Topographical changes in photo-responsive liquid crystal films: a computational analysis

Photo-activated dynamic isomerization induced large density changes in liquid crystal polymers: A molecular dynamics study

We use molecular dynamics simulations to unravel the physics underpinning the light-induced density changes caused by the dynamic trans-cis-trans isomerization cycles of azo-mesogens embedded in a liquid crystal polymer network, an intriguing experimental observation reported in the literature. We employ two approaches, cyclic and probabilistic switching of isomers, to simulate dynamic isomerization. The cyclic switching of isomers confirms that dynamic isomerization can lead to density changes at specific switch-time intervals. The probabilistic switching approach further deciphers the physics behind the non-monotonous relation between density reduction and light intensities observed in experiments. Light intensity variations in experiments are accounted for in simulations by varying the trans-cis and cis-trans isomerization probabilities. The simulations show that an optimal combination of these two probabilities results in a maximum density reduction, corroborating the experimental observations. At such an optimal combination of probabilities, the dynamic trans-cis-trans isomerization cycles occur at a specific frequency, causing significant distortion in the polymer network, resulting in a maximum density reduction.

Numerical analysis and design of a light-driven liquid crystal polymer-based motorless miniature cart

Liquid crystal polymers are a special class of soft materials that can change their shape in response to numerous stimuli such as light, heat, electric field, and chemicals. The ability to tailor the deformed shape by tuning the alignment of mesogens across the film has enabled the researchers to generate unique motions from these liquid crystal polymer thin films. Simulating such motions might allow us to understand the underlying mechanisms better and could lead to novel designs. In this paper, we analyze the kinematics of the light-driven rolling motion of wheels fabricated with azobenzene-doped glassy liquid crystal networks through a one-way coupled transient photo-mechanical model. The influence of the isomerization parameters and the alignment of mesogens through the thickness on the kinematics of the wheel is presented. The developed model is further used to assess the feasibility of a light-actuated four-wheeled cart with wheels made of azobenzene-doped liquid crystal network thin films.

Spatially and Reversibly Actuating Soft Gel Structure by Harnessing Multimode Elastic Instabilities

Autonomous shape transformation is key in developing high-performance soft robotics technology; the search for pronounced actuation mechanisms is an ongoing mission. Here, we present the programmable shape morphing of a three-dimensional (3D) curved gel structure by harnessing multimode mechanical instabilities during free swelling. First of all, the coupling of buckling and creasing occurs at the dedicated region of the gel structure, which is attributed to the edge and surface instabilities resulted from structure-defined spatial nonuniformity of swelling. The subsequent developments of post-buckling morphologies and crease patterns collaboratively drive the structural transformation of the gel part from the "open" state to the "closed" state, thus realizing the function of gripping. By utilizing the multi-stimuli-responsive nature of the hydrogel, we recover the swollen gel structure to its initial state, enabling reproducible and cyclic shape evolution. The described soft gel structure capable of shape transformation brings a variety of advantages, such as easy to fabricate, large strain transformation, efficient actuation, and high strength-to-weight ratio, and is anticipated to provide guidance for future applications in soft robotics, flexible electronics, offshore engineering, and healthcare products.

Molecular Orientation Change Nearby Topological Defects Observed by Photo-Induced Polarization/Phase Microscopy

Topological defects in liquid crystals (LCs) have been intensively studied and intentionally generated in an organized way recently because they could control the alignment and motion of LCs. We studied how the topological defects could change the molecular orientation/alignment from the observation of photo-induced orientation change of a photo-responsive LC. The photo-induced dynamics was observed by an LED-induced time-resolved polarization/phase microscopy with white light illumination. From the color image sequence, we found that the molecular orientation change started from the topological defects and the orientation change propagated as a pair of defects and was connected, and further disordering was induced as a next step after the initial orientation change finished.

Photoswitchable chevron topographies of glassy nematic coatings

We report a strategy to create photoswitchable chevron topographies via buckling of glassy nematic coatings with zigzag director alignments on soft elastic substrates. The idea is confirmed by numerical simulations where the nonlinear deformation of the coating is modeled by the Föppl-von Kármán plate theory. It is remarkable that the inclination angle of the chevron pattern may deviate significantly from the director orientation and depends on the period of director alignment. Our quantitative analysis shows that the phenomena are caused by in-plane shear stress which alters the direction of maximum principal stress in the coating and decreases monotonically with decreasing period of the director distribution.

Alignment Control of Nematic Liquid Crystal using Gold Nanoparticles Grafted by the Liquid Crystalline Polymer with Azobenzene Mesogens as the Side Chains

The gold nanoparticles highly grafted by a liquid crystalline polymer (LCP) with azobenzene mesogens as the side chain (denoted as Au@TE-PAzo NPs) are successfully designed and synthesized by the two-phase Brust-Schiffrin method. The chemical structures of the monomer and polymer ligands have been confirmed by nuclear magnetic resonance, and the molecular weight of the polymer is determined by gel permeation chromatography. The combined analysis of transmission electron microscopy and thermogravimetric analysis shows that the size of the nanoparticles is 2.5(±0.4) nm and the content of the gold in the Au@TE-PAzo NPs is ca. 17.58%. The resultant Au@TE-PAzo NPs can well disperse in the nematic LC of 5CB. The well-dispersed mixture with appropriate doping concentrations can automatically form a perfect homeotropic alignment in the LC cell. The homeotropic alignment is attributed to the brush formed by Au@TE-PAzo NPs on the substrate, wherein the Au@TE-PAzo NPs gradually diffuse onto the substrate from the mixture. On the contrary, the pure side chain LCPs cannot yield vertical alignment of 5CB, which indicates that the alignment of 5CB is ascribed to the synergistic interaction of the nanoparticles and the grafted LCPs. Moreover, Au@TE-PAzo NPs show excellent film-forming property on account of their periphery of high densely grafted LCPs, which can form uniform thin film by spin-coating. The resultant thin film also can prompt the automatical vertical alignment of the nematic 5CB. Further, upon alternative irradiation of UV and visible light, the alignment of 5CB reversibly switches between vertical and random orientation because of the trans-cis photoisomerization of the azobenzene group on the periphery of Au@TE-PAzo NPs. These experimental results suggest that this kind of nanoparticles can be potentially applied in constructing the remote-controllable optical devices.