous solutions of CNCs generated from tunicate up to 17.5 T (high fields). The birefringence reached its maximum at 17.5 T, suggesting that almost all nanocrystals originated with their long axis parallel to the field.

To appropriately regulate crystal alignment, the authors suggested utilizing a magnetic field, mechanical deformation, or neither an electric field nor a magnetic field. After extensive annealing at ambient temperature in a magnetic field, shear-induced morphology disappeared, and a distinctive nematic “schlieren texture” appeared [158]. A string of irregular director orientation points characterizes the “schlieren texture” of nematic LCs. From the end, these dots correspond to disclination lines. Other liquid crystalline materials have these “schlieren textures,” but they are created by physics and superstructure rather than chemistry [158,159,160]. After some time, the alignment caused by a high magnetic field (0.25 T) could be observed; domains with discrete LC orientations that were initially torn apart by disclinations were subsequently reoriented and joined into larger domains. Due to the low magnetic repulsion of the bare platelets, perfect alignment required many hours. The same magnetic field strength was used to adorn the platelets with iron oxide (Fe₂O₃), which preserved liquid crystallinity in an aqueous environment and finished alignments caused by filing swiftly. Crystallinity may be preserved if the suspended liquid is coupled with a polymer matrix. When polarizers were crossed, nanocomposites made of poly (acrylic acid) (PAA) and graphene oxide showed birefringent schlieren texture. The scientists discovered that platelets were precisely aligned using hand drawing and SEM.

With the gap among nematic planes along the cholesteric axis being less than the spacing among rods in a nematic plane [139], microfibril suspension exhibits anisotropic chiral nematic ordering in a 2.4 T magnetic field. Kvien and Oksman [161] used PVA with CNC made from micro-crystalline CNC. The authors claimed that in that circumstance, CNC alignment was accomplished using a powerful 7 T magnetic field. They developed translucent, birefringent films with a final concentration of 2 weight percent CNC (4.21 GPa) and discovered that it was considerably lower than the storage modulus determined in the transverse direction (6.19 GPa). This result verified that CNCs are parallel to the applied field.

The shear flow will also give uniaxial CNC alignment, another manipulating tool for directing the direction of the unwinding helix. The films with uniform nanorod alignment produced by shearing and the X-ray examination of the LC CNC alignment during shear flow displayed complicated behaviour with significantly different outcomes depending on the shear rate [162,163]. A microfluidic chip’s CNC orientation has also resulted in the complete orientation of each CNCs [164]; however, there is no information on orienting LCs in the same manner.

Under intense hydrodynamic shear, the cholesteric order (chiral nematic) breaks down and transforms into nematic [165,166]. According to studies by Lagerwall and associates [127,167], a small circular shear applied to a drying solution in a circular dish resulted in a locally increased vertical cholesteric orientation. However, when it got closer to the edge in the later example, the helicoidal structure started to sag. The reaction of anisotropic fluids to shear is challenging and intricate, especially for polymeric and colloidal fluids where viscoelastic processes may be prominent [168]. Contrary to popular belief, the transition from nematic (shear-induced) to chiral nematic states (and vice versa) is thought to involve only a simple twisting or untwisting of the chiral nematic axis. It was found that shear generated chiral structural unwinding in an elastomer with CNC embedded in it and that this process induced the CNC nanoparticles to align all along the flow axis. However, as soon as the flow was cut off, the chiral nematic form fully recovered [169]. The effect of shear on the unwinding of tactoids of CNC liquid crystals can also be examined through models such as Folgar-Tucker set models [170] that are developed based on the Jeffery equation [171]; account of the application of these models on CNC orientation prediction can be found in ref. [12].

2.6. Pitch Length

The pitch of LC is the very important distinguishing factor that controls colour in photonic crystals made from this self-assembly process. A helix is characterized by its periodicity defined mathematically as $q=\frac{2\pi }{p}$ as well as its handedness, i.e., a clockwise twist means right-handed while a negative denotes left-handed or counter-clockwise. Chirality is extendable to particles (individuals) for CNC and CNF; chirality is right-handed [30]. The wavelength of reflected light is controlled by the pitch of LC [172], and it is confirmed to be the case for CNC using experiments done in ref [173,174]. The wavelength is describable by the equation proposed by Bragg $\lambda =nP\sin \left(\theta \right)$ [175,176], where $\lambda$ is the reflected wavelength, P is the helical pitch, $\theta$ is the angle (°) of the incident light, $\sin \left(\theta \right)=1$, where incident light is perpendicular to crystal plane, n also signifies a value that is the average refractive index of the material bombarded by the light [177].

