The effect of shear‐flow on critical correlations in colloidal systems: Microstructure, turbidity, and dichroism

Geometric percolation of hard-sphere dispersions in shear flow

We combine a heuristic theory of geometric percolation and the Smoluchowski theory of colloid dynamics to predict the impact of shear flow on the percolation threshold of hard spherical colloidal particles, and verify our findings by means of molecular dynamics simulations. It appears that the impact of shear flow is subtle and highly non-trivial, even in the absence of hydrodynamic interactions between the particles. The presence of shear flow can both increase and decrease the percolation threshold, depending on the criterion used for determining whether or not two particles are connected and on the Péclet number. Our approach opens up a route to quantitatively predict the percolation threshold in nanocomposite materials that, as a rule, are produced under non-equilibrium conditions, making comparison with equilibrium percolation theory tenuous. Our theory can be adapted straightforwardly for application in other types of flow field, and particles of different shape or interacting via other than hard-core potentials.

Nonequilibrium Structure Diagram of Pendular Suspensions under Large-Amplitude Oscillatory Shear

For pendular suspensions with particles in contact with immiscible secondary liquid bridges, the shear field significantly influences particle aggregates and networks. In this work, we study the structure of the pendular network and how the structure changes under large-amplitude-oscillatory shear. Using rheology and optical microscopy, we found unique network destruction followed by reconstruction with increasing strain. Two processes show different shear-field dependencies, strain-rate dependency for destruction and strain dependency for reconstruction. A nonequilibrium state diagram is constructed to show the phase behavior, where the critical particle concentration of sol-gel transition is dependent on the shear history and may depend on shear strain nonmonotonically. Two different mechanisms, shear-induced network breakdown at low strain and shear-induced agglomeration at high strain, are suggested to describe the nonmonotonic critical concentration under the upward strain sweep quantitatively.

Microstructural response of a near-critical colloid-polymer mixture to shear flow

Long-ranged microstructural order of a near-critical colloid-polymer mixture under stationary and oscillatory shear flow is studied by means of time-resolved small-angle light scattering. The distance from the critical point, the shear rate, and the frequency of oscillation are systematically varied. Unexpected shear-induced distortions of critical microstructural order is observed in directions perpendicular to the flow direction, more so on closer approach of the gas-liquid critical point. The measured distortion of long-ranged order can be quantitatively understood on the basis of an approximate solution of the Smoluchowski equation, provided that a nonanalytical distortion of short-ranged microstructural order is assumed. These short-ranged microstructural changes, induced by the flow, account for the observed distortion perpendicular to the flow direction, and are responsible for the shear-induced shift of the location of the gas-liquid critical point. The importance of short-ranged correlations renders the behavior of critical systems under shear flow nonuniversal. The origin of the nonanalytic dependence of short-ranged distortions on the shear rate remains unclear. Including these short-ranged microstructural distortions, we find a good agreement between theory and experiment for both stationary and oscillatory shear flow.

Flow dichroism in critical colloidal fluids

Due to long-range correlations and slow dynamics of concentration fluctuations in the vicinity of the gas-liquid critical point, shear flow is very effective in distorting the microstructure of near-critical fluids. The anisotropic nature of the shear-field renders the microstructure highly anisotropic, leading to dichroism. Experiments on the dichroic behavior can thus be used to test theoretical predictions on microstructural order under shear flow conditions. We performed both static and dynamic dichroism and turbidity measurements on a colloid-polymer mixture, existing of silica spheres (radius 51 nm) and polydimethylsiloxane polymer (molar weight 204 kg/mol). Sufficiently far away from the critical point, in the mean-field region, the experimental data are in good agreement with theory. Very close to the critical point, beyond mean field, for which no theory exists yet, an unexpected decrease of dichroism on approach of the critical point is observed. Moreover, we do not observe critical slowing down of shear-induced dichroism, right up to the critical point, in contrast to the turbidity.