(450f) Probe Rheology Molecular Simulations of Nanocolloidal Suspensions

Probe rheology has evolved into a reliable experimental technique for investigating the rheological behavior, in particular, the non-Newtonian rheology of complex fluid systems. In this work, we have used an analogous molecular dynamics (MD) simulations technique that was recently developed to study the linear viscoelasticity of nanocolloidal suspension systems, whose rheology is governed by the interparticle interactions. Specifically, we have employed both the active and the passive rheology simulation techniques to obtain the dynamic moduli (G^*) values over a wide range of colloidal volume fractions. For this purpose, an explicit solvent model that was recently shown by us to exhibit the well-known rheological features of the colloidal suspensions was used. At all volume fractions and frequencies investigated, the distribution of colloidal particles around the probe in the active rheology simulations is found to be symmetric which is consistent with the system being in the linear regime. Furthermore, the mean-squared displacement (MSD) of the probe in the passive rheology simulations exhibits both the ballistic and the diffusive regimes over the time-scales studied in this work. The motion of the probe in the colloidal systems is analyzed using the inertial generalized Stokes-Einstein relation (IGSER). The resulting dynamic moduli values obtained from the active and the passive probe rheology techniques are found to be in good agreement with those obtained from the non-equilibrium MD (NEMD) simulations and the literature experiments. The role of probe-colloid size ratio in determining the values of moduli obtained by probe rheology will also be discussed.