Figure 5a depicts the chiral structure of CNC under POM, showing the fingerprint texture of the chiral nematic structure [178]. Figure 5b displays the maximal reflection wavelength at 360 nm, and the CNC iridescent film was violet. Figure 5c display of FE-SEM of the structure at a higher resolution displaying the periodicity of the developed structure and the spectra of the CNC film recorded by a reflectance spectrometer and transformed into a Commission on Illumination (CIE) chromaticity value (x = 0.175, y = 0.006) as shown in Figure 5d [178]. The graphic depicts visible colours as a function of chromaticity coordinates, which are x (red) and y (green) components. Maximum reflection was 360 nm at 43 percent relative humidity; increasing humidity to 75, 86, and 99 percent induced a significant shift in reflection maxima to 451, 513, and 525 nm, respectively [178]. Therefore, this assembly based on UV-vis spectra analysis can act as a humidity sensor.

Interestingly, when CNC was treated with ionic liquid, it developed a distinctive assembly behaviour and showed no signs of phase separation since tactoid and isotropic solutions coexisted at concentrations between 1.0 and 4 wt%. In contrast to CNCs, the concentration of treated CNCs increased the pitch of chiral nematic tactoids [179]. Polymers like polyethylene glycol (PEG) can be added to alter pitch size. For example, the half-pitch dimension increased with PEG addition from 103 nm to 143 nm with an increase in PEG content from 10 to 20 wt% [180]. Similar to chiral structures, the helical pitch increases as polymer concentration rises, and polymer addition causes the wavelength of the reflected wave to increase from 332 nm to 625 nm [181].

The rate at which films dry out under the CNC anisotropic environment can change pitch length because phase separation takes time, and the evolution of chiral structure depends on time. In ref. [182], CNC suspensions that were dried slowly had the smallest pitch size, which was 330 nm, whereas samples dried quickly had a larger pitch size, which was 440 nm. This study signified the impact of time given to form tactoids from the original isotropic suspension into a phase-separated suspension (top isotropic and bottom being liquid crystalline) and the impact of drying rate on developed pitch sizes.

The average reported pitch of CNC films was measured as roughly 1.88 micrometres with SEM, although it was substantially smaller than POM pictures (approximately 349 nm) in ref. [183]. The discrepancy found between dry and wet states is important as drying, if done through regular evaporation, can lead to an increase in concentration over time and therefore pitch size changes; the differences between POM measurement and SEM can become smaller if samples become super critically dried thus freezing the developed microstructure [68]. This variation in drying rate can also produce discrepancies with pitch size measurements from POM and SEM images. According to the drying mechanism, CNCs become biphasic and start to form tactoids as the concentration of the suspension increases over time. These tactoids expand until they reach a threshold volume, mixing with nearby tactoids to form larger anisotropic domains [29].

Salt can impact on the electrostatic attraction between CNCs in chiral systems. It has been demonstrated that salt eventually turns CNC films black and white. The development might be explained by two variables. First, if the pitch size is at a region that does not reflect the visible electromagnetic spectrum, the film may appear transparent. Another factor is the breakdown of chiral structures caused by salt, which can result in microstructures that are either completely collapsed or partially damaged [183]. Furthermore, circular dichroism was unaffected by the presence of salt since CNC films with even high concentrations of salt-preserved positive circular dichroism [183]. In presence of salt tested, i.e., NaCl, KCl, MgCl₂, and CaCl₂, CNC film maintained positive circular dichroism over the wavelength examined; therefore, the handedness of the cholesteric helix is invariant to the presence of electrolyte and ionic strength. The results showing how salt affects pitch size are consistent with those showing how pitch size changes in response to negative charges on CNCs, where pitch size increases as the zeta potential of charged CNCs increases with an increase in ionic strength [184].

Though phase transition and pitch sizes are unaffected by the type of additional coagulant cations [20], bigger ions have been proposed to enhance pitch sizes [185], whereas adding polyvalent ions results in lower pitches [186]. Finally, it is possible to alter the surface charge of carboxylated CNC or, for example, heat sulfated-CNC; pitch drop is noticeable after sulfuric acid removal [134].

If additional nanorods are used in co-assembly, their presence may affect the pitch size in CNCs. For example, gold nanorod assembly with CNC using SANS experiments were described [187]; findings highlight that a low concentration of gold nanorods scarcely affects CNC liquid crystal phase development. A tighter packing of the CNCs in the liquid crystalline phase may be caused by possible columbic interaction due to the adsorption of gold nanorods, according to SANS results that showed an increase in correlation distance compared to pure CNC systems. In another report [188], to change the magnetic response of the CNC, Fe₃O₄ nanoparticles were dispersed within the clean CNC. A 7 mT magnetic field was then applied to fine-tune the self-assembly and microstructure of the CNCs. The pitch decreased as the magnetic film thickness increased from 302 to 206 nm, with the application field’s strength increasing from 7 to 15 mT. In contrast to the researcher’s finding that pitch increased with magnetic field intensity, this discovery showed that the helicoidal structure was being compressed rather than unwound.

