Traction Rheoscopy

Abstract

Micron sized colloidal particles, which interact with nearly hard-sphere potentials, can be used to form dense amorphous packings using a variety of well established techniques. Due to the large size and slow dynamics of colloidal particles, confocal microscopy can be used to directly measure the 3D structure and dynamics of these densely packed colloidal glasses. For example, previous results have directly visualized both the inhomogeneous particle level rearrangements that occur within a colloidal glass under applied shear strain, and the emergence of continuum strain fields surrounding these particle rearrangements as a function of applied shear strain. While microscopic visualization of colloidal glasses provides real-space information not readily obtainable in atomic systems, measuring the stress response of colloidal glasses, in addition to visualization, is a significant challenge. The large micron size of the colloidal particles and thermal interaction energies necessarily give rise to solids with exceptionally small elastic moduli on the order to 1Pa. We introduce a new technique, traction rheoscopy, to directly measure the mechanical response of colloidal glasses while simultaneously visualizing microstructure. The method consists of a bilayer of colloidal glass sitting atop a well calibrated soft polymer gel of comparable shear modulus. The composite bilayer is sheared and the shear stresses are inferred from the displacement of embedded tracer particles in the calibrated polymer gel. Using these stress measurements, we show that under applied shear the colloidal glass goes through as sequence microscopic rearrangements as the stress develops correlated spatial heterogeneities. The stress measurements with simultaneous real space visualization of the particle dynamics corroborate previously reported microscopic flow mechanisms and present new insight into how colloidal glass responds to applied shear. This thesis presents experimental techniques and protocols developed for traction rheoscopy, along with results on the mechanical response of colloidal glass under increasing applied shear strains ranging from∼ 0.1%to10%.