Shear-Induced Microscopic and Macroscopic Behavior of a Hard Sphere Glassy Binary Colloidal Silica Suspension Using Confocal Rheology

Abstract

Many consumer products such as toothpaste, paint, and peanut butter exhibit both solid-like and liquid-like behavior. These materials are hard to classify because of their rich mix of both solid and liquid-like properties which makes them fascinating but also poorly understood. In many cases, these amorphous materials termed colloidal suspensions are made of particles with a wide distribution of sizes, which can be used to tune their properties. In addition, colloidal suspensions possess striking similarities to molecular glasses, and as such, serve as a valuable model to study the structure and dynamics of the glass transition. As the number of particles or packing fraction of colloidal suspensions is increased, the suspension jams, i.e.becomes a disordered solid, and develops a yield stress. However, this jammed state can be made to flow if a shear stress greater than the yield stress of the system is applied. I study how the bulk behavior of a colloidal silica glass is related to the nature of the dynamics of the particles that make up the system. In particular, I study the particle size segregation behavior in dense colloidal suspensions.

The particles of granular media can spontaneously size segregate when continuously driven. However, in jammed colloidal suspensions, this phenomenon is not well investigated. I present results of size segregation of a continuously sheared binary colloidal

suspension well above point J, through the application of a highly controlled shear at a constant shear rate with the use of a rheometer. By coupling the rheometer with a high-speed laser scanning confocal microscope, I directly image the structure and flow profiles of the suspension as it un-jams. Under oscillatory measurements, the bulk rheology exhibits a solid-like behavior for small strains and yields for strains above &sim15 %. A decrease in the yield strain is observed with an increase in the particle size ratio of the species in the suspension. Microscopically, I see evidence of both affine and shear banding flows and a migration of the small and large species; large particles move to the top while the small particles move toward the bottom, conserving the total volume fraction in all regions of the sample. An exponential relationship describes the behavior in the segregation process for both the small and large particle species. The data shows that the segregation time scale is dependent on both the particle size ratio and the applied shear rate. These results are consistent with kinetic sieving predictions and the proposed void filling and squeeze expulsion mechanism in granular systems. I also show that the bulk mechanical properties, particularly the storage and loss modulus are not dependent on the particle segregation behavior. In addition, preliminary results of shear banding behavior and their correlation to the particle distribution profile are presented. The results indicate that, shear band formation in jammed binary colloidal suspensions is closely associated with a non-uniform distribution in the local volume fraction along the gradient axis.