11 research outputs found

    Self-sustained actuation from heat dissipation in liquid crystal polymer networks

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    Liquid crystal polymer networks (LCNs) lead the research geared toward macroscopic motion of materials. These actuators are molecularly programed to adapt their shape in response to external stimuli. Non-photo-responsive thin films of LCNs covered with heat absorbers (e.g., graphene or ink) are shown to continuously oscillate when exposed to light. The motion is governed by the heat dissipated at the film surface and the anisotropic thermal deformation of the network. The influence of the LC molecular alignment, the film thickness, and the LC matrix on the macroscopic motion is analyzed to probe the limits of the system. The insights gained from these experiments provide not only guidelines to create actuators by photo-thermal or pure photo-effects but also a simple method to generate mechanical oscillators for soft robotics and automated systems

    Preparation of liquid crystal networks for macroscopic oscillatory motion induced by light

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    \u3cp\u3eA strategy based on doped liquid crystalline networks is described to create mechanical self-sustained oscillations of plastic films under continuous light irradiation. The photo-excitation of dopants that can quickly dissipate light into heat, coupled with anisotropic thermal expansion and self-shadowing of the film, gives rise to the self-sustained deformation. The oscillations observed are influenced by the dimensions and the modulus of the film, and by the directionality and intensity of the light. The system developed offers applications in energy conversion and harvesting for soft-robotics and automated systems. The general method described here consists of creating free-standing liquid crystalline films and characterizing the mechanical and thermal effects observed. The molecular alignment is achieved using alignment layers (rubbed polyimide), commonly used in the display manufacturing industry. To obtain actuators with large deformation, the mesogens are aligned and polymerized in a splay/bend configuration, i.e., with the director of the liquid crystals (LCs) going gradually from planar to homeotropic through the film thickness. Upon irradiation, the mechanical and thermal oscillations obtained are monitored with a high-speed camera. The results are further quantified by image analysis using an image processing program.\u3c/p\u3

    Self-sustained actuation from heat dissipation in liquid crystal polymer networks

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    \u3cp\u3eLiquid crystal polymer networks (LCNs) lead the research geared toward macroscopic motion of materials. These actuators are molecularly programed to adapt their shape in response to external stimuli. Non-photo-responsive thin films of LCNs covered with heat absorbers (e.g., graphene or ink) are shown to continuously oscillate when exposed to light. The motion is governed by the heat dissipated at the film surface and the anisotropic thermal deformation of the network. The influence of the LC molecular alignment, the film thickness, and the LC matrix on the macroscopic motion is analyzed to probe the limits of the system. The insights gained from these experiments provide not only guidelines to create actuators by photo-thermal or pure photo-effects but also a simple method to generate mechanical oscillators for soft robotics and automated systems.\u3c/p\u3

    A four-blade light-driven plastic mill based on hydrazone liquid-crystal networks

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    \u3cp\u3eThe first light-driven plastic mill is developed, which converts the incoming light directly into a continuous rotation. This device is composed of four blades made of hydrazone-based liquid crystal polymer films able to bend under focused light irradiation and to create a force causing the rotation of the mill. The mechanism of motion originates from the fast photo-thermal isomerization around the C[dbnd]N bond of hydrazones. We show that by accelerating the rate of the thermal Z to E back-isomerization of hydrazones, macroscopic deformation with fast strain rate can be obtained. The rapid motion of the film is the key factor in obtaining the continuous rotatory motion of the mill. These results broaden the range of molecular switches available for macroscopic motion of light-driven organic devices and offer new insights for single-step energy conversion in soft robotics and automated systems.\u3c/p\u3

    A rewritable, reprogrammable, dual light-responsive polymer actuator

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    \u3cp\u3eWe 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.\u3c/p\u3

