219 research outputs found

    Travelling waves on photo-switchable patterned liquid crystal polymer films directed by rotating polarized light

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    \u3cp\u3eNature employs travelling waves to generate propulsion of fluids, cells and organisms. This has inspired the development of responsive material systems based on different external triggers. Especially light-actuation is suitable because of its remote control and scalability, but often complex, moving light sources are required. Here, we developed a method that only requires flood exposure by rotating the linear polarization of light to generate propagating surface waves on azobenzene-modified liquid crystalline polymer films. We built a photomechanical computational model that accounts for the attenuation of polarized light and trans-to-cis isomerization of azobenzene. A non-uniform in-plane distribution of the liquid crystal molecules allows for the generation of travelling surface waves whose amplitude, speed and direction can be controlled through the intensity, rotation direction and rotation speed of the linear polarization of a light source. Our method opens new avenues for motion control based on light-responsive topographical transformations for application in microfluidic lab-on-chip systems and soft robotics.\u3c/p\u3

    Annual Reports of the Board of Selectmen Treasurer, and Supervisor of Schools of the Town of Greene, for the Year Ending March 6, 1896

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    An in situ method for sealing an array of pre-filled micro-cavities, such as encountered in electrophoretic displays, is presented. The technique, which is based on photoembossing, forms a hermetic seal between the cover and the cavity walls. The seal locations are defined by ultraviolet exposure through a photolithographic mask, forming a latent image overlapping with the locations of the cavity walls. During a thermal development step, while the cover is mounted on top of the micro-cavities, the seal evolves and makes firm contact with the cavity walls. This technology is demonstrated to be insensitive to small deviations in cavity height, flatness of the cover and thin fluid films remaining between the cover and the top of the cavity walls. In the past, these aspects made it difficult to effectively seal large-area devices

    Photoswitchable ratchet surface topographies based on self-protonating spiropyran-NIPAAM hydrogels

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    in this work, self-protonating spiropryan based poly(N-isopropylacrylamide) polymer networks are prepared. These photoresponsive hydrogel coatings can change their surface topography upon exposure with visible light in neutral environment. Photoresponsive surface constrained films have been fabricated of which the swelling behaviour can be controlled in a reversible manner. In a first step, symmetrical switchable surface topologies with varying cross-link density are obtained by polymerization-induced diffusion. Under light exposure, the areas with low cross-link density swell more than the areas with high crosslink density thus forming a corrugated surface. Asymmetric ratchet-like photoresponsive surfaces have been prepared on pre-structured asymmetric substrates. Due to thickness variations of the surface confined hydrogel layer as asymmetric swelling behavior is obtained. Depending on the cross-link density of the hydrogel it is possible to switch between a ratchet and flat surface topography or even an inverse ratchet surface by light

    Phototriggered Complex Motion by Programmable Construction of Light-Driven Molecular Motors in Liquid Crystal Networks

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    Recent developments in artificial molecular machines have enabled precisely controlled molecular motion, which allows several distinct mechanical operations at the nanoscale. However, harnessing and amplifying molecular motion along multiple length scales to induce macroscopic motion are still major challenges and comprise an important next step toward future actuators and soft robotics. The key to addressing this challenge relies on effective integration of synthetic molecular machines in a hierarchically aligned structure so numerous individual molecular motions can be collected in a cooperative way and amplified to higher length scales and eventually lead to macroscopic motion. Here, we report the complex motion of liquid crystal networks embedded with molecular motors triggered by single-wavelength illumination. By design, both racemic and enantiomerically pure molecular motors are programmably integrated into liquid crystal networks with a defined orientation. The motors have multiple functions acting as cross-linkers, actuators, and chiral dopants inside the network. The collective rotary motion of motors resulted in multiple types of motion of the polymeric film, including bending, wavy motion, fast unidirectional movement on surfaces, and synchronized helical motion with different handedness, paving the way for the future design of responsive materials with enhanced complex functions

    Liquid crystal elastomer coatings with programmed response of surface profile

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    Stimuli-responsive liquid crystal elastomers (LCEs) with a strong coupling of orientational molecular order and rubber-like elasticity, show a great potential as working elements in soft robotics, sensing, transport and propulsion systems. We demonstrate a dynamic thermal control of the surface topography of LCE coatings achieved through pre-designed patterns of in-plane molecular orientation. These patterns determine whether the LCE coating develops elevations, depressions, or in-plane deformations. The deterministic dependence of the out-of-plane dynamic surface profile on the in-plane orientational pattern is explained by activation forces. These forces are caused by two factors: (i) stretching-contraction of the polymer networks driven by temperature; (ii) spatially varying orientation of the LCE. The activation force concept brings the responsive LCEs into the domain of active matter. The demonstrated relationship can be used to design programmable coatings with functionalities that mimic biological tissues such as skin

    Reaction-diffusion model for the preparation of polymer gratings by patterned ultraviolet illumination

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    A model is developed to describe the migration mechanism of monomers during the lithographic preparation of polymer gratings by ultraviolet polymerization. The model is based on the Flory‚ÄďHuggins theory: a thermodynamic theory that deals with monomer/polymer solutions. During the photoinduced polymerization process, monomer migration is assumed to be driven by a gradient in the chemical potential rather than the concentration. If the chemical potential is used as the driving force, monomer migration is not only driven by a difference in concentration, or volume fraction, but also by other entropic effects such as monomer size and the degree of crosslinking of the polymer network, which is related to the ability of a polymer to swell. Interaction of the monomers with each other or the polymer is an additional energetic term in the chemical potential. The theoretical background of the model is explained and results of simulations are compared with those of nuclear microprobe measurements. A nuclear microprobe is used to determine the spatial monomer distribution in the polymer gratings. It is shown that two-way diffusion is expected if the monomers are both difunctional and have the same size. In some cases, if one monomer is considerably smaller than the other, it can eventually have a higher concentration in the illuminated regions, even when it has a lower reactivity. The model is used to simulate the grating formation process. This results in a calculated distribution of the monomer volume fractions as a function of position in polymer gratings. An excellent agreement with the nuclear microprobe measurements is obtained. ¬©2004 American Institute of Physics
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