Surprisingly, in ref [189], pitch length reduced as virus length increased, according to the author’s tuning of fd-viruses with varied lengths. The same argument may be made for CNCs. The aspect ratio of the CNC significantly impacts the rate of phase separation in biphasic suspension. Long CNCs exhibit quick phase separation with gravity and achieve their final equilibrium in a day, whereas shorter CNCs did not, even after three months [43]. As a result, it is conceivable to hypothesize that rate of phase separation can also be used as an extra tool for characterization by indirectly correlating it to the aspect ratio. This unique suspension can be used in coatings thanks to the rigorous supervision of the CNCs author in ref. [43], who also generated mosaic-free films that were consistent in colour.

2.7. Confinement

The effects of refinement, advanced issues centred on liquid crystal forms and their specific applications, as well as certain under-researched areas in the literature will all be addressed in addition to the themes that have already been briefly discussed here.

For the study of soft matter, it is crucial to do basic research on the geometric confinement of nanoparticles, molecules, and liquid crystals. The symmetry of their equilibrium states may be broken by confinement in 1D, 2D, and 3D dimensions, stabilising topological faults and producing distinctive structural patterns (such as disinclination). As a result of the restriction of liquid crystals in 2 dimensions to round droplets [190] and small capillaries, technological advances in optics and sensing are anticipated [191,192]. This study may also open up new research into liquid crystalline formations similar to those in living organisms like the cornea [193,194] and insect exoskeletons [195,196]. Confinement Monitoring the evolution of the pitch as a function of the concentration over a much larger range is made possible by observing the diminishing microdroplet. As a result, in microspheres, contraction is more in pitch length, which is expected as opposed to cases of 2-D contraction in a film [197,198]. The pitch at low concentration matches pretty well with the bulk concentration and follows a power law of approximately $p\propto c^{-1}$ above a certain connection attributed to the kinetic arrest scaling law correlating pitch to concentration changes as $p\propto c^{-1/3}$. Additionally, containment in spherical particles with a diameter of less than 90 micrometres led to the development of an isotropic core and cholesteric shell [199]; when the diameter was decreased below 20 micrometres, concentric layers were formed parallel to the interface, changing the morphology to bipolar planar [200] Latex, gold, magnetic nanoparticles, and other substances were segregated into an isotropic core. Like these findings, scientists in reference reported splitting fluorescent latex nanoparticles into an isotropic core [201] during confinement in capillaries. This method of partitioning, which adopts the formation of liquid crystals in one phase, can be beneficial for the natural separation of colloids from one another.

3. Applications

3.1. Responsive Materials

The cholesteric helix can be tuned to provide stimuli response materials that may match the complexity of biological systems, which is a big opportunity (may operate on moisture, heat, phase transition, etc.). Low-cost biodegradable optical sensors are made possible by implementing sustainable enhancements like CNC because the underlying nanostructure regulates how the reflected colour structurally responds to an external stimulus. It has been demonstrated that responsive materials can measure pressure in [202], detect humidity in [203,204], solvents in [202], ultraviolet (UV) light in [205], and temperature in [205].

For instance, CNCs-based films have an advantage over other inclusions with comparable properties because of their inherent sensitivity to water. Their low water resistance, which results in the loss of their prized iridescent colour even after being gently inflated by water, restricts their use in humid environments. However, because of their sensitivity to humidity, they make excellent humidity sensors. Recently, the co-assembly of CNC with oxidised starch and tannic acid was researched as a replacement for humidity variations to improve CNC’s solvent sensitivity. A full day of submersion in water did not affect the composite’s structural integrity, vibrant colour, or mechanical characteristics; cross-links were made possible using tannic acid [206]. Starch or tannic acid additions also kept the self-assembly mostly unaltered. As a result of the information provided here, the chirality sensitivity of CNCs is adjustable; that is, the degree of assembly with other substances, such as starch or tannic acid, can affect how sensitive the chirality is to humidity.

He et al. [207] produced CNC composite sheets that responded to moisture and mechanical stress using glycerol as a plasticiser. The structural colours of the films could have their chiral structures altered [207]. There was a reversible colour change when the film was exposed to relative humidity values between 16 and 98 percentBy altering its iridescent hue, the material can also monitor compression pressure quantitatively. In addition to responding to humidity and formaldehyde gas, films created using CNC technology can also change their structural colour and have gas detection capabilities [178]. At RHs of 43, 75, 86, and 99%, the maximum reflection wavelengths of CNC films were measured to be 360, 451, 513, and 525 nm, respectively. Due to their abundance of hydroxyl groups and porosity, CNC can also react to gas [208]. Formaldehyde gas concentration was varied to achieve maximum reflection wavelengths of 378, 404, and 470 nm, respectively, in order to evaluate the response of chiral structures to the gas.

