Traction Rheoscopy of Colloidal Glass

Micron-sized hard-sphere colloidal particles can be used to form dense amorphous packings. Due to the large size and slow dynamics of colloidal particles, confocal microscopy can be used to investigate the 3D structure and dynamics of these glasses at the particle level. Measuring the stress in colloidal glasses during deformation, however, is a significant challenge; due to the large particle size and thermal interaction energies, colloidal solids have small elastic moduli. 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 atop a calibrated polymer gel of comparable elastic modulus. The bilayer is sheared at constant strain rate. Using a confocal microscope, shear stresses are inferred from the displacement of embedded tracer particles in the polymer gel and both the local strains and the applied strain on the sample are measured. Using these mechanical measurements, we show that under applied shear strains of less than 1%, the colloidal glass exhibits non-monotonic mechanical response. We will share results relating this mechanical response to the locally heterogenous response of the colloidal glass.