Liquid Crystal Research and Novel Applications in the 21st Century

Liquid crystals (LCs) are largely known as the materials used in flat panel displays, from simple pocket calculator displays to laptop screens, all the way to large TVs [1]. The technology is, in principle, still based largely on the original work from half a century ago, albeit of course with numerous advances and improvements [2] in materials, addressing, electrode patterns, alignment, and processing. While excellent displays with great viewing angle characteristics and video capability have been available since the turn of the millennium, fundamental display research has slowed down and progressed towards 3D vision, holographic displays, augmented and virtual reality devices (AV-VR) [3], and flexible displays.

In fundamental research of liquid crystals, interest has somewhat shifted from structure–property relationships like ferroelectric and antiferroelectric LCs (FLC and AFLC) [4,5,6,7], Twist Grain Boundary Phase (TGB) [8,9,10], or bent-shaped liquid crystals [11,12,13,14] towards a broader variety of questions, which are more closely related to other fields of research, like fundamental physics, optics and photonics, topology, biology, and nanotechnology. One can also observe an increasing overlap of topics, which indicates a growing synergy and multi-disciplinarity. An example would be the combination of liquid crystals and polymers to LC elastomers, which show promise in the application for soft robotics. Yet this should not take away from the interesting developments in the synthesis of new materials showing novel phases, particularly those of the nematic state, for example, the twist-bend phases, which by now have been studied extensively through materials [15,16] and theory [17]. Probably even more of interest in the future will be the recent discovery of ferroelectric nematic liquid crystals [18,19], the long sought “holy grail” of LC materials.

A topic that has largely raised interest from the fundamental and application point of view are rubber-like materials, elastomers, with liquid crystalline order [20]. These can be exploited as multifunctional materials, especially when prepared as composites [21]. Applications are mainly proposed for their use as smart, soft actuators [22], for example, in the field of soft, small-scale robotics [23], but also other applications as we shall see below. Into a similar category falls the technique of fibre drawing from liquid crystals, often from lyotropic phases. Such fibres have been known for quite some time in the form of Kevlar^® or Nomex^®, but more recently additional functionalities besides high tensile strength have been incorporated through the use of 2D materials such as graphene oxide [24,25] or MXenes [26]. These functionalities can comprise conductivity, so that one can weave high-tensile fabrics with the possibility to generate power.

Another trend that has set off an increasing activity in liquid crystal-related work is the development of advanced functional materials, using the self-organisation and self-assembly of liquid crystals together with properties that are electric, magnetic, mechanical, or optic in nature [27]. Liquid crystals used can be thermotropic or lyotropic, and the functionality is often added via dispersion of colloids. The interactions between liquid crystal and colloidal particles lead to deformations of the director field and the observation of point or line defects. Furthermore, these defects may interact to form two- or three-dimension colloidal crystals [28]. Colloids of different shape and form, like plates, knots, or helices, embedded in a liquid crystal matrix can produce artificial materials of great complexity [29,30]. Other examples can be found by dispersing low-dimensional carbon allotropes in thermotropic liquid crystals [31], or the formation of lyotropic liquid crystal phases by dispersing shape-anisotropic colloids in isotropic solvents [32]. These materials open a whole new field of tunable and switchable devices in liquid crystal-aided nanotechnology and nanoscience [33].

Colloidal liquid crystals are closely related to active liquid crystals [34], where motile bacteria, viruses, phages, or spindle-shaped cells, self-propelled, and often biological colloids are dispersed in liquid crystals [35]. Again, the latter can be of the thermotropic or the lyotropic type but are often lyotropic to assure a longer survival time. Similarly, such colloidal materials in an isotropic solvent, (often water), can also lead to the formation of liquid crystal phases, depending on concentration. Active liquid crystals or living liquid crystals are gaining increasing interest as topological defects, for example, allow the steering of viruses or bacteria. Due to their monodispersion, they can also be exploited as model systems in soft matter physics [36].

Liquid crystals also increasingly draw on self-organisation mechanisms observed in nature, bio-inspired liquid crystals and materials. Structures formed are often chiral [37]-like helical cholesteric structures found in certain beetles, which demonstrate quite astonishing optical properties, such as the reflection of circular polarised light, but which could also be used as active media in bio-lasers. Crosslinking of dispersed bifunctional monomers can lead to bicontinuous films with widened selective reflection, or tunable reflection wavelength, for example, in the use of smart glass, privacy windows, or scattering displays, which can also be produced with flexible substrates. But also, Blue Phases and lyotropic systems can be achieved from nature-inspired materials. Examples of the latter are of the colloidal type, for instance, liquid crystal phases of DNA or from cellulose nanocrystals (CNC), which can also be dried into solid films with selective reflection properties [37]. One cannot only observe the overlap with a range of photonic properties, but also in the production of multifunctional materials as they were discussed above, for example, as actuators and sensors in soft bio-inspired robotics [38] and generally as nature-inspired liquid crystal-actuator materials [39]. Such actuators can be driven by electric or magnetic fields, light, temperature, or pressure [40], ideal for soft robots. Cellulose nanocrystals have attracted much attention in recent years as colloidal liquid crystals on the basis of biological structures [41]. Their inherent chirality leads to lyotropic cholesteric liquid crystals for which the pitch can be varied by a large variety of parameters, such as concentration, aspect ratio, crystallite surface treatment, or the addition of salt. This varies the rheology and optical properties of the liquid crystal. Additionally, CNCs may be combined with polymers or 2D materials in composites, varying mechanical properties.

