Effects of Operating Mechanical Conditions and Polymer Networks of Nematic Elastomers on Photo-Induced Mechanical Performances.

Photoresponsive liquid-crystalline elastomers (LCEs) are promising candidates for light-controlled soft actuators. Photoinduced stress/strain originates from the changes in mechanical properties after light irradiation. However, the correlation between the photoinduced mechanical performance and in-use conditions such as stress/strain states and polymer network properties (such as effective crosslink density and dangling chain density) remains unexplored for practical applications. Here, isometric photo-induced stress or isotonic strain is investigated at different operating strains or stresses, respectively, on LCEs with polymer network variations, produced by different amounts of solvent during polymerization. As the solvent volume increases, the moduli and photoinduced stresses decrease. However, the photo-induced strain, fracture strain, fracture stress, and viscosity increase. The optical response performance initially increases with the operating strain/stress, peaks at a higher actuation strain/stress, and then, decreases depending on the polymer network. The maximum work densities, which also depend on the operating stress, are in the range of ≈200-300 kJm-3. These findings, highlighting the significant variations in the mechanical performance with the operating stress/strain ranges and amount of solvent used in the synthesis, are critical for designing LCE-based mechanical devices.

Photo-controllable azobenzene microdroplets on an open surface and their application as transporters.

The control of droplet motion is a significant challenge, as there has been no simple method for effective manipulation. Utilizing light for the control of droplets offers a promising solution due to its non-contact nature and high degree of controllability. In this study, we present our findings on the translational motion of pre-photomelted droplets composed of azobenzene derivatives on a glass surface when exposed to UV and visible light sources from different directions. These droplets exhibited directional and continuous motion upon light irradiation and this motion was size-dependent. Only droplets with diameters less than 10 µm moved with a maximum velocity of 300 µm min-1. In addition, the direction of the movement was controllable by the direction of the light. The motion is driven by a change in contact angle, where UV or visible light switched the contact angle to approximately 50° or 35°, respectively. In addition, these droplets were also found to be capable carriers for fluorescent quantum dots. As such, droplets composed of photoresponsive molecules offer unique opportunities for designing novel light-driven open-surface microfluidic systems.

Light-Driven Liquid Conveyors: Manipulating Liquid Mobility and Transporting Solids on Demand.

The intelligent transport of materials at interfaces is essential for a wide range of processes, including chemical microreactions, bioanalysis, and microfabrication. Both passive and active methods have been used to transport droplets, among which light-based techniques have attracted much attention because they are noncontact, safe, reversible, and controllable. However, conventional light-driven systems also involve challenges related to low transport ability and instability. Because of these shortcomings, technologies that can transport and manipulate droplets and microsolids on the same surface have yet to be realized. The present work demonstrates a light-driven system referred to as a liquid conveyor that enables the transport of both water droplets and microsolids. After the incorporation of an azobenzene-based molecular motor capable of undergoing photoisomerization into the surface liquid layer of this system, an isomerization gradient was induced by exposure to ultraviolet light emitting diodes that induced flow in this layer. Various parameters were optimized, including the concentration of the molecular motor compound, the light intensity, the viscosity of the liquid layer, and the droplet volume. This process eventually achieved the horizontal transport of droplets in any direction at varied rates. As a consequence of the limited heat buildup, the lack of droplet deformation, and extremely small contact angle hysteresis in this system, microsolids on droplets were also transported. This liquid conveyor is a promising platform for high-throughput omni-liquid/solid manipulation in the fields of biotechnology, chemistry, and mechanical engineering.

Photo-Rewritable Glaring Patterns Composed of Stripe Domains in Nematic Elastomers.

Dynamic ordered micropatterns in polymeric materials provide an effective approach for the on-demand tuning of optical properties toward a smart optical material. In this study, it is shown that glaring patterns exhibiting strong anisotropic light diffusion can be developed at specific locations in nematic liquid-crystal elastomers with light-sensitive azobenzene units. Glaring originates from the stripe domains of the nematic directors that self-organize in light-irradiated regions after a simple uniaxial stretching and releasing process without any complicated lithographic technique. The nematic order transiently reduced by the photo-induced cis azobenzene isomers unlocks entropic elasticity, which induces local uniaxial shrinkage that causes buckling of the directors forming stripe domains. The written pattern on the film is tangibly visible with the backlight owing to the difference in anisotropic light diffusion. Furthermore, this pattern can be erased by light irradiation or thermal annealing. These films can be applied to optical elements for achieving augmented luminaries, security labeling, and sign-sheeting applications.

