Fragile dense suspensions under shear rotation

While quasi-Newtonian complex fluids under steady-state flow, dense non-Brownian suspensions exhibit complex "macro-fragile" behaviors under varying flow conditions, as revealed by abrupt shear inversions (reversals). Here, we introduce an experimental setup to systematically explore their macro-fragile response to shear rotations, where one suddenly rotates the principal axes of shear by an angle θ. This reveals a transient decrease of the shear stress under shear rotation. Moreover, the orthogonal shear stress, which vanishes in steady state, takes non-negligible values with a rich θ-dependence, changing qualitatively with ϕ, and resulting in a force that tends to reduce or enhance the direction of flow for small or large ϕ. These findings are confirmed and rationalized by particle-based numerical simulations and a recently proposed constitutive model. We show that the angular dependence of the orthogonal stress results from an interplay between hydrodynamic and contact stresses, which balance changes with ϕ.