8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P5.108Wed 711:10-14:00Hydrodynamic Rayleigh-Taylor-like instabilities insedimenting colloidal mixturesKristina Milinkovic, 1 Johan T. Padding, 2 and Marjolein Dijkstra 11 Debye Institute, Utrecht University, Princetonplein 1, 3584 CC, Utrecht, Netherlands2 IMCN, Université catholique de Louvain, Louvain-la-Neuve, BelgiumWe present a simulation study of the sedimentation of an initially inhomogeneous distribution ofbinary hard-sphere like colloidal mixtures confined to a slit. The simulation technique that weuse is a coarse-grained hybrid molecular dynamics and stochastic rotation dynamics method thatincorporates both long ranged hydrodynamic interactions and Brownian motion [1, 2]. We find thatthe sedimentation results in the formation of the Rayleigh-Taylor-like hydrodynamic instabilityand we investigate both the process of the formation of the instability and the particle mixing andseparation effects. The motion of a sedimenting colloid is characterized by the Peclet number,which is the ratio between the time a particle needs to diffuse over its own radius and the time itneeds to sediment over the same distance. By studying a range of Peclet numbers, set separatelyfor each of the species, we vary the strength of the gravitational field that drives the instability.Examining the transient network-like structures that form as a consequence of the instability, wefind that the structure is hardly affected by the composition of the mixture, but does depend onthe Peclet numbers of the species. The spatial velocity correlation functions we calculate enableus to follow the time evolution of the instability, and they show correlations developing at shortdistances and anti-correlations at larger distances which is due to the formation of swirls. We alsocalculate the growth rates of the undulations as a function of the corresponding wave numbers.[1] A. Malevanets and R. Kapral, J. Chem. Phys. , 112, 7260 (1999).[2] J. T. Padding and A. A. Louis, Phys. Rev. E, 74, 031402 (2006).108