Friction and Flow of Granular Emulsions

We experimentally investigate the role of friction in dense non-Brownian suspensions. We perform rheological and tribological experiments using hydrogels. Similar to the droplets in an emulsion, our hydrogel particles are soft, so we can make a dense suspension. However, as in a granular material, the particles are solid and thus interact through friction. We use hydrogels with varying chemical composition to make the particles, and measure the friction coefficient of the materials with a custom tribometer. We link the microscopic friction of the particles to the rheological behavior of the suspension, by combining rheological measurements with optical techniques in a 3D printed Couette geometry. We find that the friction coefficient of the material sets the stress ratio (shear stress/pressure) of the suspension, provided the friction coefficient is sufficiently high. This suggests two dissipation mechanisms contribute to the shear resistance of the suspension.