Dissertation Defense: Will Buchholtz

Candidate: Will Buchholtz

Major: Physics

Advisor: Emanuela Del Gado, Ph.D.

Title: A Microstructural Analysis Of Dense Suspension Rheology

Suspensions are mixtures of particles in a solvent in which the particles do not dissolve. Many suspensions, such as cement slurries or ooblek, exhibit shear thickening – their viscosity grows dramatically upon increasing an applied shear rate. Understanding the origins of thickening in suspensions has important applications to industry where damage can occur to equipment if the viscosity of a material unexpectedly increases. Novel uses, such as for protective fabrics and body armor, have also been proposed. Understanding the microscopic origins of shear thickening in suspensions and predicting their macroscopic behavior from the microscopic physics is a tremendous challenge. Over the last couple of decades a picture has emerged of shear thickening that emphasizes the role of frictional contacts between the particles. This thesis builds on this new understanding and insight by investigating how the microstructure of frictional particles evolves with shear rate in a simulated 2D suspension of soft disks. The softness of the particles is key, since it produces a thinning regime (where the viscosity decreases with increasing shear rate) that follows the Newtonian (constant viscosity) and thickening flow at lower rates, enabling the comparison of microstructure across all three regimes. I first show how the thickening regime corresponds to the rapid growth in the number of contacts, three disk chains, and triangles. During the thinning, the rate of contact growth decreases and there is a buckling of the chains and triangles. The behavior of these local structures confirms the growth of force networks during the thickening. I find that strong force networks of over-constrained particles (those in which each particle has at least 3 contacts with a force above the peak of the force distribution) show evidence of a percolation transition during the thickening. These networks subsequently de-percolate during the thinning. To conclude the analysis, I study the evolution of microscopic dynamics during the different flow regimes, finding that thickening coincides with an increase in non-affine motion of the particles. Localized clusters of disks with large non-affine speeds are also found to increase during high stress events.