Inter-particle adhesion induced strong mechanical memory in a dense granular suspension

Repeated/cyclic shearing can drive amorphous solids to a steady state encoding a memory of the applied strain amplitude. However, recent experiments find that the effect of such memory formation on the mechanical properties of the bulk material is rather weak. Here, we study the memory effect in a yield stress solid formed by a dense suspension of cornstarch particles in paraffin oil. Under cyclic shear, the system evolves toward a steady state showing training-induced strain stiffening and plasticity. A readout reveals that the system encodes a strong memory of the training amplitude (γ_T) as indicated by a large change in the differential shear modulus. We observe that memory can be encoded for a wide range of γ_T values both above and below the yielding albeit the strength of the memory decreases with increasing γ_T. In situ boundary imaging shows strain localization close to the shearing boundaries, while the bulk of the sample moves like a solid plug. In the steady state, the average particle velocity v inside the solid-like region slows down with respect to the moving plate as γ approaches γ_T; however, as the readout strain crosses γ_T, v suddenly increases. We demonstrate that inter-particle adhesive interaction is crucial for such a strong memory effect. Interestingly, our system can also remember more than one input only if the training strain with smaller amplitude is applied last.