Shear-switched bi-stability in binary suspensions

Concentrated binary suspensions (or slurries) containing large grains suspended in a colloidal gel background are ubiquitous in applications such as lithium-ion battery catholytes. For efficient and effective processing, it is vital to understand how they flow. We uncover surprising complexity in the rheology of a well-characterised model system: non-Brownian repulsive silica particles suspended in a background of attractive silica particles that are ten times smaller. Routine laboratory operations revealed that our suspensions could be switched reversibly by shear between solid- and liquid-like states. Confocal rheometry showed respectively large particles homogeneously dispersed in a background colloidal gel matrix in the solid state, and compact clusters of small particles dispersed amongst large particles in the liquid state. Mapping out a rheological state diagram uncovered a very low shear regime in which the system became strongly history dependence. These complexities disappeared when the large particles became small enough to penetrate the fractal clusters forming the background colloidal gel. We enunciate design principles for tuning the microstructure and rheology of such binary suspensions.