Microstructure response of concentrated suspensions to flow reversal

We study experimentally at the macroscopic and microstructure scale a dense suspension ofnon-Brownian neutrally buoyant spherical particles experiencing periodic reversals of flow at constant ratebetween parallel plates and tracked individually. We first characterize the quasi-steady state reached atthe end of half periods. The volume fraction of particles increases from the walls to the center as a resultof migration induced by the nonuniform strain rate. Except very close to the walls and the center, theparticle pair distribution is fore-aft asymmetric with depletions of pairs in the extensional quadrants,similar to that reported for shear flows of same volume fraction as the local one. The dynamics of theperiodic rearrangements occurring after each flow reversal are characterized by a microstructure tensorcomponent. The relaxation time characterizing the reorganization increases from the walls to the centerdue to the inhomogeneous strain rate. On the other hand, the local accumulated strain required for thisreorganization decreases with the volume fraction, like for viscosity measurements in uniform strain rateconditions. However, the variation of the microstructure with the accumulated strain is faster than that ofthe viscosity, showing the complementarity of the two measurements.