Contact tribology also affects the slow flow behavior of granular emulsions

Abstract

Recent work on suspension flows has shown that contact mechanics plays a role in suspension flow dynamics. The contact mechanics between particulate matter in dispersions should depend sensitively on the composition of the dispersed phase: evidently emulsion droplets interact differently with each other than angular sand particles. We therefore ask: what is the role of contact mechanics in dispersed media flow? We focus on slow flows, where contacts are long-lasting and hence contact mechanics effects should be most visible. To answer our question, we synthesize soft hydrogel particles with different friction coefficients. By making the particles soft, we can drive them at finite confining pressure at all driving rates. For particles with a low friction coefficient, we obtain a rheology similar to that of an emulsion, yet with an effective friction much larger than expected from their microscopic contact mechanics. Increasing the friction coefficient of the particles, we find a flow instability in the suspension. Particle level flow and fluctuations are also greatly affected by the microscopic friction coefficient of the suspended particles. The specific rheology of our "granular emulsions" provides further evidence that a better understanding of microscopic particle interactions is of broad relevance for dispersed media flows.

Uncovering friction dynamics using hydrogel particles as soft ball bearings

“…Hydrogels have become immensely popular in the past decades and these materials have, more recently, been the subject of a broad range of frictional studies. 1,[12][13][14][15] Hydrogels exhibit unique frictional behavior and friction coefficients as low as 0.01 have been reported for hydrogel-on-hydrogel friction (Fig. 1b).…”

Shear-driven flow of athermal, frictionless, spherocylinder suspensions in two dimensions: Stress, jamming, and contacts

“…This is a simplification compared to real physical suspensions, which may include hydrodynamic forces [48], lubrication forces [49][50][51], and inertial effects [52]. More recently, frictional contact interactions have been proposed to become important when the lubrication layer breaks down upon close contact of particles near jamming, and this has been proposed as a possible mechanism for shear thickening [53][54][55][56][57][58][59].…”

Connecting the Drops: Observing Collective Flow Behavior in Emulsions

“…It should be noted that the HB description of structured fluids is not always applicable. Suspensions of soft frictional particles have an effective friction coefficient that can get smaller with rate [64]. Such flow instabilities have been observed in multiple systems [65,66] but were then linked to shear-induced weakening: the shear induces so much fluctuations that the yield stress is affected, even creating an additional critical point [67].…”

“…To illustrate the complexity of friction in structured fluids studies, it is helpful to consider that at the same time, numerous simulations have shown that even when one sets the interparticle friction coefficient to zero, the macroscopic friction coefficient of a structured fluid remains finite [91][92][93][94]. Even though numerical simulations of comparable systems have found contradicting relationships between the microscopic and macroscopic friction coefficient [93][94][95], the emergence of a non-zero macroscopic friction coefficient, where microscopically it is negligible, has been been confirmed in experiments [64]. Non-sphericity can further complicate the relation between the microscopic contact laws and the emergent frictional properties [92] but should be less important in slowly sheared emulsions, as the Laplace pressure will keep droplets in emulsions mostly spherical.…”

“…The coupling of the mean flow and its fluctuations also gives rise to different types of fluctuations [15,101]. One might also believe that viscous damping suppresses the coupling between mean flow and fluctuations and the associated self-induced weakening, but the viscous damping needs to be strong enough: glass bead suspensions with water also show a logarithmic negative rate dependence [64]. Emulsions can also be composed of droplets that are smaller than a micrometer.…”