Colloids and Interfaces in Printing Technology

Vesicles have recently found widespread use in applications such as conditioning of textiles, paper and hair, as well as transdermal drug delivery. The mode of treatment in several such cases involves the application of droplets of aqueous dispersions of vesicles onto dry porous substrates like paper and textiles. One of the factors which affects the performance of such treatments is the rate at which the droplets spread and imbibe on the porous substrate. Depending upon the specific purpose of the treatment either a fast or slow droplet spreading kinetics could be desired. Therefore, it is important to have a good understanding of the droplet spreading process and the factors which influence it. In this work, an experimental investigation of the simultaneous spreading and imbibition of vesicle dispersion droplets on cellulose filter papers is carried out. Two different types of vesicles which are composed of similar lipid molecules but exhibit contrasting lipid bilayer phase behavior are used. Two different grades of filter papers with comparable porosities but different thicknesses are used as porous substrate. It is found that the droplet spreading behavior is of the “complete wetting” type on the thicker porous substrate, whereas it is of the “partial wetting” type on the thinner substrate. Furthermore, it is observed that the spreading of droplets containing vesicles with liquid-crystalline phase bilayers occurs faster than that of vesicles with solid-gel phase bilayers. The secondary radial penetration which commences after the initial droplet spreading is complete is also investigated and discussed. Full article

Aerogel objects inspired by plant cell wall components and structures were fabricated using extrusion-based 3D printing at cryogenic temperatures. The printing process combines 3D printing with the alignment of rod-shaped nanoparticles through the freeze-casting of aqueous inks. We have named this method direct cryo writing (DCW) as it encompasses in a single processing step traditional directional freeze casting and the spatial fidelity of 3D printing. DCW is demonstrated with inks that are composed of an aqueous mixture of cellulose nanocrystals (CNCs) and xyloglucan (XG), which are the major building blocks of plant cell walls. Rapid fixation of the inks is achieved through tailored rheological properties and controlled directional freezing. Morphological evaluation revealed the role of ice crystal growth in the alignment of CNCs and XG. The structure of the aerogels changed from organized and tubular to disordered and flakey pores with an increase in XG content. The internal structure of the printed objects mimics the structure of various wood species and can therefore be used to create wood-like structures via additive manufacturing technologies using only renewable wood-based materials. Full article

Surface-tension induced flows may have a significant impact on the surface topography of thin films or small printed structures derived from polymer solution processing. Despite a century of research on Marangoni convection, the community lacks quantitative experimental flow field data, especially from within drying solutions. We utilize multifocal micro particle tracking velocimetry (µPTV) to obtain these data and show a calibration routine based on point spread function (PSF) simulations as well as experimental data. The results account for a varying sample refractive index, beneficial cover-glass correction collar settings as well as a multifocal lens system. Finally, the calibration procedure is utilized exemplarily to reconstruct a three-dimensional, transient flow field within a poly(vinyl acetate)-methanol solution dried with inhomogeneous boundary conditions. Full article

We considered pattern formation, i.e. viscous fingering, in the ink splitting process between an elastic flexographic printing plate and the substrate. We observed an unexpected scaling behavior of the emerging pattern length scale (i.e., finger width) as a function of printing velocity, fluid viscosity, surface tension, and plate elasticity coefficients. Scaling exponents depended on the ratio of the capillary number of the fluid flow, and the elastocapillary number defined by plate elasticity and surface tension. The exponents significantly differed from rigid printing plates, which depend on the capillary number only. A dynamic model is proposed to predict the scaling exponents. The results indicate that flexo printing corresponded to a self-regulating dynamical equilibrium of viscous, capillary, and elastic forces. We argue that these forces stabilize the process conditions in a flexo printing unit over a wide range of printing velocities, ink viscosities, and mechanical process settings. Full article

In this study, multilayer organic light-emitting diodes (OLEDs) consisting of three solution-processed layers are fabricated using slot die coating, gravure printing, and inkjet printing, techniques that are commonly used in the industry. Different technique combinations are investigated to successively deposit a hole injection layer (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)), a cross-linkable hole transport layer (N,N′-bis(4-(6-((3-ethyloxetan-3-yl)methoxy)-hexyloxy)phenyl)-N,N′-bis(4-methoxyphenyl)biphenyl-4,4′-diamin (QUPD)), and a green emissive layer (TSG-M) on top of each other. In order to compare the application techniques, the ink formulations have to be adapted to the respective process requirements. First, the influence of the application technique on the layer homogeneity of the different materials is investigated. Large area thickness measurements of the layers based on imaging color reflectometry (ICR) are used to compare the application techniques regarding the layer homogeneity and reproducible film thickness. The total stack thickness of all solution-processed layers of 32 OLEDs could be reproduced homogeneously in a process window of 30 nm for the technique combination of slot die coating and inkjet printing. The best efficiency of 13.3 cd A⁻¹ is reached for a process combination of slot die coating and gravure printing. In order to enable a statistically significant evaluation, in total, 96 OLEDs were analyzed and the corresponding 288 layers were measured successively to determine the influence of layer homogeneity on device performance. Full article

