Mud as a soft matter system: Stability of aggregates under time-periodic flow*

Mud, a mixture of fine-grained sediments (such as clays and sand), organic material, and water, exhibits complex rheological behavior while being important to many environmental processes. The cohesive nature of clay minerals dominates the rheology of mud, since these constituents tend to aggregate through collisions and bonding of clay particles. Aggregation of cohesive sediment is highly influenced by hydrodynamic forces, such as that experienced in rivers, in ways that are far from understood. This motivates our experimental study, in which we model cohesive sediment by investigating a granular medium of spherical polystyrene grains that can aggregate due to interfacial deformation as they float at an air-liquid interface. The grains are introduced to well-characterized, two-dimensional time-periodic flows which are controlled by Lorentz forcing. We thereby describe the formation, breakup, and transport of granular aggregates due to the relative importance of shear stresses and interparticle forces.