Steady-state rheology and structure of soft hybrid mixtures of liquid crystals and magnetic nanoparticles

GP Shrivastav and HN Siboni and SHL Klapp, SOFT MATTER, 16, 2516-2527 (2020).

DOI: 10.1039/c9sm02080b

Using non-equilibrium molecular dynamics simulations, we study the rheology of a model hybrid mixture of liquid crystals (LCs) and dipolar soft spheres (DSS) representing magnetic nanoparticles. The bulk isotropic LC-DSS mixture is sheared with different shear rates using Lees-Edwards periodic boundary conditions. The steady-state rheological properties and the effect of the shear on the microstructure of the mixture are studied for different strengths of the dipolar coupling, lambda, among the DSS. We find that at large shear rates, the mixture shows a shear-thinning behavior for all considered values of lambda. At low and intermediate values of lambda, a crossover from Newtonian to non-Newtonian behavior is observed as the rate of applied shear is increased. In contrast, for large values of lambda, such a crossover is not observed within the range of shear rates considered. Also, the extent of the non-Newtonian regime increases as lambda is increased. These features can be understood via the shear-induced changes of the microstructure. In particular, the LCs display a shear-induced isotropic-to-nematic transition at large shear rates with a shear-rate dependent degree of nematic ordering. The DSS show a shear-induced nematic ordering only for large values of lambda, where the particles self-assemble into chains. Moreover, at large lambda and low shear rates, our simulations indicate that the DSS form ferromagnetic domains.

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