Shear fronts in shear-thickening suspensions

Abstract

We study the fronts that appear when a shear-thickening suspension is submitted to a sudden driving force at a boundary. Using a quasi-one-dimensional experimental geometry, we extract the front shape and the propagation speed from the suspension flow field and map out their dependence on applied shear. We find that the relation between stress and velocity is quadratic, as is generally true for inertial effects in liquids, but with a pre-factor that can be much larger than the material density. We show that these experimental findings can be explained by an extension of the Wyart-Cates model, which was originally developed to describe steady-state shear-thickening. This is achieved by introducing a sole additional parameter: the characteristic strain scale that controls the crossover from start-up response to steady-state behavior. The theoretical framework we obtain unifies both transient and steady-state properties of shear-thickening materials. arXiv:1711.02196v1 [cond-mat.soft]

Dynamic jamming of dense suspensions under tilted impact

“…Previous impact experiments at normal incidence [24,31] have shown that as U p increases, k L and k T each approach an asymptotic plateau that is independent of U p . For the suspension we used here, U p = 0.2 m/s was fast enough to be in this plateau regime.…”

“…Now let us revisit the anisotropy in the dimensionless front propagation speed k. Similar to the case of the front width, the ratio between k L and k T is roughly 2 as well. In previous work [24,31], we have shown that in the high stress regime (fast impact), the speed with which a dense suspension shear jams is limited by having to build up the finite shear strain for rearranging the particles into a jammed configuration. We assume that this threshold strain is a scalar: when the suspension approaches this strain scale locally, its viscosity increases dramatically and develops towards a jammed solid.…”

“…Instead, the formation of the solid-like region is initiated on the side of the line that is closer to the impactor. Its specific position on the z = 0 plane will depend on the size of the system, because when such a jamming front propagates, its dimension grows linearly with time, but its width ∆ remains invariant [31]. As discussed above, the effect of a solid boundary starts to appear when its distance to the front becomes comparable to ∆.…”

“…Consequently, the left and right lobes of the front now accumulate strain at different rates. Since shear jamming of transient flow not only depends on the applied stress, but also the accumulated shear strain [24,31], higher shear rate results in a shorter time to jam. We suspect that this is the reason why in Fig.…”

“…Lastly, how about jamming by shear? Our earlier work [31] showed that to create a dynamic jamming front by shear, a local strain needs to be applied to shear the suspension out of the uniform, isotropic initial state, and bring the particles into contact networks. This requires a strain of order one that is applied along a particular direction.…”