We report on the fabrication of a rewritable and reprogrammable dual‐photoresponsive liquid crystalline‐based actuator containing an azomerocyanine dye that can be locally converted into the ...hydroxyazopyridinium form by acid treatment. Each dye absorbs at a different wavelength giving access to programmable actuators, the folding of which can be controlled by using different colors of light. The acidic patterning is reversible and allows the erasing and rewriting of patterns in the polymer film, giving access to reusable, adjustable soft actuators.
Bending patterns: Rewritable and reprogrammable actuators can be obtained by using the local activation of an initially chemically homogenous film. The locally activated material is exposed to UV light, which results in the precise folding of the actuator. By switching the wavelength, a different shape can be obtained. Furthermore, the pattern can be erased and a new one can be written, giving access to a multitude of actuated shapes.
Liquid crystals are the basis of a pervasive technology of the modern era. Yet, as the display market becomes commoditized, researchers in industry, government and academia are increasingly examining ...liquid crystalline materials in a variety of polymeric forms and discovering their fascinating and useful properties. In this Review, we detail the historical development of liquid crystalline polymeric materials, with emphasis on the thermally and photogenerated macroscale mechanical responses--such as bending, twisting and buckling--and on local-feature development (primarily related to topographical control). Within this framework, we elucidate the benefits of liquid crystallinity and contrast them with other stimuli-induced mechanical responses reported for other materials. We end with an outlook of existing challenges and near-term application opportunities.
This work presents an approach to create mechanical undulations at a solid organic coating surface under the influence of an electric field. The coating is fabricated through polymerization of chiral ...reactive mesogens aligned in their fingerprint mode on top of interdigitated electrodes. The fingerprint mode gives a corrugation of the surface perpendicular to the helix axes. When a lateral alternating electric field is applied, the order parameter of the helicoidally packed mesogens is reduced. This simultaneously leads to an inversion of the fingerprint heights, an overall thickness increase, and a chaotic and fast surface oscillation. These three effects work in concert stimulating wavy deformation figures at the coating surface. The process is fast and reversible; the dynamics of the topographic textures stop immediately when the electric field is switched off. The continuous generation of surface undulations sustains transport of species at the coating surface. It removes dust and debris providing an active dust control.
A self‐cleaning coating is developed that keeps its surface clean, even in absence of water. A chiral‐nematic polymer coating on interdigitated electrodes is electrically activated. The AC field inverts the characteristic fingerprint surface topography and makes dust and sand particles slide off.
Chiral‐nematic polymer network coatings form a “fingerprint” texture through self‐assembly. For this purpose the molecular helix of the coating is oriented parallel to the substrate. The coating has ...a flat surface but when actuated by light in the presence of a copolymerized azobenzene compound, 3D fingerprint structures appear in the coating. The helix forms protrusions at the positions where the molecules are aligned parallel to the surface and withdraws at the positions where the orientation is perpendicular. This process proceeds rapidly and is reversible, that is, the fingerprint‐shaped protrusions disappear when the light is switched off. The texture in the on‐state resembles that of a human fingerprint and is used to manipulate the gripping friction of a robotic finger. The friction coefficient drops by a factor of four to five when the fingerprint switched on because of reduced surface contacts.
Science friction: Artificial fingerprints are switched between a flat “off” state and a protruding “on” state. The switching principle is based on a modulated change of liquid‐crystal chiral‐nematic order, thus creating geometric changes. When illuminated with UV light, the 3D fingerprints are activated by azobenzene and appear in the coating. The surface friction decreases when the fingerprints form and increases when they are switched off.
Chemical networks and molecular switches dominate the area of research geared toward macroscopic motion of materials. A counter‐intuitive approach to create self‐sustained oscillation by light ...irradiation of ordinary photostabilizers in splay‐aligned liquid‐crystalline networks made from commercial mesogens is developed. Photostabilizers or any molecules that are able to quickly dissipate the absorbed light through heat, by vibrational and/or rotational modes, can reach self‐oscillating macroscopic motion where self‐shadowing plays a critical role. The mechanical self‐oscillation is linked to temperature oscillations and the asymmetric response over the film thickness. Only a localized responsive zone, acting as hinge, activates the oscillation of a beam‐shaped device. The outcome of this research is extended from UV to near‐IR actuation, making bulk applications to convert sunlight into mechanical work within reach.
