(530e) Computational Studies of Colloidal Dynamics In Entropic Force Fields
The ability of colloidal particles to self-organize suggests that colloidal particles could be used as precursors for advanced materials via the generation of complex microstructures. The precise control of colloidal dispersions rests upon our knowledge of the forces that arise between particles and surfaces of various shapes. An important class of inter-particle forces is induced by the presence of other colloidal species and arises solely as a result of entropic considerations. These entropic forces can promote order-disorder transitions in the dispersion microstructure and may be responsible for a disorder-disorder transition. Furthermore, passive structures etched into the walls of the container can create entropic force fields of sufficient range and magnitude so that the motion and position of large colloids can be controlled, thereby generating various two-dimensional fluid-like and solid-like phases on chosen templates.
By providing new and potentially simple routes for the directed self-assembly of novel mesoscopic structures, the use of entropic force fields to create various complex microstructures is a promising approach to the production of advanced materials. Various issues concerning the feasibility of such methods, however, need to be addressed. For example, the dynamics of colloidal particles diffusing through an entropic force field is not well known. Since the entropic forces that arise within colloidal dispersions become repulsive at intermediate separations, large repulsive barriers may kinetically stabilize suspensions even though coagulation/deposition is thermodynamically favored. In some instances, these repulsive barriers may prevent the desired deposition or coagulation.
We investigate in detail the dynamics of hard-sphere colloids moving above and onto surfaces of various shapes via the use of two computational methods: molecular dynamics (MD) and stochastic rotation dynamics (SRD). SRD, which is a method for coarse-graining fluid interactions while still including the correct hydrodynamic interactions, such as the important lubrication forces, allows us to determine the relative influence of hydrodynamic and entropic effects on particle deposition. We find good agreement between our calculated and previously measured (via experiments ) normal and transverse diffusion coefficients of a colloid particle located near a hard wall. A comparison of MD and SRD results also reveals that SRD captures interesting solvent behavior when the gap distance between colloids or between colloids and surfaces become quite small. While no simulation method is optimal for all systems of interest, our studies indicate that SRD is a robust computational tool that should be applicable to a reasonable variety of other technologically relevant colloidal dispersions. In addition to simulation techniques, we model hard-particle colloidal dispersions operating in entropic potentials through the use of Fokker-Planck- and Smoluchowski-type equations for the probability densities of both colloidal position and momentum coordinates.
Ultimately, we are interested in elucidating the pathways that colloidal particles, while operating within these entropic force fields, take to self-assemble or deposit onto a specified surface and to obtain the various time-scales for these processes. To this end, we utilize both MD and SRD to study the deposition of colloidal particles onto substrates of various simple geometries (e.g., corner, step ledge, etc.). We extract the time scale for the migration of a colloidal particle initially located at a position in a uniform, bulk fluid to a particular position on the substrate. Furthermore, we investigate the importance of Brownian fluctuations and hydrodynamic interactions relative to the entropically generated forces. By comparing the results of both MD and SRD simulations, we determine the influence of hydrodynamic interactions on the structures that form, as well as analyze the time scale for the development of the full (equilibrium) entropic potential force-field. The information gleaned from these computational studies will be of importance for the tuning or design of surfaces that can be used to control with sufficient precision the rate of self-assembly and, perhaps more importantly, the final assembled structure.
 Oetama, Ratna J. and John Y. Walz. J. Coll. Inter. Sci. 284 (2005) 323-331