CNCs can be utilised as solvent sensors since their chiral-optical properties can be altered in relation to solvents. Based on this, the researchers developed a unique technique for producing tuneable-colour mesoporous CNC films. Giese and colleagues [202] created mesoporous photonic cellulose (MPC) film by treating a composite of CNCs and a urea-formaldehyde (UF) resin with an alkaline solution. This allowed for quick and reversible structural colour changes in the visible light spectrum. They discovered that the peak reflection wavelength of the composite material was 430 nm in 100 percent ethanol but 840 nm in pure water. The colour will “redshift” when there is more water present. The synthetic cellulose films displayed excellent flexibility due to their reduced crystallinity (in comparison to the initial CNC films’ crystallinity) and the mesoporous structure produced by super-critical drying. Furthermore, the fast and reversible colour change this MPC film undergoes upon swelling makes it ideal for pressure sensing. These new active mesoporous cellulose materials could be useful for biosensing, functional membranes, tissue engineering, and chiral separation.

Instances of materials where PEG-induced “depletion affinity” has been acknowledged are DNA and lyotropic LC [209,210]. CNCs with asymmetric nematic structures can alter their optical properties by altering the PEG’s molecular weight [180]. As pure CNC films are delicate, several water-soluble polymers, such as poly(vinyl alcohol) [12], PEG, and polyurethane [181], have been added to increase the elasticity and good mechanical of the film. With these water-soluble polymers, the pitch size can also be changed. Due to depletion forces, PEG has two effects. First, it enhances the CNC film’s mechanical characteristics. Second, it alters the pitch size due to the effects of depletion.

However, the inclusion of polymers can also alter mechanical characteristics. By altering the polymer fraction, it is possible to change the depletion effect’s iridescent colours from red to blue. The concurrent improvement of mechanical properties enables a wide range of applications, including pressure sensors [113], humidity sensors, and anti-counterfeiting sheets [48]. For example, the reflectance spectra of pure CNC films show a peak at 242 nm, which rises to 361 nm when the PEG weight fraction reaches 30 wt%. Additionally, home furniture and other products can be created by altering the reflection spectrum [180,211] since the reflectance band grows when the PEG level exceeds 25 weight percent. The half-pitch diameter increases from 103 nm to 143 nm when the PEG weight percentage increases from 10% to 20%. Similar conduct was displayed in reference [180]. When the molecular weight of the non-adsorbing polymer rises, the pitch size can also change.

The creation of thin films on a Petri plate because of the evaporation of a tiny volume of an iridescent solution is the most basic demonstration of LC in confinement. Brilliant structural colours can still be created even when the thickness of such films is only a few orders of magnitude greater than the pitch length [212,213]. A planar orientation of the cholesteric nanostructure is easily produced in such thin-film confinement [214], even under quick-drying conditions when disclinations in the cholesteric order are supposed to be kinetically confined. The discontinuous pitch shift brought on by these disclinations becomes critical in very thin films (about 1–2 nm) and can produce an appealing colour mosaic [174]. According to some reports, confinement conceals translational order in smectic crystals [215] and turns the bulk isotropic-to-nematic transition into a continuous ordering from an isotropic to a nematic phase.

In addition, due to uneven film production on various surfaces, suspension can offer a variety of iridescent colours that can be adjusted. The effect of the substrate on the formation of the CNC cholesteric phase was investigated in ref. [216]. When CNC dispersion was dropped on the substrates, the influence of the substrate on the fading out of CNC suspension and its LC formation behaviour was investigated. On glass and stainless steel (SS), CNCs demonstrated initial contact angles of 37.83° and 57.32°, respectively. On the hydrophilic glass surfaces, the cholesteric phase self-assembled from the droplet’s bottom centre and diffused to the edges [217]. The iridescent coating films created on polystyrene (PS), SS, glass, Cu-Zn alloy, and Cu-Ni alloy exhibit distinctive “coffee rings” as a result of the edge-centre ordered drying procedure [125]. The study’s conclusions can be used to spray coat a variety of substrates, but more investigation is required to see whether the surfaces in question inherently display shimmering colour. When utilised as a stimulus-response material, developed iridescence may also be used to determine the surface’s composition. Additionally, coated components and a chemical resistance polymer can produce a vibrant, long-lasting coating, making this a suitable colouring technique for urban development.