Much related to bio- or nature-inspired liquid crystals is the use of these materials for biomedical applications [42], due to their inherent biocompatibility and responsiveness. This includes all areas of biomedicine, not only technical aspects such as bioimaging and biosensing, also through wearable technologies, but also implants and tissue engineering, and of course drug delivery. For the encapsulation, transport, and delivery of drugs, cubosomes have proven to be of substantial potential [43,44,45]. These are liquid crystalline structures formed from the cubic phases of lipid molecules. Choice of lipid and amphiphiles, liquid crystalline phase, alky-chain length, stabilisers, and many more parameters allow a large variety of tuning mechanisms, which is beneficial for the development of the ideal structure for transport, delivery, and release of specific drugs.

One of the general topics of mathematical physics that has found its way into liquid crystal research over the last decade or so, and which is attracting increasing interest, is that of topology [46]. Also a topic in the wider field of soft matter, topology and knot theory, as well as defects and solitons, can be found in descriptions of polymers, DNA, proteins, living matter, or colloids. Examples involving liquid crystals are the following: (1) linear chains or 2D and 3D aggregates of colloids being held together by knots [28,47,48]; (2) motile bacteria in lyotropic nematic liquid crystals being steered by topological defects; or (3) the topology of liquid crystal director fields confined to droplets or shells. Closely related systems are topological solitons observed in liquid crystals [49,50]. The latter field has evolved quite rapidly over recent years with the experimental observation and description of skyrmions and skyrmion bags, hopfions, directrons and heliknotons, transformations between these solitons, interactions, and their dynamics and collective behaviour.

From a more applicational point of view, the alignment of liquid crystals on surfaces seems to have experienced a renaissance with the development of increasingly sophisticated techniques of producing patterned substrates, for example, through photo-patterned substrates for the alignment of liquid crystals for photonic applications [51]. But also, bio-inspired surfaces [52] can be used to tune the wettability of liquid crystals on surfaces to produce semi-droplets or droplets for micro-photonics; substrates can be patterned for tunable meta-materials or for the use in liquid crystal-based sensors for gases, solvents, or biomolecules.

Non-display applications of liquid crystals have taken hold of a considerable amount of efforts, now that large-area, high-performance screens are readily available at a decent price. We have already mentioned elastomers and their potential for robotics, as well as polymer stabilised- and polymer-dispersed liquid crystals for use in privacy windows, smart glass, and as environmental building materials for heat regulation. Other applications of liquid crystal polymers have been proposed as membranes in fuel cells [53]. Different liquid crystal phases can be used in self-assembled, tunable, soft photonics [54], or as light-driven materials in photochromic, photo-stimulated, or photo-modulated applications [55]. Of interest is certainly also the development of tunable lasers of liquid crystal-based materials [56,57], which comprise band-edge lasers of cholesteric and Blue Phases, and random lasers of nematics, polymer-dispersed liquid crystals (PDLC), quantum dot-doped LCs, and other nanoparticles dispersed in the liquid crystal. An interesting development can also be seen in micro-lasers, with whispering gallery modes observed from liquid crystal droplets [58]. A further field of LC applications that has generated significant impact is that of chemical and biochemical sensors on the basis of texture transitions of liquid crystals, either from homeotropic to planar orientation or vice versa [59,60,61,62,63]. These offer the opportunity to mass produce sensor devices at a reasonable price, which are easy to use and transportable, offering a potential self-test opportunity for the next pandemic.

At last, one also needs to mention that the field of computer modelling and simulation of liquid crystals has made some significant steps forward in recent years [64]. Of course, research can still not perform full atomic simulations of thermodynamic ensembles, neither for thermotropic liquid crystals and certainly not for lyotropic ones, which face the additional difficulty of the large number of solvent molecules present. But nevertheless, bent-core phases, the newly discovered nematic phases—like twist-bend nematics—are obtained, as are lyotropic systems and chromonics, as coarse-graining techniques have evolved and improved.

We have listed several aspects of liquid crystal research, together with a range of corresponding recent review articles, which I feel have held a preeminent position of interest in the last decade and which promise to continue to do so in the short- and medium-term future. This list is obviously somewhat biassed, and I am sure that readers may have other topics that they feel deserve mentioning as well. In any case, the breadth of research around liquid crystals has clearly increased over recent years, which is also reflected by the articles submitted to this Special Issue on liquid crystal research and applications of the 21st century. These can be arranged into five general areas, which are often interconnected, reflecting the multi-disciplinarity of the topics, also in relation to aspects mentioned above:

A: Liquid crystal elastomers and polymer-modified liquid crystals, covering the role of elastomers in biological applications [65] and multi-mode shape-morphing [66].

B: Photonics and sensors, summarising Tamm plasmons in liquid crystal devices [67], density functional theory calculations to interpret experimental IR spectra of chiral mesogens [68] and photo-physical properties of liquid crystals [69].

C: Topology and solitons in liquid crystals, reviewing chiral, topological, and knotted colloids in liquid crystals [48], and studying topological defect annihilation of colloid-doped LCs by machine learning [70].

D: Interactions with substrates and alignment, giving accounts of polarisation coupling between ferroelectric liquid crystals and solid ferroelectrics [71], a detailed investigation of the cholesteric lying helix state [72], and effects of photo-patterning conditions on anchoring strength [73].

E: Phase structure of thermotropic and lyotropic liquid crystals, providing a structural study of the nematic phase [74], and lyotropic structures from sphere-rod amphiphilic compounds in a solvent [75].

I hope that this Special Issue of Crystals conveys the large range of research topics covered by liquid crystal-based systems, from chemistry, physics, and mathematics to biology and engineering. I hope that it also conveys the excitement of timely research topics available, ranging from topology and solitons to defect dynamics, from elastomers and photonics to biomaterials, and phase structures to subtle interactions with surfaces. I would like to thank all contributors for their time and engagement with this issue on liquid crystal research and novel applications in the 21st century.