arXiv subjects

Vianney Gimenez-Pinto

Publications and source records attributed to Vianney Gimenez-Pinto.

3 recordsLinked to original sources

arXiv2019

Three-dimensional modeling of chiral nematic texture evolution under electric switching

Chiral nematic liquid crystals exhibit both a helical planar ground state with uniform twist and a metastable defect-rich focal conic texture, and can be switched between the two microstructures via application of transient voltage pulses. In this work, we model these electrically-induced texture transitions using finite difference methods to examine resulting microstructural evolution, the first time this transition has been modeled in three dimensions. We analyze the planar to focal conic, focal conic to planar, and planar to planar transitions depending on voltage pulse magnitude. We consider first the special case of chiral nematics with matched twist and bend elastic constants. Results show a variety of defect-rich morphologies in the disordered focal conic texture and demonstrate a fast recovery of the planar ground state on switching without formation of a transient planar state. We evaluate both texture microstructural evolution as well as cell capacitance. Beyond the single elastic constant approximation, we evaluate the planar to transient-planar as well as the planar to Helfrich-deformed transitions in simulations of a liquid crystal compound with different elastic constants. Our methods represent the evolving microstructure as a uniaxial director field, with relaxation dynamics calculated from a tensor representation so that half charge disclination defects are not suppressed. We discuss potential application of these computationally efficient three-dimensional modeling approaches for design and optimization of chiral nematic devices.

cond-mat.mtrl-sci↗

arXiv2016

Modeling Defects, Shape Evolution, and Programmed Auto-origami in Liquid Crystal Elastomers

Liquid crystal elastomers represent a novel class of programmable shape-transforming materials whose shape change trajectory is encoded in the material's nematic director field. Using three-dimensional nonlinear finite element elastodynamics simulation, we model a variety of different actuation geometries and device designs: thin films containing topological defects, patterns that induce formation of folds and twists, and a bas-relief structure. The inclusion of finite bending energy in the simulation model reveals features of actuation trajectory that may be absent when bending energy is neglected. We examine geometries with a director pattern uniform through the film thickness encoding multiple regions of positive Gaussian curvature. Simulations indicate that heating such a system uniformly produces a disordered state with curved regions emerging randomly in both directions due to the film's up-down symmetry. By contrast, applying a thermal gradient by heating the material first on one side breaks up-down symmetry and results in a deterministic trajectory producing a more ordered final shape. We demonstrate that a folding zone design containing cut-out areas accommodates transverse displacements without warping or buckling; and demonstrate that bas-relief and more complex bent-twisted structures can be assembled by combining simple design motifs.

cond-mat.soft↗

arXiv2013

Shape and Chirality Transitions in Off-Axis Twist Nematic Elastomer Ribbons

Using both experiments and finite element simulations, we explore the shape evolution of off-axis nematic elastomer ribbons as a function of temperature. The elastomers are prepared by cross-linking the mesogens with planar anchoring of the director at top and bottom surfaces with a 90 degree left-handed twist. Shape evolution depends sensitively on the off-axis director orientation at the sample mid-plane. When the director at midplane is parallel to either the ribbon's long or short axes, ribbons form either helicoids or spirals depending on the aspect ratio and temperature. If the director at midplane is more than 5 degrees off-axis, then they form only spiral ribbons. Samples in all these geometries show a remarkable transition from right- to left-handed chiral shapes on change of temperature. Simulation studies provide insight into the mechanisms driving shape evolution and enable engineering design of these materials for future applications.

cond-mat.soft↗