Internal constraints and arrested relaxation in main-chain nematic elastomers.

Nematic liquid crystal elastomers (N-LCE) exhibit intriguing mechanical properties, such as reversible actuation and soft elasticity, which manifests as a wide plateau of low nearly-constant stress upon stretching. N-LCE also have a characteristically slow stress relaxation, which sometimes prevents their shape recovery. To understand how the inherent nematic order retards and arrests the equilibration, here we examine hysteretic stress-strain characteristics in a series of specifically designed main-chain N-LCE, investigating both macroscopic mechanical properties and the microscopic nematic director distribution under applied strains. The hysteretic features are attributed to the dynamics of thermodynamically unfavoured hairpins, the sharp folds on anisotropic polymer strands, the creation and transition of which are restricted by the nematic order. These findings provide a new avenue for tuning the hysteretic nature of N-LCE at both macro- and microscopic levels via different designs of polymer networks, toward materials with highly nonlinear mechanical properties and shape-memory applications.

Light-Driven Dynamic Adhesion on Photosensitized Nematic Liquid Crystalline Elastomers.

In liquid crystal elastomers (LCEs), the internal mechanical loss increases around the nematic-isotropic phase transition and remains high all through the nematic phase, originating from the internal orientational relaxation related to the so-called "soft elasticity". Because the viscoelastic dissipation of the materials affects their adhesion properties, the nematic-isotropic phase transition can cause dramatic changes in the adhesion strength. Although the phase transitions can generally be induced by heat, here, we demonstrate the light-driven transition in dynamic adhesion in dye-doped nematic LCE. The special dye is chosen to efficiently generate local heat on light absorption. The adhesion strength is lowered with fine tunability depending on the light power, which governs the effective local temperature and through that the viscoelastic damping of the system. We demonstrate the light-assisted dynamic control of adhesion in a 90°-peel test and in pick-and-release of objects, which may lead to the development of stimuli-responsive adhesive systems with fine spatio-temporal controls.

Enhanced Dynamic Adhesion in Nematic Liquid Crystal Elastomers.

Smart adhesives that undergo reversible detachment in response to external stimuli enable a wide range of applications in household products, medical devices, or manufacturing. Here, a new model system for the design of smart soft adhesives that dynamically respond to their environment is presented. By exploiting the effect of dynamic soft elasticity in nematic liquid crystal elastomers (LCE), the temperature-dependent control of adhesion to a solid glass surface is demonstrated. The adhesion strength of LCE is more than double in the nematic phase, in comparison to the isotropic phase, further increasing at higher detachment rates. The static work of adhesion, related to the interfacial energy of adhesive contact, is shown to change very little within the explored temperature range. Accordingly, the observed enhanced adhesion in the nematic phase is primarily attributable to the increased internal energy dissipation during the detachment process. This adhesion effect is correlated with the inherent bulk dynamic-mechanical response in the nematic LCE. The reported enhanced dynamic adhesion can lead to the development of a new class of stimuli-responsive adhesives.

Site-specific attraction dynamics of surface colloids driven by gradients of liquid crystalline distortions.

The site-specific migration dynamics of small particles at the air-nematic liquid crystal (LC) interface is investigated, using a LC film with a unique gradient of LC distortions. This gradient has been identified as the direct origin of the site-specific migration, while the elastic multipole interaction between the pre-existing distortion and that induced around particles is negligible. The results reveal a basic behavior of small particles with weak anchoring strength in LC distortions, which is often hidden in prominent elastic multipole interactions typically found with larger particles. Moreover, the present mechanism of the site-specific attraction of particles is specifically relevant to directed patterning and manipulations of nano-particles in nematic LCs with distortions.

