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

Abstract

Utilizing molecular dynamics simulations, we report a nonmonotonic dependence of the shear stress on the strength of a continuously increasing (i.e., time-varying) external magnetic field (H) in a liquid-crystalline mixture of magnetic and nonmagnetic anisotropic particles. We relate the origin of this nonmonotonicity of the transient dynamics 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 Fréederickz-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 nonmagnetic ones occurs, which leads to a drop in shear stress, hence creating a nonmonotonic 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.