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, λ, among the DSS. We find that at large shear rates, the mixture shows a shear-thinning behavior for all considered values of λ. At low and intermediate values of λ, a crossover from Newtonian to non-Newtonian behavior is observed as the rate of applied shear is increased. In contrast, for large values of λ, such a crossover is not observed within the range of shear rates considered. Also, the extent of the non-Newtonian regime increases as λ 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 λ, where the particles self-assemble into chains. Moreover, at large λ and low shear rates, our simulations indicate that the DSS form ferromagnetic domains.
Highlights
In recent years, composites of liquid crystals (LCs) and magnetic nanoparticles (MNPs) have been established as an important new class of soft ‘‘hybrid’’ materials
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
For the understanding of the rheology, the main quantity of interest is the shear stress, sxz, which we calculate via the Irving–Kirkwood expression,[22,62]
Summary
Composites of liquid crystals (LCs) and magnetic nanoparticles (MNPs) have been established as an important new class of soft ‘‘hybrid’’ materials. The findings were found to be consistent with nanorheological experiments.[50] Still, it is clear that more particle-based investigations are needed to elucidate the behavior of magnetic hybrid systems with complex matrices under shear, especially at larger dipolar coupling strengths From this perspective, we consider the model of GB and DSS particles considered in the present study as an archetypal example of a soft magnetic hybrid system.
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