Hydrodynamic Simulations of Colloidal Suspensions with Short-Range Attraction and Long-Range Repulsion: Microstructure and Rheology

Michael Bybee, University of Illinois

Suspensions of colloidal particles (typically a few nanometers to a few microns in radius) are of great importance in a variety of industrial applications including paints, coatings, foodstuffs, pharmaceuticals, and personal care products. Additional applications arise in the fabrication and development of novel electronic materials, microscale biosensors, and other nanostructured materials. Colloidal suspensions exhibit a wide variety of phase transitions and rheological behavior that can be far more complex than for normal fluids. The ability to understand, manipulate, and predict the behavior of these systems is important for designing and optimizing novel materials and manufacturing processes.

One of the most intriguing aspects of colloidal suspensions is the ability to tune the interparticle interactions, which have a direct influence on the suspension microstructure and mechanical and rheological properties. This work focuses on suspensions of particles with short-range depletion attraction and long-range screened electrostatic repulsion. Equilibrium predictions and dynamic simulations are employed to study the effects of these interactions on suspension microstructure and rheology. Dynamic simulations include the effects of many-body hydrodynamic interactions and Brownian motion through a method called fast lubrication dynamics, a reduced version of Stokesian dynamics.

Simulations exhibit stable fluid phases, metastable fluid phases, cluster phases, fluid-crystal phase separation by homogeneous nucleation and growth, and gelation by dynamic arrest. Crystals are only observed at the lowest strengths of repulsion. For all systems studied, increasing the strength of attraction results in gelation, marked by both structural and dynamical transitions. Gels are composed of connected networks of particle chains. These chains become thinner with increasing repulsion, consistent with both kinetic and thermodynamic arguments. By simply tuning the interparticle potential, a wide variety of suspension microstructures can be achieved that will result in a wide range of mechanical and rheological properties.

Abstract Author(s): Michael Bybee