(587d) Cluster Dynamics in the Presence of Surfaces

Clusters of particles are important entities in the study of colloidal dispersions; they play an important part in many areas of complex fluids. Although clusters have been extensively studied, the relative effects of hydrodynamic interactions on colloidal aggregates are not necessarily well-understood. Moreover, the effects that bounding surfaces have on cluster dynamics is not necessarily well-characterized. To probe these effects, we explore the dynamics of pre-formed clusters interacting hydrodynamically with planar walls and corners via two separate, but complementary, routes. Due to the fact that colloidal particles have length scales that are large compared to the fluid in which they are dispersed, a hierarchy of time scales is introduced that needs to be resolved in any simulation of colloidal dispersions. Since it is impossible to perform full-scale molecular dynamics simulations of all of these time scales, we make a compromise and utilize meso-scopic simulation techniques, such as Stochastic Rotation Dynamics (SRD)¹. Using SRD, we are able to coarse-grain the fluid degrees of freedom so that the fluid is represented in an averaged way, enabling the re-production of hydrodynamic fields around the clusters. One branch of our investigation, then, is to simulate colloidal aggregates with SRD to understand how hydrodynamics affects the dynamics of colloidal aggregates. While meso-scopic simulations enable the long time scale simulation of truly colloidal aggregates, the coarse-graining of the fluid necessarily includes a loss of detail in the fluid. It is thought that the local fluid structure, which cannot be represented in meso-scopic simulations, plays an important role in the stability of colloidal clusters. One additional drawback of meso-scopic simulations is that they cannot capture hydrodynamic fields on small length scales. To this end, our second line of investigation is to utilize fully-atomistic molecular dynamics simulations to explore colloidal aggregates in the same configurations as we have investigated with SRD. With these simulations, we are able to recover the relative importance of fluid structure to hydrodynamics on short length scales.

1: J.T. Padding and A.A. Louis. Phys. Rev. E., 74:031402 (2006).