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

TL;DR

This study uses molecular dynamics simulations to explore how shear rate and dipolar coupling strength affect the rheology and microstructure of liquid crystal and magnetic nanoparticle mixtures, revealing shear-induced phase transitions and microstructural changes.

Contribution

It provides new insights into the shear-dependent rheological behavior and microstructural evolution of LC-DSS hybrid mixtures, highlighting the role of dipolar interactions.

Findings

Shear-thinning behavior observed at high shear rates.

Crossover from Newtonian to non-Newtonian behavior at low/intermediate dipolar coupling.

Shear-induced isotropic-to-nematic transition and chain formation of DSS.

Abstract

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.