Tacking: Shear fragility and geometry reduces the dissipation for dense suspensions

Dense suspensions tend to shear jam at large packing fractions. However, it has recently been shown that various oscillation protocols can unjam as well as reduce viscosity and dissipation per strain. In this numerical work, we show that alternating shear rotations protocols also reduce the dissipation per strain similar to shear protocols with a steady primary shear and superimposed cross oscillations, even though the latter's viscosity reduction is more considerable. Furthermore, we find that alternating between primary and perpendicular oscillations yields a much higher dissipation than the two protocols mentioned above, yet has similar viscosity as the cross-oscillatory one. While self-organization has been shown to minimize viscosity, our findings challenge the idea that random organization is the underlying mechanism for reducing the dissipation per strain. Instead, we attribute it to shear “fragility” combined with geometry, which explains the counterintuitive decoupling of the minima of viscosity and dissipation for the alternating shear rotation protocol. This work paves the way for a new class of highly energy-efficient flow protocols. Published by the American Physical Society 2024