Rheology of an Inverted Cholesteric Droplet under Shear Flow

Abstract

Abstract:The dynamics of a quasi two-dimensional isotropic droplet in a cholesteric liquid crystal medium under symmetric shear flow is studied by lattice Boltzmann simulations. We consider a geometry in which the flow direction is along the axis of the cholesteric, as this setup exhibits a significant viscoelastic response to external stress. We find that the dynamics depends on the magnitude of the shear rate, the anchoring strength of the liquid crystal at the droplet interface and the chirality. While low shear rate and weak interface anchoring the system shows a non-Newtonian behavior, a Newtonian-like response is observed at high shear rate and strong interface anchoring. This is investigated both by estimating the secondary flow profile, namely a flow emerging along the out-of-plane direction (absent in fully-Newtonian fluids, such as water) and by monitoring defect formation and dynamics, which significantly alter the rheological response of the system.

Lattice Boltzmann Modeling of Cholesteric Liquid Crystal Droplets Under an Oscillatory Electric Field

“…(2.8) and (2.9) by a standard lattice Boltzmann method [45]. We incidentally note that, although a hybrid LB machinery has been already successfully employed to study binary [52] and ternary fluids [49], active gels [53][54][55] and liquid crystals, such as nematics [56,57] and cholesterics [30,31,[58][59][60][61], in this work the applicability of the method has been further extended to study liquid crystal emulsions in the presence of a surfactant whose dynamics is explicitly solved.…”

“…Indeed, under confinement, the typical helical structure of the cholesteric may conflict with that imposed at the boundaries, often favouring the formation of topological defects (or disclinations) whose nature can decisively condition mechanical and optical properties of the liquid crystal [15,20,21]. While, over the years, considerable efforts have been addressed to theoretically investigate the physics of cholesteric droplets and their associated defect structure at equilibrium [15,16,[22][23][24][25][26][27][28], only recently a number of numerical works have been dedicated to pinpointing their response under an external driving, such as a heat flux [29] or an electric field [30,31]. Such works have been inspired by experiments showing for example that, if subject to a temperature gradient, cholesteric drops are set into rotation due to either a thermomechanical torque mechanism [32][33][34] or to Marangoni flows [29,35].…”