Dynamic Contact Guidance of Myoblasts by Feature Size and Reversible Switching of Substrate Topography: Orchestration of Cell Shape, Orientation, and Nematic Ordering of Actin Cytoskeletons.

Biological cells in tissues alter their shapes, positions, and orientations in response to dynamic changes in their physical microenvironments. Here, we investigated the dynamic response of myoblast cells by fabricating substrates displaying microwrinkles that can reversibly change their direction within 60 s by axial compression and relaxation. To quantitatively assess the collective order of cells, we introduced the nematic order parameter of cells that takes not only the distribution of cell-wrinkle angles but also the degree of cell elongation into account. On the subcellular level, we also calculated the nematic order parameter of actin cytoskeletons that takes the rearrangement of actin filaments into consideration. The results obtained on substrates with different wrinkle wavelengths implied the presence of a characteristic wavelength beyond which the order parameters of both cells and actin cytoskeletons level off. Immunofluorescence labeling of vinculin showed that the focal adhesions were all concentrated on the peaks of wrinkles when the wavelength is below the characteristic value. On the other hand, we found focal adhesions on both the peaks and the troughs of wrinkles when the wavelength exceeds the characteristic level. The emergence of collective ordering of cytoskeletons and the adaptation of cell shapes and orientations were monitored by live cell imaging after the seeding of cells from suspensions. After the cells had reached the steady state, the orientation of wrinkles was abruptly changed by 90°. The dynamic response of myoblasts to the drastic change in surface topography was monitored, demonstrating the coordination of the shape and orientation of cells and the nematic ordering of actin cytoskeletons. The "dynamic" substrates established in this study can be used as a powerful tool in mechanobiology that helps us understand how cytoskeletons, cells, and cell ensembles respond to dynamic contact guidance cues.

A two-step method for fabricating large-area textile-embedded elastomers for tunable friction.

Recently, shape-tunable wrinkles formed on an elastomeric sheet with a textile finely embedded in proximity to the surface have been developed for in situ control of friction depending on various situations. For their actual uses, sheets with a large area are desired. A key challenge on their fabrication is to overcome the non-uniformity of the vertical position of the textile embedded within the elastomeric sheet, which causes substantial reduction in the tunable range of friction. The defect originates from the increased difficulty, as the sheet area is scaled up, of squeezing a viscoelastic precursor liquid due to the use of a deformable elastomeric surface. Here, we report a new two-step method for a textile-embedded elastomeric sheet that avoids using the soft elastomeric surface on the squeezing process and requires post-joining to an elastomeric base sheet. The obtained sheet with a large area (180 × 180 mm), was uniform and showed a large change of friction on its strain-induced transformation between flat and wrinkled states. The relationship between the experimentally controllable parameters and the squeeze film hydrodynamics is theoretically discussed, which is generally applicable to precise embedding micro-objects at the elastomer surface.

Uncovering different states of topological defects in schlieren textures of a nematic liquid crystal.

Topological defects are ubiquitously found in physical systems and therefore have been an important research subject of not only condensed matter physics but also cosmology. However, their fine structures remain elusive because of the microscopic scales involved. In the case of a liquid crystal, optical microscopy, although routinely used for the identification of liquid crystal phases and associated defects, does not have resolution high enough to distinguish fine structures of topological defects. Here we show that polarised and fluorescence microscopy, with the aid of numerical calculations on the orientational order and resulting image distortions, can uncover the structural states of topological defects with strength m = ±1 in a thin cell of a nematic liquid crystal. Particularly, defects with m = +1 exhibit four different states arising from chiral symmetry breaking and up-down symmetry breaking. Our results demonstrate that optical microscopy is still a powerful tool to identify fine states of liquid crystalline defects.

Photo-enhanced Aqueous Solubilization of an Azo-compound.

We previously showed that disruption of intermolecular interactions, e.g., by lowering the molecular planarity and/or introducing bent structures, improves the aqueous solubility of compounds, and based upon that work, we hypothesized that azobenzene trans-to-cis photoswitching could also be utilized to enhance the aqueous solubility of compounds. Here, we demonstrate that UV/visible light irradiation can reversibly switch the aqueous solubilization of an anti-cancer candidate drug, a low-molecular-weight kinase inhibitor bearing an azobenzene moiety. The increase of solubilization associated with UV-induced trans-to-cis conversion may have clinical relevance, because the time-scale of thermal cis-to-trans reversion at 37 °C is longer than that of oral absorption.