Thermotropic LCs can act as a thermal switch and affect the average refractive index of periodic structures by being associated with the chiral structure of CNCs. This has already been done in silica films [218]. Thermotropic LCs can also undergo orientation due to temperature fluctuations. The film became colourless as the temperature increased because the thermotropic agent, originally nematic, changed into an isotropic state around 40 °C. On the other hand, films were iridescent at room temperature. As there was no indication of a reflection signal in the UV-vis spectra, these visual cues were coupled with the optical properties of LC-loaded films. In reference [219], the texture of the suspension of the CNC-grafted poly(N, N-dimethylamino ethyl methacrylate was also altered by temperature. This colour-changing technique has remained largely unexplored.

Pitch was measured using SEM on CNC film cross-sections and optical microscopy on CNC photonic films; the outcomes revealed a low-temperature dependency. The cholesteric stripe’s bright and dark margins were not equal, and their difference varied greatly with temperature and nematic phase. The change from one biaxial cholesteric to (calamitic cholesteric, discotic cholesteric and biaxial cholesteric), the crossovers between biaxial cholesteric and calamitic cholesteric as well as discotic cholesteric and biaxial cholesteric were seen using optical microscopy [220]. Changing the optical properties of photonic substances is a crucial objective in the creation of reflecting displays, screens, and sensors. The optical properties of photonic materials can be modified by adjusting their periodicity or refractive index contrast. Visitors that allow stimuli-induced changes in refractive index and hence fluid modulation of the optical properties of the composite may be present in a chiral nematic mesoporous host. As a result, temperature changes in CNC chiral structures can be programmed.

3.2. Energy Storage Applications

Due to its exceptional biocompatibility, adaptable surface chemistry, renewable and carbon-neutral nature, unsurpassed optical and mechanical capabilities, and great biocompatibility, nanocellulose is becoming increasingly popular [8]. This study’s objective is to present a current assessment of recent nanomaterial advancements and their prospective uses in soft robotics, energy storage, and medical research; therefore, the discussion on energy storage applications fits very well.

As cutting-edge technology (such as portable electronic gadgets, electric cars, and big intermittent battery systems) is integrated into our everyday lives, the need for sophisticated energy storage systems with high energy density, high power density, and extended lifespan has continued to rise [221]. Due to their lengthy lifespan, superior performance, and dependable stability, lithium ion batteries (LIBs) have been the most extensively used candidate systems in commercial electronic products [222,223]. Due to their extended cycle life, high specific power, and energy density, rechargeable lithium-ion batteries (LIBs) are also viable options for sustainable energy storage devices [224,225]. Since the introduction of commercial LIBs in 1991, carbonaceous materials have received a great deal of attention as candidate anode materials due to their respectable theoretical capacity (372 mAh·g⁻¹) [226], good electrical conductivity, and exceptional mechanical-chemical stability. Examples include graphite [227,228], CNT [229], and its associated composites [230,231]. At the same time, environmental concerns have sparked a lot of interest in adopting ecologically benign materials for Li/Na ion batteries sourced from sustainable resources such as [232,233] and [234]. The idea of employing carbon produced from fungi as an anode material for LIBs was put out by Tang et al. [235]. Several resources have been used as carbon sources and have shown outstanding electrochemical performances for Li/Na ion batteries. These resources include banana peels [236], packing peanuts [237], wheat [238], and numerous others [239,240]. The capacity of a battery is measured in milliamp hours (mAH). For example, if a battery has 250 mAH capacity and delivers 2 mA average current to a load, it should last 125 h.

However, the investigations have mostly concentrated on modifying carbon precursors or enhancing material composition. These material-oriented methodologies have not considered additional potential characteristics, such as electrode shape, dispersion, and alignment, that might impact the overall electrochemical kinetics and cycle stability of battery electrodes. For rechargeable energy storage devices, research into the implications of carbon precursors’ structural orientation (i.e., alignment)has thus far attracted little attention. Even if the fundamental elements that make up the electrode are the same, it is crucial to note that the architecture (or alignment) of materials can greatly alter the electrochemical kinetics and stability in energy storage applications [241,242]. For instance, Liu et al. looked at how three inorganic fillers with various alignments affected the ionic conductivity of composite polymer electrolytes [243]. Aligned inorganic nanowires (NW) have ten times higher ionic conductivity than randomly dispersed NWs. In this case, inorganic nanowires of different orientations were mainly utilized as fillers to improve the mobility of Li ions inside the polymer medium and to compensate for the drawbacks of polymer electrolytes. There is no evidence that the structural orientation of inorganic (or organic) components has a direct impact on the overall electrochemical performances of LIBs. To tackle the intriguing issues stated above, cellulose nanocrystals (CNCs) films with different structural orientations were utilized as carbon precursors for this experiment.

