Introduction
When suspended particles move in a background fluid, they experience random kicks (Brownian motion), but also generate long-ranged hydrodynamic flows. In this project we use a new computer simulation method, stochastic rotation dynamics (SRD) that includes both these effects, to study flow properties of colloidal fluids. On-going projects include:Velocity fluctuations in sedimenting suspensions
We study the steady-state sedimentation of hard sphere particles. In classical Brownian dynamics simulations hydrodynamic interactions between different particles are neglected, and only a friction with a static background remains. It is often argued that this is appropriate for Peclet numbers smaller than one. We show however that, even when the hydrodynamic interactions are an order of magnitude weaker than Brownian forces (Pe=0.1), they still induce backflow effects that dominate the reduction of the average sedimentation velocity with increasing particle packing fraction. Velocity fluctuations, on the other hand, begin to show non-equilibrium hydrodynamic character for Pe>1. The next movies show 2D sedimentation of 240 hard spheres in a system with walls at all sides. The first movie shows classical Brownian dynamics results. In the second movie we have included O(10^5) SRD particles to represent the thermohydrodynamic solvent. Note that only the hard spheres are shown.- Movie of 2D sedimentation, classical Brownian dynamics (no hydrodynamic interactions), 33MB.
- Movie of 2D sedimentation, including hydrodynamic interactions, 42MB.
Laning in oppositely driven colloids
We study the formation of lanes of oppositely driven colloids. Laning has been observed in classical Brownian dynamics simulations, but has proven to be very difficult to be observed experimentally. We study the influence of hydrodynamic interactions on the laning by computer simulations. These hydrodynamic interactions cause velocity fluctuations and swirls which are not present in the classical Brownian dynamics approach (see movies below). We study to what extent the hydrodynamic fluctuations hinder the formation of lanes.- Movie of 2D oppositely driven colloids, classical Brownian dynamics (no hydrodynamic interactions), 29MB.
- Movie of 2D oppositely driven colloids, including hydrodynamic interactions at the same driving force as above, 30MB.
Reversibility of Stokes flow in the presence of thermal fluctuations
In this project we combine theory and simulation to explore the extent to which time-velocity reversibility of Stokes flow remains valid in the presence of thermal fluctuations. [Project has just started.]
Sedimentation of 7800 colloidal spheres (yellow) in a thermohydrodynamic solvent consisting of 9.000.000 SRD particles (not shown). Periodic boundaries are used in all directions.