Jamming distance: a physics-informed design parameter for dense suspension rheology*

Dense suspensions are ubiquitous in everyday life: with applications ranging from household products to geophysical flows. These suspensions are complex in nature, with constituent particles exhibiting polydispersity, roughness, and interactions. By decoupling the effects of two complexities: surface roughness and interparticle interactions, we a propose a single parameter - jamming distance - that can unify the flow behavior in dense suspensions. Jamming distance can be defined as the spatial distance of a given suspension from its jamming point. From a physical viewpoint, jamming point is the concentration where the flow properties, namely the suspension viscosity, diverges. The effect of the surface roughness on dense suspension flow mechanics was studied using model smooth and rough PMMA colloids by performing rheological experiments at low strains (linear regime) and high shear rates (non-linear regime). At low strains, close to the suspension jamming point, the viscoelastic moduli of rough suspensions were thousand fold higher compared to the smooth suspensions due to the enhanced hydrodynamic lubrication interactions between the rough surface asperities in close contact. At high shear rates, dense suspensions shear thickened and we observed a universal correlation between the rate of shear thickening and jamming distance in colloidal suspensions. Using confocal rheometry, we characterized one of the first experimental 3D contact networks and found that jamming distance represented a scaled volume to rearrange particles during shear. Next, we studied real-life dense mixtures which are heterogenous in nature and have complex interactions. We found that steady shear flow curves of natural debris suspensions can be explained using a simple Bingham-fluid model, if the jamming distance-dependent viscous stresses are taken into account. Thus, our work eluciates that a single parameter, jamming distance, as a powerful tool to design and study flow mechanics in dense suspensions, from model suspension mixtures to natural geophysical flows.