Commonly used photostabilizers and dyes are added to liquid‐crystalline polymeric materials to bring out‐of‐equilibrium deformation under continuous input of energy. Molecules able to quickly dissipate the absorbed light through heat, by vibrational and/or rotational modes, can reach self‐oscillating macroscopic motion where self‐shadowing plays a critical role. The mechanical self‐oscillation is linked to thermal oscillation, demonstrating the key function of the photothermal effect.
Monolithically ordered liquid crystal polymer networks are formed by the photoinitiated polymerization of multifunctional liquid crystal monomers. This paper describes the relevant principles and ...methods, the basic structure–property relationships in terms of mesogenic properties of the monomers, and the mechanical and optical properties of the polymers. Strategies are discussed to control the molecular orientation by various means and in all three dimensions. The versatility of the process is demonstrated by two examples of films with a patterned molecular order. It is shown that patterned retarders can be made by a two-step polymerization process which is successfully employed in a transflective display principle. A transflective display is a liquid crystal display that operates in both a reflective mode using ambient light and a transmissive mode with light coming from a backlight system. Furthermore, a method is discussed to create a patterned film in a single polymerization process. This film has alternating planar chiral nematic areas next to perpendicularly oriented (so-called homeotropic) areas. When applied as a coating to a substrate, the film changes its surface texture. During exposure to UV light, it switches from a flat to a corrugated state.
Photoactivated generation of disorder in a liquid crystal network produces free volume that leads to the controlled formation of dynamic corrugations at its surface. The liquid crystal order ...amplifies the deformation of copolymerized azobenzene, which takes place on molecular length scales, to a micrometre-sized macroscopic phenomenon based on changes in density. We postulate a new mechanism in which continuous oscillating dynamics of the trans-to-cis isomerization of the azobenzene overrules the net conversion, which is currently considered as the origin. This is supported by a significant local density decrease when both the trans and cis isomers are triggered simultaneously, either by dual-wavelength excitation or by the addition of a fluorescent agent converting part of the light to the cis-actuating wavelengths. This new insight provides a general guideline to boost free volume generation leading not only to larger macroscopic deformations but also to controllable and faster non-equilibrium dynamics.
Many publications report on stimuli responsive coatings, but only a few on the controlled release of species in order to change the coating surface properties. A sponge‐like coating that is able to ...release and absorb a liquid upon exposure to light has been developed. The morphology of the porous coating is controlled by the smectic liquid crystal properties of the monomer mixture prior to its polymerization, and homeotropic order is found to give the largest contraction. The fast release of the liquid can be induced by a macroscopic contraction of the coating caused by a trans to cis conversion of a copolymerized azobenzene moiety. The liquid secretion can be localized by local light exposure or by creating a surface relief. The uptake of liquid proceeds by stimulating the back reaction of the azo compound by exposure at higher wavelength or by thermal relaxation. The surface forces of the sponge‐like coating in contact with an opposing surface can be controlled by light‐induced capillary bridging revealing that the controlled release of liquid gives access to tunable adhesion.
A photosponge consisting of a porous liquid crystal network with smectic molecular order, crosslinked with an azobenzene molecule is reported. The pores are filled with liquid cyanobiphenyl. UV light irradiation shrinks the network, resulting in liquid secretion by the coating, while visible‐light irradiation triggers reabsorption of the liquid. The coating regulates by light‐induced adhesion and friction at its surface.
Nature provides much inspiration for the design of materials capable of motion upon exposure to external stimuli, and many examples of such active systems have been created in the laboratory. ...However, to achieve continuous motion driven by an unchanging, constant stimulus has proven extremely challenging. Here we describe a liquid crystalline polymer film doped with a visible light responsive fluorinated azobenzene capable of continuous chaotic oscillatory motion when exposed to ambient sunlight in air. The presence of simultaneous illumination by blue and green light is necessary for the oscillating behaviour to occur, suggesting that the dynamics of continuous forward and backward switching are causing the observed effect. Our work constitutes an important step towards the realization of autonomous, persistently self-propelling machines and self-cleaning surfaces powered by sunlight.
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