Tune it your way: activity induced, stress controlled bifurcation of the rheology of a dense 3D Quincke rotor suspension*

We experimentally investigate how activity can be used to tune the rheology of a dense suspension of Quincke rotors in 3D and connect our observations to changes in the 3D microstructure. Under a constant stress, the suspension viscosity shows a clear bifurcation in response to a constant electric field being applied, depending on the magnitude of the applied stress. For low stresses, the viscosity increases with increasing electric field strength, eventually reaching a jammed state for larger field strengths. This is associated with the local densifications of particles into strong percolating structures that can withstand the shear, induced by the random rotation of the particles. For high stresses, the viscosity decreases with increasing electric field strengths, and this is associated with the shear induced degeneracy of the direction of rotation in this large shear limit. Our study reveals how exactly activity interacts with shear in 3D to result in this novel bifurcation mechanism for tuning the suspension flow.