Non-monotonic response of a sheared magnetic liquid crystal to an external field

TL;DR

This study uses molecular dynamics simulations to reveal a non-monotonic shear stress response to external magnetic fields in a magnetic liquid crystal mixture, driven by competing alignment effects and demixing phenomena.

Contribution

It uncovers a novel non-monotonic magnetic field dependence of shear stress in magnetic liquid crystals, contrasting with traditional ferrofluid behavior, and explains it through particle orientation competition and entropy effects.

Findings

Shear stress varies non-monotonically with magnetic field strength.

Magnetic particles undergo a Fréedericksz-like transition affecting shear stress.

Demixing of magnetic and non-magnetic particles causes the non-monotonic response.

Abstract

Utilizing molecular dynamics simulations, we report a non-monotonic dependence of the shear stress on the strength of an external magnetic field ($H$) in a liquid-crystalline mixture of magnetic and non-magnetic anisotropic particles. This non-monotonic behavior is in sharp contrast with the well-studied monotonic $H$-dependency of the shear stress in conventional ferrofluids, where the shear stress increases with $H$ until it reaches a saturation value. We relate the origin of this non-monotonicity to the competing effects of particle alignment along the shear-induced direction, on the one hand, and the magnetic field direction, on the other hand. To isolate the role of these competing effects, we consider a two-component mixture composed of particles with effectively identical steric interactions, where the orientations of a small fraction, i.e.\ the magnetic ones, are coupled to the external magnetic field. By increasing $H$ from zero, the orientations of the magnetic particles show a Fr\'{e}ederickz-like transition and eventually start deviating from the shear-induced orientation, leading to an increase in shear stress. Upon further increase of $H$, a demixing of the magnetic particles from the non-magnetic ones occurs which leads to a drop in shear stress, hence creating a non-monotonic response to $H$. Unlike the equilibrium demixing phenomena reported in previous studies, the demixing observed here is neither due to size-polydispersity nor due to a wall-induced nematic transition. Based on a simplified Onsager analysis, we rather argue that it occurs solely due to packing entropy of particles with different shear- or magnetic-field-induced orientations.