We describe photo-thermo-mechanical actuation and its dynamics in thin films of a liquid crystal networks (LCN) under near infrared (NIR) illumination through experiments and simulations. Splay ...aligned films of different thicknesses (25 μm to 100 μm) were obtained by crosslinking a mixture of mono-functional and bi-functional liquid crystal monomers. The NIR-driven thermo-mechanical actuation was achieved by adding an NIR dye to the monomer mixture. The absorption of incoming radiation by the dye molecules raises the local temperature of the film causing an order-disorder (nematic-isotropic) transition, thereby resulting in a macroscopic shape change. We have investigated the effect of film thickness, NIR laser power and dye concentration on the tip displacement of the films in a cantilever configuration. The experimental findings and finite element simulation results are in reasonably good quantitative agreement. Despite using lower NIR powers than typically employed, the films show high actuation and large displacements. After achieving saturation in actuation, the films exhibit a flutter behavior which is discussed in light of the observed overshoot in the tip displacement for certain intensities and thicknesses. Finally, using a solar simulator, we also show the visible light response of the film.
We describe photo-thermo-mechanical actuation and its dynamics in thin films of a liquid crystal network (LCN) under near infrared (NIR) illumination through experiments and simulations.
A triangular shaped liquid crystal network is shown to undergo a continual, rocking chair-like oscillatory chaotic motion upon exposure to a surface of constant temperature. With the addition of an ...azobenzene chromophore, dual response is achieved, extending the actuation freedom towards a film that shows rocking and rolling motion.
Liquid crystalline networks of specific geometry are observed to undergo thermally triggered chaotic continual rocking motion and light triggered rolling.
Achieving oscillatory motion in polymers without requiring on/off switching of stimuli is a current challenge. Hereby, a free‐standing liquid crystal polymer (LCP) is demonstrated to undergo a ...sustained oscillatory motion when triggered by light, moving back and forth, resembling the motion of a rocking‐chair. Two polymer films having different azobenzene photo‐switches have been studied, revealing photoswitch requirements as well as illumination conditions necessary to sustain oscillations. The motion presented here shows how feedback loops involving light‐triggered actuation, self‐shadowing and a shifting center of gravity can be utilized to achieve self‐sustained motion in free‐standing polymers.
Sustained oscillations in stimuli‐responsive actuators will increase the autonomy of future devices. Here, a liquid crystal polymer is shown to undergo continuous light‐driven oscillation without requiring on/off switching of the light source.
Liquid crystalline networks have been fashioned into thin films with tapered thicknesses, revealing the possibility of rolling up extremely tightly when triggered thermally or with light. Compared to ...the often limited bending shown previously in liquid crystal network actuators, these tapered films curl up several hundreds of degrees. Finite element results of simulated functionally graded thin films with tapered thicknesses corroborate well with experimental work.
Tight bending bio-inspired soft actuators were fashioned in liquid crystalline networks by using a novel tapered film geometry.
Soft actuators allowing multifunctional, multishape deformations based on single polymer films or bilayers remain challenging to produce. In this contribution, direct ink writing is used for ...generating patterned actuators, which are in between single- and bilayer films, with multifunctionality and a plurality of possible shape changes in a single object. The key is to use the controlled deposition of a light-responsive liquid crystal ink with direct ink writing to partially cover a foil at strategic locations. We found patterned films with 40% coverage of the passive substrate by an active material outperformed “standard” fully covered bilayers. By patterning the film as two stripes, a range of motions, including left- and right-handed twisting and bending in orthogonal directions, could be controllably induced in the same actuator. The partial coverage also left space for applying liquid crystal inks with other functionalities, exemplified by fabricating a light-responsive green reflective actuator whose reflection can be switched “on” and “off”. The results presented here serve as a toolbox for the design and fabrication of patterned actuators with dramatically expanded shape deformation and functionality capabilities.
Recently researchers have developed soft actuators capable of morphing into complex shapes taking inspiration from nature. In this paper, we have developed a splay‐nematic liquid crystal polymer ...network tapered actuator that can morph from a flat film to a cone, mimicking the blooming of single petal Calla Lily flower. We have demonstrated the formation of conical tubes through finite element simulations and experiments. The influence of tapering and alignment orientations with respect to the edge of the film on the cones is analyzed through simulations. The design with tapering and splayed alignments oriented at 45° to the edge is found to be the optimal choice for forming conical tubes.
•A transient light-attenuation photo-mechanical model including the viscoelastic nature of liquid crystal polymers characterized using Dynamic Mechanical Analysis is presented.•The developed model is ...used to understand the interaction between the molecular cis/trans isomerization time scale, the material relaxation (viscoelastic) time scale and the external illumination time scale.•Surface waves on liquid crystal thin films are simulated and the effects of the time scales on the amplitude, frequency and the phase lag are analyzed.•A master curve relating the amplitude and phase lag with the three different time scales is obtained from the simulation results which can assist in the design of surface waves.•The paper demonstrates the possibility to generate complex dynamic surface topographies on liquid crystal surfaces for applications such as controlled wettability, friction control and particle transport through an illustration of a sinusoidal pattern.
In this paper we present a computational model to simulate surface waves on photo-responsive liquid crystal thin films under dynamic illumination. The influence of light attenuation is included to model the dynamic photo-mechanical response as a result of the forward (trans to cis) and the backward (cis to trans) isomerization of the embedded azobenzene molecules. The viscoelastic nature of the liquid crystal films is also estimated and incorporated in this study. The temporal response of the material under static spot illumination is analyzed first to understand the underlying mechanisms behind the observed response. The relation between the fraction of transformed molecules and the macroscopic light induced strains and stresses through the thickness is investigated to establish the significance of light attenuation. Subsequently, the model liquid crystal film is exposed to a rectangular illumination waveform to simulate a moving light source and thereby generate surface waves. Results are presented that relate the wave attributes, (i.e., the amplitude, frequency and phase lag) to the time scales associated with the moving light source, the molecular trans-cis isomerization reaction and the viscoelastic response of the material. A design guideline for producing surface waves with specific features (amplitude, phase) is presented.
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