There is a big gap between idealized colloid science and the needs of a printing ink formulator. This often leads to formulations based on intuition and experience rather than good science. By bringing together the most relevant colloid and interface science with the capabilities of modern apps, it is possible to bridge the gap between theory and reality to the benefit of both the colloid science community and those who need the science to improve their formulations. The process of making current science usable also exposes the limitations of available theories. This suggests a methodology for highlighting the grand challenges to the colloid science research community, including the challenge of making any new science more usable. Full article

Polylactic acid (PLA) filaments are very popular as a thermoplastic source used in the 3D printing field by the “Fused Deposition Modeling” method in the last decade. The PLA market is expected to reach 5.2 billion US dollars in 2020 for all of its industrial uses. On the other hand, 3D printing is an expanding technology that has a large economic potential in many industries where PLA is one of the main choices as the source polymer due to its ease of printing, environmentally friendly nature, glossiness and multicolor appearance properties. In this review, we first reported the chemical structure, production methods, general properties, and present market of the PLA. Then, the chemical modification possibilities of PLA and its use in 3D printers, present drawbacks, and the surface modification methods of PLA polymers in many different fields were discussed. Specifically, the 3D printing method where the PLA filaments are used in the extrusion-based 3D printing technologies is reviewed in this article. Many methods have been proposed for the permanent surface modifications of the PLA where covalent attachments were formed such as alkaline surface hydrolysis, atom transfer polymerization, photografting by UV light, plasma treatment, and chemical reactions after plasma treatment. Some of these methods can be applied for surface modifications of PLA objects obtained by 3D printing for better performance in biomedical uses and other fields. Some recent publications reporting the surface modification of 3D printed PLA objects were also discussed. Full article

The spreading of small liquid drops over thin and thick porous layers (dry or saturated with the same liquid) is discussed in the case of both complete wetting (silicone oils of different viscosities over nitrocellulose membranes and blood over a filter paper) and partial wetting (aqueous SDS (Sodium dodecyl sulfate) solutions of different concentrations and blood over partially wetted substrates). Filter paper and nitrocellulose membranes of different porosity and different average pore size were used as a model of thin porous layers, sponges, glass and metal filters were used as a model of thick porous substrates. Spreading of both Newtonian and non-Newtonian liquid are considered below. In the case of complete wetting, two spreading regimes were found (i) the fast spreading regime, when imbibition is not important and (ii) the second slow regime when imbibition dominates. As a result of these two competing processes, the radius of the drop goes through a maximum value over time. A system of two differential equations was derived in the case of complete wetting for both Newtonian and non-Newtonian liquids to describe the evolution with time of radii of both the drop base and the wetted region inside the porous layer. The deduced system of differential equations does not include any fitting parameter. Experiments were carried out by the spreading of silicone oil drops over various dry microfiltration membranes (permeable in both normal and tangential directions) and blood over dry filter paper. The time evolution of the radii of both the drop base and the wetted region inside the porous layer were monitored. All experimental data fell on two universal curves if appropriate scales are used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer on dimensionless time. The predicted theoretical relationships are two universal curves accounting quite satisfactorily for the experimental data. According to the theory prediction, (i) the dynamic contact angle dependence on the same dimensionless time as before should be a universal function and (ii) the dynamic contact angle should change rapidly over an initial short stage of spreading and should remain a constant value over the duration of the rest of the spreading process. The constancy of the contact angle on this stage has nothing to do with hysteresis of the contact angle: there is no hysteresis in the system under investigation in the case of complete wetting. These conclusions again are in good agreement with experimental observations in the case of complete wetting for both Newtonian and non-Newtonian liquids. Addition of surfactant to aqueous solutions, as expected, improve spreading over porous substrates and, in some cases, results in switching from partial to complete wetting. It was shown that for the spreading of surfactant solutions on thick porous substrates there is a minimum contact angle after which the droplet rapidly absorbs into the substrate. Unfortunately, a theory of spreading/imbibition over thick porous substrates is still to be developed. However, it was shown that the dimensionless time dependences of both contact angle and spreading radius of the droplet on thick porous material fall on to a universal curve in the case of complete wetting. Full article