Electrocapillary Phenomena at Edible Oil/Saline Interfaces.

Interfacial tension between edible oil and saline was measured under applied electric fields to understand the electrocapillary phenomena at the edible oil/saline interfaces. The electric responses of saline droplets in edible oil were also observed microscopically to examine the relationship between the electrocapillary phenomena and interfacial polarization. When sodium oleate (SO) was added to edible oil (SO-oil), the interfacial tension between SO-oil and saline decreased. However, no decrease was observed for additive-free oil or oleic acid (OA)-added oil (OA-oil). Microscopic observations suggested that the magnitude of interfacial polarization increased in the order of additive-free oil < OA-oil < SO-oil. The difference in electrocapillary phenomena between OA- and SO-oils was closely related to the polarization magnitude. In the case of SO-oil, the decrease in interfacial tension was remarkably larger for saline (pH 5.4~5.6) than that for phosphate-buffered saline (PBS, pH 7.2~7.4). However, no difference was observed between the electric responses of PBS and saline droplets in SO-oil. The difference in electrocapillary phenomena for PBS and saline could not be simply explained in terms of polarization magnitude. The ratio of ionized and non-ionized OA at the interfaces changed with the saline pH, possibly leading to the above difference.

Fluorescence microscopy reveals molecular localisation at line defects in nematic liquid crystals.

Topological defects easily form in liquid crystals (LCs) as a result of frustrations in spatially dependent anisotropic molecular ordering, and have been regarded as promising tools for facilitating manipulation of relatively large non-LC materials such as colloids. However, it remains unclear whether low-molecular-weight (LMW) impurities that do not aggregate or self-assemble in bulk LCs because of the dominance of entropy can localise at LC defects. Here, by fluorescence microscopy, we directly show the localisation of LMW molecules at the topological line defects of a nematic LC. It is theoretically explained that excess free energy density of nematic ordering at the defect core allows LMW solutes to accumulate at a non-negligible level overcoming the entropy leading to their uniform distributions. Our results demonstrate the usefulness of LC defects as a bottom-up field that enables micromanipulation of LMW molecules and realisation of transformable three-dimensional micro-architectures composed of versatile small functional molecules.

Formation of Hydroxyapatite Skeletal Materials from Hydrogel Matrices via Artificial Biomineralization.

Several kinds of hydrogels were prepared as mimics for the collagen/acidic protein hydrogel employed as the polymer matrix for mineralization in natural bone formation. The hydrogels prepared as mineralization matrices were employed for synthesizing artificial bones. The artificial bone made from a network of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) prepared by heating (PVA/PAA-h-network) exhibited mechanical properties comparable with those of fish scales. To elucidate the formation mechanism of the artificial bone, we synthesized four further kinds of matrix. Artificial bones were obtained from both a PVA/PAA network prepared by repeated freezing and thawing (PVA/PAA-ft-network) and a chitosan/PAA network, in which hydrogen bonding exists between the two constituent polymers, similar to that observed in a natural collagen/acidic protein network. The artificial bone made from the chitosan/PAA network was confirmed to be formed by the phase transformation of a cartilaginous precursor by a process similar to the transformation of cartilaginous tissue to natural bone. In addition, skeletal phase material, i.e., a homogeneous solid phase of hydroxyapatite/polymers, was formed in the cartilaginous phase, i.e., the hypercomplex gel. The skeletal phase grew thicker at the expense of the cartilaginous phase until it formed the entirety of the composite. Artificial bones were also obtained from a gelatin/PAA network and a poly[N-(2-hydroxyethyl)acrylamide]-co-(acrylic acid) network. These experimental results suggested that the coexistence of proton donor and proton acceptor functions in the hydrogel is a key factor for bone formation. The hydroxyapatite content of our artificial bones was almost conterminous with those of natural bones.

