Condensed Matter Theory

Abstract:

A formulation is developed for the calculation of the electromagnetic-fluctuation forces for dielectric objects of arbitrary geometry at small separations, as a perturbative expansion in the dielectric contrast. The resulting Lifshitz energy automatically takes on the form of a series expansion of the different many-body contributions. The formulation has the advantage that the divergent contributions can be readily determined and subtracted off, and thus makes a convenient scheme for realistic numerical calculations, which could be useful in designing nanoscale mechanical devices.

Abstract:

We explore the conditions under which colloids can be stabilized by the addition of smaller particles. The largest repulsive barriers between colloids occur when the added particles repel each other with soft interactions, leading to an average accumulation near the colloid surfaces. At lower densities these diffuse layers of mobile particles (nanoparticle halos) result in stabilization, but when too many are added, the interactions become attractive again. We systematically study these effects -accumulation repulsion, re-entrant attraction, and bridging - by accurate and flexible integral equation techniques[1], which faithfully reproduce recent computer simulations of the same effect[2]. We can explain recent experiments[3], and moreover show that there is a very substantial parameter regime where nanoparticle halos lead to colloidal stabilisation. We argue that this new mechanism should be widely applicable and complimentary to exisiting steric and charge stabilization techniques. It may should also be relevant for smaller scale biological interactions.

Abstract:

We adapt stochastic rotation dynamics, a mesoscopic computer simulation method, to colloidal suspensions, making sure length and time-scales are carefully separated to generate the correct coarse-grained physical properties[1]. This allows us to study the interplay between hydrodynamic and Brownian fluctuations during steady-state sedimentation of hard sphere particles for Peclet numbers (Pe) ranging from 0.1 to 15. Even when the hydrodynamic interactions are an order of magnitude weaker than Brownian forces, they still induce backflow effects that dominate the reduction of the average sedimentation velocity with increasing particle packing fraction. Velocity fluctuations, on the other hand, begin to show nonequilibrium hydrodynamic character for Pe > 1. We also explore the effects of hydrodynamics on driven lane-formation and aggregation of colloidal suspensions.