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See detailFacile Anisotropic Deswelling Method for Realizing Large‐Area Cholesteric Liquid Crystal Elastomers with Uniform Structural Color and Broad‐Range Mechanochromic Response
Kizhakidathazhath, Rijeesh UL; Geng, Yong UL; Jampani, Venkata UL et al

in Advanced Functional Materials (2019)

Cholesteric liquid crystal elastomers (CLCEs) are soft and dynamic photonic elements that couple the circularly polarized structural color from the cho- lesteric helix to the viscoelasticity of rubbers ... [more ▼]

Cholesteric liquid crystal elastomers (CLCEs) are soft and dynamic photonic elements that couple the circularly polarized structural color from the cho- lesteric helix to the viscoelasticity of rubbers: the reflection color is mechani- cally tunable (mechanochromic response) over a broad range. This requires uniform helix orientation, previously realized by long-term centrifugation to ensure anisotropic deswelling, or using sacrificial substrates or external fields. The present paper presents a simple, reproducible, and scalable method to fab- ricate highly elastic, large-area, millimeters thick CLCE sheets with intense uni- form reflection color that is repeatably, rapidly, and continuously tunable across the full visible spectrum by stretching or compressing. A precursor solution is poured onto a substrate and allowed to polymerize into a 3D network during solvent evaporation. Pinning to the substrate prevents in-plane shrinkage, thereby realizing anisotropic deswelling in an unprecedentedly simple manner. Quantitative stress–strain–reflection wavelength characterization reveals behavior in line with theoretical predictions: two linear regimes are identified for strains below and above the helix unwinding threshold, respectively. Up to a doubling of the sample length, the continuous color variation across the full visible spectrum repeatedly follows a volume conserving function of the strain, allowing the CLCE to be used as optical high-resolution strain sensor. [less ▲]

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See detailMicrometer-Scale Porous Buckling Shell Actuators Based on Liquid Crystal Networks
Jampani, Venkata UL; Mulder, Dirk; Reguengo de Sousa, Kevin UL et al

in Advanced Functional Materials (2018), 28(31), 1801209

Micrometer‐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 ... [more ▼]

Micrometer‐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. [less ▲]

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See detailCurrent Understanding of Van der Waals Effects in Realistic Materials
Tkatchenko, Alexandre UL

in ADVANCED FUNCTIONAL MATERIALS (2015), 25(13, SI), 2054-2061

Van der Waals (vdW) interactions arise from correlated electronic fluctuations in matter and are therefore present in all materials. Our understanding of these relatively weak yet ubiquitous quantum ... [more ▼]

Van der Waals (vdW) interactions arise from correlated electronic fluctuations in matter and are therefore present in all materials. Our understanding of these relatively weak yet ubiquitous quantum mechanical interactions has improved significantly during the past decade. This understanding has been largely driven by the development of efficient methods that now enable the modeling of vdW interactions in many realistic materials of interest for fundamental scientific questions and technological applications. In this work, the physics behind the currently available vdW methods are reviewed, and their applications to a wide variety of materials are highlighted, ranging from molecular assemblies to solids with and without defects, nanostructures of varying size and dimensionality, as well as interfaces between inorganic and organic materials. The origin of collective vdW interactions in materials is discussed using the concept of topological dipole waves. Focus is placed on the important observation that the full many-body treatment of vdW interactions becomes crucial in the investigation and characterization of materials with increasing complexity, especially when studying their response properties, including vibrational mechanical, and optical phenomena. Despite significant recent advances many challenges still remain in the development of accurate and efficient methods for treating vdW interactions that will be broadly applicable to the modeling of functional materials at all relevant length and timescales. [less ▲]

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See detailTowards efficient dispersion of carbon nanotubes in thermotropic liquid crystals
Schymura, Stefan; Kühnast, Martin; Lutz, Vanessa et al

in Advanced Functional Materials (2010), 20(19), 3350-3357

Motivated by numerous recent reports indicating attractive properties of composite materials of carbon nanotubes (CNTs) and liquid crystals (LCs) and a lack of research aimed at optimizing such composites ... [more ▼]

Motivated by numerous recent reports indicating attractive properties of composite materials of carbon nanotubes (CNTs) and liquid crystals (LCs) and a lack of research aimed at optimizing such composites, the process of dispersing CNTs in thermotropic LCs is systematically studied. LC hosts can perform comparably or even better than the best known organic solvents for CNTs such as N-methyl pyrrolidone (NMP), provided that the dispersion process and choice of LC material are optimized. The chemical structure of the molecules in the LC is very important; variations in core as well as in terminal alkyl chain influence the result. Several observations moreover indicate that the anisotropic nematic phase, aligning the nanotubes in the matrix, per se stabilizes the dispersion compared to a host that is isotropic and thus yields random tube orientation. The chemical and physical phenomena governing the preparation of the dispersion and its stability are identified, taking into account enthalpic, entropic, as well as kinetic factors. This allows a guideline on how to best design and prepare CNT–LC composites to be sketched, following which tailored development of new LCs may take the advanced functional material that CNT–LC composites comprise to the stage of commercial application. [less ▲]

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