In a different study, the sol-gel technique was used to integrate Germanium (IV) oxide (GeO₂) onto chiral nematic CNCs. The procedure maintained the original arrangement of the chiral nematic CNC aerogels. It led to hybrid aerogels with a large proportion of randomly dispersed GeO₂ nanoparticles and concentrated regions up to 705 m²/g. Carbonizing the composite material produced a crystalline structure material with really no pressure collapse and good form restoration following release. By fusing the carbonaceous skeleton’s electrochemical double-layer capacitance with the pseudocapacitive contribution of the GeO₂ nanoparticles, materials with a maximum capacitance (Cp) of 113 F/g and high capacitance retention were created [244]. Similar to this, in work by Lizundia et al. [245], conductive polymers were applied to modified chiral nematic CNC sheets on polymerising pyrrole in place. The TEMPO-oxidation, acetylation, desulfation, and cationization processes did not affect the chiral structures. These innovative materials offer appealing possibilities for eco-friendly sensors and energy storage devices since they are simple to make. The chiral structure was unaffected by TEMPO oxidation, acetylation, desulfation, or cationization.

Synthetic chiral material created by nano-micro-sized matrices has aided chemical synthesis, chiral sensing, chiral catalysis, and metamaterial-based improved optical devices. Both hard template methods and soft template approaches are often used to create chiral substances. This method could be used to produce new chiral nanostructured materials. (1) Chiral material synthesis typically uses mesoporous hosts as hard templates to transfer their nanostructure to other materials. (2) Soft template method: material is created by partially removing the host template. The template uses techniques including hyper molecular aggregation, rapid self-assembly, and molecular evaporation. For nano-mesoporous materials, soft template creation is more flexible than hard template preparation. Chiral compounds based on cellulose have grown in prominence as chiral research shifts from the molecule to the nanoscale.

Template synthesis from nano colloidal LCs in limited shape can be used as a model for the organization of nanoparticles (chiral structure guides their assembly in 3-D space). In reference [201], 3-D confinement of cholesteric LC under two-dimensional containment was investigated. The phase-separated cholesteric shell was built with an isotropic inner thread parallel to the capillary’s long axis and a helicoidal axis parallel to the inner surface of the walls. The generated core-shell LCs might be used as optical waveguides since the geometry of the LCs changed as the amount of confinement increased. The structure was examined over time using POM, and it was discovered that an isotropic core had been precisely constructed after six hours. It took its core 168 h to begin to relax. The time-consuming nature of isotropic and cholesteric CNC rearranging was thus highlighted [201] The average pitch recorded in the core-shell structure of optical waveguides in ref. [201] after switching substrates to a glass capillary was about 9 micrometres, but the structure created in Teflon tubes was 5.8 micrometres under the same conditions. As a result, the pitch object served in the glass capillary was much larger than the one created in Teflon. The mismatch results from surface energy variations between the two surfaces.

In ref. [246], mesoporous black titanium dioxide (TiO_2-X) with the chiral nematic structure of core-shell nanorod was created utilising the templating method once more. Carbonized TiO₂/CNC helical materials were created by chiral depositing TiO₂ nanoparticles onto gelatine functionalized CNCs and calcinating to recover TiO_2-X copies after the carbon is removed. The black TiO₂ was composed of nanorods of chiral nematic crystalline-amorphous TiO₂ and had a mesoporous semiconducting structure. Highly porous nanocarbon networks support the chiral black TiO_2-X nanocrystals that make up the anode electrodes of lithium-ion batteries. In addition to the current efforts, these black TiO₂-_X materials and composites might be useful in fields including energy storage and catalysis. Using silica precursors and evaporation-induced self-assembly of CNC, reference [247] claims that nanocomposites containing chiral nematic structures can be created. Following the pyrolysis and etching of the silica, chiral nematic mesoporous carbon films are produced. Using a specific capacitance of 170 F·g⁻¹ at 230 mA·g⁻¹, mesoporous carbon sheets displayed nearly ideal capacitor behaviour in a symmetric capacitance with H₂SO₄ as the electrolyte. It has also been described in the literature [248,249] to produce energy storage devices employing cellulose filaments and other novel nanomaterials.

To facilitate different interactions with light, templating can also be used. Due to the scripting technique [36,250], the helical shape can be applied to a variety of media. Compared to a dried CNC film, the inorganic replica’s optical sensitivity may differ. While an optically isotropic material displays continuously shifting refractive indices due to rotation of the birefringent CNC rods, an amorphous inorganic film with a template exhibits recurrent discontinuous differences between the refractive indices of air and the substance. By introducing an index-matching fluid into the gaps and allowing it to evaporate, the reflection can be activated and deactivated [36,251]. If a thermotropic nematic was used to replace the gaps, the material’s absence from the gaps would cause its refractive index to change whenever an electric field is applied; a thermotropic nematic loses index matching, which causes the film to seem coloured. A birefringent matrix material lacked an index matching fluid, which prevents such ON/OFF switching of pho-tonic crystal characteristics even when the original dried CNC film is porous. Inorganic and air layer thicknesses, as well as their combined thickness (which establishes the local optical period), are likely to have a greater impact on the colours produced than visible light wavelengths. One potential use for transparent CNC-templated inorganic materials is cholesteric-based mirrorless lasing [36].

