Colloidal gels displaying both flow-aligned and vorticity-aligned flocs under shear

To connect the structural and flow properties of colloidal gels we experimentally probe colloid-polymer mixtures on a rheometer with an optical microscopy attachment. We study mixtures of a polymer and temperature-responsive hydrogel particles (pNIPAM) which undergo fluid-fluid phase separation or gelation at room temperature depending on concentrations. At higher temperatures (above about 32 °C) the hydrogel particles form a gel. We observe these mixtures as they are sheared in a parallel-plate geometry with an optically transparent base plate and a small gap thickness. At room temperature, we observe flow-aligned domains with shear-rate-dependent dimensions. Above 32 °C, the stronger interparticle attraction leads to vorticity-aligned flocs. We can cycle between flow-aligned and vorticity-aligned domains by varying the temperature by just a couple degrees. This allows us to investigate the formation of these shear-induced domains and to connect their spatial characteristics with the bulk sample flow properties.