Switching dynamics in cholesteric liquid crystal emulsions

Abstract

In this work we numerically study the switching dynamics of a 2D cholesteric emulsion droplet immersed in an isotropic fluid under an electric field, which is either uniform or rotating with constant speed. The overall dynamics depend strongly on the magnitude and on the direction (with respect to the cholesteric axis) of the applied field, on the anchoring of the director at the droplet surface and on the elasticity. If the surface anchoring is homeotropic and a uniform field is parallel to the cholesteric axis, the director undergoes deep elastic deformations and the droplet typically gets stuck into metastable states which are rich in topological defects. When the surface anchoring is tangential, the effects due to the electric field are overall less dramatic, as a small number of topological defects form at equilibrium. The application of the field perpendicular to the cholesteric axis usually has negligible effects on the defect dynamics. The presence of a rotating electric field of varying frequency fosters the rotation of the defects and of the droplet as well, typically at a lower speed than that of the field, due to the inertia of the liquid crystal. If the surface anchoring is homeotropic, a periodic motion is found. Our results represent a first step to understand the dynamical response of a cholesteric droplet under an electric field and its possible application in designing novel liquid crystal-based devices.

Rheology of an Inverted Cholesteric Droplet under Shear Flow

“…Due to the interplay between the orientation of the cholesteric phase near the walls and the imposed perpendicular anchoring, topological defects emerge. Although in cholesterics four different types of defects may occur [36,41,42], here we observe λ ±m and τ ±m disclinations defects (m is the topological charge, an integer or semi-integer number). While in λ defects the director field is continuous (it is the local cholesteric pitch axis, winding around the defect, that is discontinuous) and flips into the third dimension at the λ line, τ defects are true disclinations as the director drops down at the defect core.…”

“…For a nematogen without chirality (q 0 = 0), the phase is isotropic if ζ(φ) < ζ c = 2.7, otherwise it is cholesteric. By following previous studies [17,29,36] we set ζ(φ) = ζ 0 + ζ s φ, where ζ 0 and ζ s control the boundary of the coexistence region. The remaining terms of Eq.…”

“…Equations (8) (11) (12) (13) are solved numerically by means of a hybrid lattice Boltzmann method [36,39], which uses a combination of a standard lattice Boltzmann approach to solve the Navier-Stokes and the continuity equation, and a finite-difference scheme to solve Eqs. (8) and (11).…”

Theory and simulation of objects in liquid crystals

“…As such interfaces can exhibit other fascinating properties, this could be an interesting line of study. The electroactivity of liquid crystals also opens up a variety of novel applications, such as particle sorting [104] and the effect of droplets on switching dynamics [105].…”

Direct measurements of structural forces and twist transitions in cholesteric liquid crystal films with a surface force apparatus

“…19,20 Moreover, SFA can be used as a general platform to study confinement-induced and dynamic phenomena in CLC, e.g. helix inversion [53][54][55][56] , defect nucleation/annihilation and defect-mediated colloidal interactions, 57 and permeative flow 48,[58][59][60][61] . We anticipate that the behavior of the CLC observed during surface approach was very different from the one observed during separation.…”