(650f) Computational Studies of Colloidal Dynamics in Entropic Fields

An important class of inter-particle forces, known as depletion forces, is induced by the presence of other colloidal species and arises solely as a result of entropic considerations. Passive structures etched into the walls of a surface can create entropic force fields of sufficient range and magnitude so that the motion and position of large colloids can be controlled. By providing potentially simple routes for the directed self-assembly of novel mesoscopic structures, the use of entropic force fields is a promising approach to the production of advanced materials. Various issues concerning the feasibility of such methods need to be addressed, however, given that the dynamics of colloidal particles diffusing through an entropic force field is not well known. Since entropic forces become repulsive at intermediate colloidal separations, large repulsive barriers may kinetically stabilize suspensions or prevent deposition. Moreover, the total depletion interaction emerges as a net result of the equilibrium distribution of secondary species and, hence, evolves in time; it remains unclear as to the time scale over which the depletion interaction develops and how the interstitial fluid affects this timescale. We therefore investigate the dynamics of hard-sphere colloids moving above and onto surfaces of various shapes via the use of two mesoscopic simulation techniques, stochastic rotation dynamics (SRD) and dissipative particle dynamics (DPD). While usage of mesoscopic techniques is essential to the computational exploration of colloidal behavior, the effect of coarse-graining, where there is some loss of detail regarding the fluid, raises some issues regarding the fluid that, previously, have not been satisfactorily answered. Although techniques like SRD and DPD have been shown to correctly replicate hydrodynamic effects in a bulk fluid, there has been some uncertainty as to how well these techniques are able to capture fluid effects in the presence of a surface. More specifically, the time scale over which fluid memory effects develop and decay, and, by extension, how these effects correlate with the definition of colloidal local self-diffusion coefficients has not been well understood. We clarify these points and develop a systematic interpretation for defining position-dependent diffusion coefficients. Furthermore, using the coarse-grained techniques, we determine the relative influence of hydrodynamic and entropic effects on particle deposition and agglomeration. Such key issues need to be explored in sufficient detail before an effort can emerge to describe the dynamics and evolution of depletion potentials through a direct appeal to the Fokker-Planck, or probabilistic, description of the underlying stochastic process.