    Coupled liquid crystalline oscillators in Huygens’ synchrony

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    In the flourishing field of soft robotics, strategies to embody communication and collective motion are scarce. Here we report the synchronized oscillations of thin plastic actuators by an approach reminiscent of the synchronized motion of pendula and metronomes. Two liquid crystalline network oscillators fuelled by light influence the movement of one another and display synchronized oscillations in-phase and anti-phase in a steady state. By observing entrainment between the asymmetric oscillators we demonstrate the existence of coupling between the two actuators. We qualitatively explain the origin of the synchronized motion using a theoretical model and numerical simulations, which suggest that the motion can be tuned by the mechanical properties of the coupling joint. We thus anticipate that the complex synchronization phenomena usually observed in rigid systems can also exist in soft polymeric materials. This enables the use of new stimuli, featuring an example of collective motion by photo-actuation

    Micrometer-scale porous buckling shell actuators based on liquid crystal networks

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    \u3cp\u3eMicrometer-scale liquid crystal network (LCN) actuators have potential for application areas like biomedical systems, soft robotics, and microfluidics. To fully harness their power, a diversification in production methods is called for, targeting unconventional shapes and complex actuation modes. Crucial for controlling LCN actuation is the combination of macroscopic shape and molecular-scale alignment in the ground state, the latter becoming particularly challenging when the desired shape is more complex than a flat sheet. Here, one-step processing of an LCN precursor material in a glass capillary microfluidic set-up to mold it into thin shells is used, which are stretched by osmosis to reach a diameter of a few hundred micrometers and thickness on the order of a micrometer, before they are UV crosslinked into an LCN. The shells exhibit radial alignment of the director field and the surface is porous, with pore size that is tunable via the osmosis time. The LCN shells actuate reversibly upon heating and cooling. The decrease in order parameter upon heating induces a reduction in thickness and expansion of surface area of the shells that triggers continuous buckling in multiple locations. Such buckling porous shells are interesting as soft cargo carriers with capacity for autonomous cargo release.\u3c/p\u3

    Making waves in a photoactive polymer film

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    \u3cp\u3eOscillating materials that adapt their shapes in response to external stimuli are of interest for emerging applications in medicine and robotics. For example, liquid-crystal networks can be programmed to undergo stimulus-induced deformations in various geometries, including in response to light. Azobenzene molecules are often incorporated into liquid-crystal polymer films to make them photoresponsive; however, in most cases only the bending responses of these films have been studied, and relaxation after photo-isomerization is rather slow. Modifying the core or adding substituents to the azobenzene moiety can lead to marked changes in photophysical and photochemical properties, providing an opportunity to circumvent the use of a complex set-up that involves multiple light sources, lenses or mirrors. Here, by incorporating azobenzene derivatives with fast cis-to-trans thermal relaxation into liquid-crystal networks, we generate photoactive polymer films that exhibit continuous, directional, macroscopic mechanical waves under constant light illumination, with a feedback loop that is driven by self-shadowing. We explain the mechanism of wave generation using a theoretical model and numerical simulations, which show good qualitative agreement with our experiments. We also demonstrate the potential application of our photoactive films in light-driven locomotion and self-cleaning surfaces, and anticipate further applications in fields such as photomechanical energy harvesting and miniaturized transport.\u3c/p\u3

    Photoresponsive sponge-like coating for on-demand liquid release

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    \u3cp\u3eMany 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.\u3c/p\u3

    Photoresponsive Nanoporous Smectic Liquid Crystalline Polymer Networks: Changing the Number of Binding Sites and Pore Dimensions in Polymer Adsorbents by Light

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    Photoresponsive nanoporous polymer films have been fabricated by adding a photoresponsive azobenzene cross-linker to a hydrogen-bonded smectic liquid crystalline polymer network A base treatment resulted in the nanoporous material which has been fully characterized. Upon exposure to UV light a decrease in the smectic layer spacing is observed, suggesting a decrease in pore size. In addition, the binding sites in the material could be changed with light, leading to light-induced adsorption of cations and cationic dyes. Finally, light could also be used to create nanoporous channels in the polymer film
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