SuspenseStory · Flow-induced anisotropic mechanical response of dense suspensions

Dense suspensions are complex fluids, like shampoo or concrete, with equal parts of particles and liquid. Large particles necessitate external forces for movement, causing mechanical properties to depend on shear history. This is currently not understood nor well characterized and is a major obstacle for the modeling of its flows with quickly varying conditions, which are ubiquitous in industry (e.g. extrusion, drilling) or in nature (e.g. landslides). This project will unravel how flow history controls the mechanical response of suspensions.The viscosity of a suspension of hard particles under a quickly varying flow can be much smaller than a steady flow because the anisotropic microstructure is too slow to evolve and provide a strong resistance. For the same reason, a jammed suspension of soft particles can retain stresses even after flow stops. So far, only shear stress has been studied, and the microstructure-stress relation remains poorly characterized. Advanced particle-based simulations, considering hydrodynamics and complex contact physics, allow precise exploration of this relationship. Separately, a new generation of microscopics-informed constitutive models show great promise to predict the evolution of the stress under arbitrary flow history but remain tested only in limited setups. We aim at characterizing the full stress tensor response under shear rotations, i.e. model time-dependent flows where the strain axes are suddenly rotated.We will determine the contribution of hydrodynamics, elastic, frictional interactions to the response, and how these depend on particle softness. We will use our results as stringent tests for current constitutive models.The last goal is to achieve quantitative match with experiments performing shear rotations on suspensions of polystyrene (hard) and Carbopol (soft) performed by collaborators, which report intriguing observations of transient tangential stresses orthogonal to the flow direction.