A liquid crystalline chirality balance for vapours.

Chiral discrimination of vapours plays an important role in olfactory perception of biological systems and its realization by artificial sensors has been an intriguing challenge. Here, we report a simple method that tangibly visualizes the chirality of a diverse variety of molecules dissolved from vapours with high sensitivity, by making use of a structural change in a periodic microstructure of a nematic liquid crystal confined in open microchannels. This microstructure is accompanied by a topological line defect of a zigzag form with equal lengths of 'zig' and 'zag.' We find that a tiny amount of vapour of chiral molecules injected onto the liquid crystal induces the imbalance of 'zig' and 'zag' depending on its enantiomeric excess within a few seconds. Our liquid-crystal-based 'chirality balance' offers a simple, quick and versatile chirality-sensing/-screening method for gas-phase analysis (for example, for odours, environmental chemicals or drugs).

Oscillating friction on shape-tunable wrinkles.

Friction on soft materials is strongly correlated with the associated deformation, which may be controlled by the surface topography. We investigate the wearless sliding friction between a rigid hemispherical indenter and a deformable textured surface, which is shape-tunable wrinkles. The size of the indenter is comparable to the wavelength of the wrinkles. We evaluate the effects on the friction of the aspect ratio of the wrinkles, the applied normal load, and the alignment direction of the wrinkles relative to the sliding direction. The frictional oscillations are observed during sliding in the direction perpendicular to the alignment using optical images and friction profiles. The correlation of friction force oscillation with deformation of the wrinkles is elucidated using Hertz contact theory. Within a cycle of frictional oscillation, the friction force increases as the front part of the indenter elastically plows the crests. When the normal load is high and/or the aspect ratio of the wrinkles is low, the indenter continues to squash the wrinkles and remains in contact with them during sliding. Consequently, the amplitude of friction force oscillation relative to the averaged friction force decreases.

Effects of surfactant concentration on formation of high-aspect-ratio gold nanorods.

The effects of surfactant concentration in a growth solution on the elongation of gold nanorods were examined. Gold nanorods were synthesized in solutions with different concentrations of hexadecyltrimethylammonium bromide (HTAB): 100, 200, 300, 400, 500, and 600 mM. The nanorods grown in a solution with higher surfactant concentrations were longer (aspect ratio ~30) than those grown in that with lower concentrations (aspect ratio <10). The self-assembled surfactant structures in the solutions were analyzed using viscosity measurement and small-angle X-ray scattering. These results showed a decrease in the inter-micellar distance with increasing surfactant concentration. Taking the chemical equilibrium for the complex formation between Au ions and HTAB micelles into account, we found that the free Au ion concentration decreases accompanied with the increase in the surfactant concentration. This decrease in the free Au ion concentration suppresses undesirable secondary nucleation of gold crystals in a growth solution, resulting in gold nanorod elongation.

±1/2 wedge disclinations stabilized by a sinusoidal boundary in a thin hybrid nematic liquid-crystal film.

As an interesting example of how geometry affects the formation of defects, we study the defect structures of a hybrid nematic liquid-crystal film in a wedge-shaped cell made up of sinusoidal microwrinkles and an elastomer sheet. When the cell thickness is larger than a threshold value h(c), +1/2 and -1/2 disclinations are simultaneously stabilized along concave grooves and convex crests, respectively. A simple theoretical analysis gives a good estimate of h(c). The disclinations also show alternating optical rotations resulting from the curved boundary and liquid-crystal elastic anisotropy.

Manipulation of liquid filaments on photoresponsive microwrinkles.

Microwrinkle grooves serve as open microchannel capillaries, where the capillary action depends on the wettability of a liquid on the groove surface. Here, we report the photoinduced capillary action of a liquid in such microwrinkle grooves. The wettability is changed through the irradiation of a photoresponsive microwrinkle surface. By utilizing micropattern light-projection apparatus, we prepare liquid filaments that fill only the microgrooves prescribed by the patterned light, with micrometer-scale spatial resolution. This new technology enables the precise spatial control of liquids on a solid surface, and thus, is applicable in the fields of micropatterning and open-channel microfluidics.