Computational studies of model colloidal dispersions

Bryce Daniel Sturtevant, Purdue University

Abstract

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 meso-scopic structures, the use of entropic force fields is a promising approach to the cost-efficient 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 understood. Moreover, the essential physics of colloidal dispersions interacting hydrodynamically with the complex boundaries that are required for the initiation of entropic potentials is not yet clear. To this end, it is the purpose of this work to establish tools for simulating the effect of bounding surfaces on the motion of colloidal dispersions and, also, determining the equilibrium structure of colloidal particles. Here, we pursue a dynamical study of colloidal motion using both molecular dynamics and a meso-scopic simulation technique, Stochastic Rotation Dynamics (SRD), as well as a study of the equilibrium structure of colloidal dispersions through development of a novel integral equation. Our initial efforts are directed at determining the local friction and ordering forces that colloidal particles experience whenever they are closely interacting with a surface. Estimates of the local friction, recovered from conducting both moving-particle and fixed-particle molecular dynamics simulations, were found to show that local ordering of the fluid produced considerable departures from the hydrodynamic approximations that are presently used. What is more, these findings are bolstered by results from a new, novel, simulation technique that was developed to extend the applicability of expensive molecular dynamics simulations to larger colloidal sizes by simulating only a small patch of the colloidal surface. We find that, even for very large colloidal particles, local ordering of the fluid plays a dominant role in the force that a colloidal particle experiences.

Degree

Ph.D.

Advisors

Corti, Purdue University.

Subject Area

Chemical engineering|Condensed matter physics|Theoretical physics

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