According to the sizes, structures, and surface chemical performances of lignocellulosic biomass, new materials and devices for energy storage should be researched. These materials and devices should use various biomass resources and their combinations as building blocks. According to reference [8], nanocellulose can be created from a wide range of sources and processes. For example, TEMPO-oxidized CNCs can be used with transparent conductors to create flexible and optically transparent electrodes [252]. Both forms of CNC contain negatively charged rod particles, and sulfuric acid hydrolysed CNC can be utilised as a template to make chiral nematic mesoporous electrode materials. Research into new materials and production techniques may lead to new opportunities in a range of applications. Even though supercapacitors and lithium-ion batteries (LIBs) have extensively used nanocellulose and its derivative materials [253], only a small number of research on the use of nanocellulose in sodium-ion and Li-S batteries has been published [254,255]. Additionally, several cutting-edge energy storage technologies, including Mg (Al, Mn)-ion batteries, have not received enough attention. Due to its physical robustness, adaptable structure, and surface/interface chemistry, nanocellulose and its derivatives are an environmentally friendly material alternative for rapidly developing energy storage devices.

3.3. Optical and Optoelectronic Applications

The colour of films (produced with liquid crystalline CNCs) is produced by reflected light, which is determined by the wavelength and angle of incoming light. The material looks colourful to the human eye when the incoming light’s wavelength is in the visible range. The usage of the perfect films is constrained by their fragility and uneven structural makeup; therefore, an additional supplementary chemical is needed to address mechanical property weakness. This shortcoming is manageable considering the optical characteristics of the CNC chiral nematic LC that cannot be matched by other rod particles; inherent properties such as handedness, sensitivity to humidity, ability to be surface modified and mixed with other chemicals and wealth of information currently available in the literature. The CNC is also excellent for wettability control and adhesion because of its built-in periodic spirally structured LC structure. As mentioned in the energy storage usage, templating may enhance optical properties. In addition to other uses, novel materials have promise for chiral separation [256], enantioselective adsorption [257], catalysis [258], sensing, optoelectronics, and lithium-ion batteries [259].

Polymers and CNC have been used to boost the mechanical qualities of naturally inspired structures, as some accounts of this research were outlined earlier. For improved mechanical or optical qualities, the insertion of CNC films into larger laminar structures has been investigated [260]. Thin sandwiched structures with bright structural colour, improved mechanical characteristics, and shape memory capabilities were developed by incorporating CNC films into the polymer. Similar to the jewel beetle chrysin resplendent [203], CNC films may be built up on either side of a birefringent membrane to enable the simultaneous reflection of left- and right-handed circularly polarised light. A similar birefringent layer was produced for the sample optical mechanism by impregnating millimetre-scale planar gaps in CNC films with a nematic LC. This layer may be triggered by heat or electric fields to modify the reflected spectrum or its polarization state [261].

Additionally, CNCs sheets only reflect light with a left circular polarisation (LCP). The odd manner chiral cellulose nanorods self-assemble is what causes this property. It is possible to control the switch from right to left-handedness by adjusting the temperature or using an electric field [262]. This method might modify the optical characteristics of different chiral LC systems, which could be used as coatings, polarizers, and filters in optical devices. Using references and highlights,

Table 3. Modulation of optical properties of CNC pure and composite films using references and highlights.

Reference	Highlight
[263]	Widening of left circularly polarization reflected band with addition of micelles
[264]	Distortion of helix during drying phase; impact of vertical compression
[184]	Molecular dynamic simulations proving that increase surface charge increases pitch size
[265]	The pitch of the LC in the box varied due to the isomerization of photosensitive molecules when exposed to alternating ultraviolet and blue light, resulting in a shift in the reflection wavelength.
[133]	Using grinded CNC iridescent film as pigments
[266]	Coffee ring effect leading to non-uniform optical characteristics.
[127]	Employing circular shear flow is applied in the drying process to improve CNC films uniformity.

presents some of the most important findings in the literature on the modification of optical characteristics of CNC pure and composite films.

