Mari Romain

Exploiting fragility of granular suspensions to enhance flowability

The mechanical response of a suspension to an applied load is strongly dependent on its microstructure, that is, the statistics of the suspended particles' relative positions. In turn, the microstructure is itself strongly dependent on the history of the applied load. Hence, when the applied load changes rapidly compared to the relaxation time of the microstructure, there is a load/microstructure mismatch. This gives rise to very rich non-steady state behaviors, a well-known example being the complex response observed during a shear reversal. For this reason suspensions have sometimes been presented as a prototype example of ``fragile matter''. In this talk I will introduce simulation results exploring the fragility of dense granular suspensions in a systematic and quantitative manner. These simulations reveal that close to the jamming transition, one can actively exploit this fragility to achieve flow at a reduced viscosity using small amplitude oscillatory shears on top of a desired primary flow. Despite the viscosity reduction though, due to the high oscillation frequencies needed the energy dissipation often appears to increase. However, after arguing that the viscosity reduction is due to the well-known “random organization”, a self-organization process that increases nearest neighbor distances, I will demonstrate that a protocol alternating phases of self-organizing crosswise oscillations and primary flow can achieve both substantial viscosity and dissipation reductions.