Can activity in a suspension cause thickening or dethickening?

We describe our investigations into the ability of active matter to tune shear thickening and jamming in colloidal suspensions. Active colloidal suspensions exhibit many interesting features including swarming, motility-induced phase separation and disordered hyperuniformity. Such features arise microscopically from various self-propulsion and self-organization mechanisms and can lead to rheological signatures such as shear thinning and super fluidity. Here, by altering the density and activity of the microparticles, we propose to tune the shear thickening, jamming and yielding behavior of an active colloidal suspension. We carry out our investigations on an active optorheological medium consisting of titanium dioxide particles which self-propel upon activation by ultraviolet light, allowing the activity and thus the shear thickening rheology of the suspension to be tuned via the light intensity and particle concentrations. The addition of activity to the parameter space governing the transition between the thickened, jammed and yielding state would provide an extremely powerful and convenient tuning mechanism to rapidly modify the shear behavior of the suspension on the go.