3.4. Advanced Applications

Liquid crystal production can be considered a purifying technique since CNC can self-partition out of a mixture of colloids. In ref. [267] the co-assembly of CNCs and solid latex nanoparticles was investigated by the author; the nanoparticles were also fluorescently labelled, and co-assembly was investigated in both solution and solid films. The creation of two types of structures—CNC-rich layers with cholesteric structures and randomly distributed latex-rich layers—was demonstrated by the CNC-latex combination. These layers were arranged on the film’s plane, and a sizeable number of latex nanoparticles were dispersed in the cholesteric area of the CNC matrix. With the use of circular dichroism spectroscopy, the chiroptical properties of films were investigated. The CD spectra of a latex-free film revealed a positive single from the deliberate transmission of right-handed circularly polarized light by CNCs. With the addition of latex particles, the volume portion of the 0.31 positive CD signal remained constant, but the peak significantly grew, indicating a wider range of pitch values. The loss of cholesteric order and the 57% drop in signal were both correlated with the CNC percentage. The position maximum also remained constant, indicating that the addition of spherical latex particles did not affect the cholesteric domains.

Following findings from earlier studies on wood-sourced CNCs, measurements of the distance among fringe lines in fingerprint texture revealed that half pitch of domains increased from 14 ± 3 micrometres in latex-free CNC suspension to 2410 micrometres in the presence of latex particles [268]. Subsequent analysis of the fluorescence signal revealed that fluorescence intensity was 30% less cholesteric phase than in the isotropic phase, meaning there was a lower concentration of spherical particulates.

In films, the pitch of the organization is as follows phase decreased from 48 micrometre to 220 ± 80 nm for samples that contain latex particles while it changed to 220 ± 60 nm in samples with no latex particles, following the knowledge that assembly within cholesteric particles occurs primarily in suspension and not in the dried-out form. This is because films formed after drying will have lower pitch values. It was determined that excluded volume effects resulting from depletion pressures between latex spherical nanoparticles [269,270] were responsible for the creation of latex-rich “islands” in CNC-latex films. Due to depletion effects, the addition of tiny colloids to a solution of larger colloids can cause flocculation of the larger colloids; as a result, bigger latex nanoparticles may exhibit attraction in the presence of CNCs [271,272].

The author went on to infer that particle geometry regulates the assembly of a binary mixture of rods and spheres in the absence of interaction [273]. Phase separation between sphere- and rod-rich phases occurs in a liquid state, but at large sphere volume fractions, only one isotropic phase emerges [273]. If an application calls for such arrangements, having spheres within rod-rich phases is crucial in situations when it is intended to prevent rod particles from forming liquid crystalline phases, such as viruses.

4. Conclusions

While CNC-based LC is still a young field of study, it is already extending into intriguing new fields. Currently, CNC-based nanomaterials can be placed artfully into unique self-assembled structures. Here, we have given a thorough overview of the relevant topic, emphasized its conception and discovery while skipping over a few subgenres of developing applications. All CNC-based materials, whether modified or unaltered, should create colloidal LC phases over a certain threshold concentration due to the exceptionally substantial shape anisotropy.

It was discovered that the main interaction stabilizing the equilibrium LC phase was repulsive contact between negatively charged CNC rods. Other research projects that were conducted afterwards led to the rediscovery of the nematic phase as well as other mesophases, including the lamellar and helically chiral LC phases. These findings suggested high-performance applications, including wet spinning of CNC fibres, in addition to reporting unique phase behaviour. Due to their ability to control the internal structure of the photonic crystal, cellulose-based LCs are amazing novel LC materials with extraordinary optical characteristics. A special kind of LC material has the potential for large-scale commercial use due to its straightforward manufacturing procedure, straightforward raw material availability, and low energy consumption. This review briefly discussed how to distinguish left- and right-handed CNCs using molecular dynamic simulations, microscopy, self-assembly, and rheology. Potential applications for producing non-fading colours and energy devices included the use of energy storage and photonic crystals. A product made from CNC and other polymers may be structurally coloured, used as a template, magnetically changeable, and able to change colour in response to humidity. Chirality may make its goods more durable by employing CNC helicoidal organization. The goods may also be made responsive by calcination or coating them with conductive materials, which makes it possible to employ them for applications like intriguing energy storage systems for batteries and a variety of other purposes.

Due to the nanostructure’s poor compatibility with the host medium, scalable fabrication of such materials using mesostructured liquids like LCs is still difficult. The photo-switchable chirality of plasmonic DNA-origami in cellulose nanofiber-based LCs has already been proven. With the help of UV light, the DNA-origami plasmonic nanostructure characteristics of the composite material may be deleted and recovered. Finally, by modifying any parameter that may affect the point at which an isotropic material transitions to a nematic one, it is feasible to modify the chirality of CNCs to suit a range of applications. Due to the wide range of applications, they could serve, iridescent inks created by CNCs are likely to become more well-known shortly.