Traction force rheology of colloidal polycrystals and glasses

Micron sized colloidal particles in a solution can be assembled and ordered into densely packed polycrystals or disordered glasses. Due to the large size and slow dynamics of colloidal particles, confocal microscopy can be used to directly measure the 3D structure of dynamics of colloidal solids. Measuring the stress response, in addition to visualization, is a challenge because large colloids (which are essential for confocal microscopy) and thermal interaction energies necessarily give rise to solid phases with exceptionally small elastic constants on the order of 10-100 mPa. We introduce a new technique, traction force rheology, to directly measure the mechanical response of colloidal polycrystals and glasses while simultaneously visualizing the microstructure. The method consists of a bilayer of colloidal solid 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 gel. To complement the direct stress measurements, we visualize, in 3D, the time evolution of the complex microstructure including dislocation and grain boundaries in colloidal polycerystals and flow defects in the colloidal glass.