Emergent biaxiality in nematic microflows illuminated by a laser beam

Abstract

Anisotropic fluids (e.g. liquid crystals) offer a remarkable promise as optofluidic materials owing to the directional, tunable, and coupled interactions between the material, flow, and the optical fields. Here we present a comprehensive in silico treatment of this anisotropic interaction by performing nonequilibrium molecular dynamics simulations. We quantify the response of a nematic liquid crystal (NLC) undergoing a Poiseuille flow in the Stokes regime, while being illuminated by a laser beam incident perpendicular to the flow direction. We adopt a minimalistic model to capture the interactions, accounting for two features: first, the laser heats up the NLC locally; and second, the laser polarises the NLC and exerts an optical torque that tends to reorient molecules of the nematic phase. Because of this reorientation the liquid crystal exhibits small regions of biaxiality, where the nematic director is one symmetry axis and the axis of rotation for the reorientation of the molecules is the other one. We find that the relative strength of the viscous and the optical torques mediates the flow-induced response of the biaxial regions, thereby tuning the emergence, shape and location of the regions of enhanced biaxiality. The mechanistic framework presented here promises experimentally tractable routes toward novel optofluidic applications based on material-flow-light interactions.

Flow-assisted self-healing of the helical structure in a cholesteric liquid crystal

“…We are interested in establishing a Poiseuille flow in our NEMD simulations. Poiseuille flow is characterized by a parabolic velocity profile where, for our specific setup, the x-component of the flow velocity needs to drop to zero at the walls [34][35][36][37][38]. To accomplish this, friction is normally required to establish and stabilize this velocity profile.…”

“…We also carry out NEMD by applying a nonhomogeneous temperature profile to our cholesteric fluid. Similar to our previous work [38], we superimpose local heating of the system through a local temperature profile given by the expression…”

“…10(a) as a solid line. The input temperature profile is imposed via the Galilean-invariant thermostat proposed by Stojanov and Groot [40] as described in detail elsewhere [38]. From an experimental perspective a realization of such a system with a temperature profile could be a liquid-crystalline sample illuminated by a laser beam.…”

“…We have analyzed a simplistic model of such an illuminated nematic liquid crystal in our previous publication [38], where we showed that if flow is coupled with local heating a synergistic effect arises in which the ordered liquid-crystalline phase exhibits local biaxial symmetry.…”

“…Because of the application of this external body force we need to thermostat the system during the entire course of the simulation. Details of this thermostatting procedure have been described elsewhere [37,38]. Because of the thermostat the sequence of microstates in phase space comports with the distribution of these states in the canonical ensemble assuming that our system is ergodic.…”