Liquid-like at rest, dense suspensions of hard particles can undergo striking transformations in behaviour when agitated or sheared. These phenomena include solidification during rapid impact, as well as strong shear thickening characterized by discontinuous, orders of magnitude increases in suspension viscosity. Much of this highly non-Newtonian behavior has recently been interpreted within the framework of a jamming transition. However, while jamming indeed induces solid-like rigidity, even a strongly shear-thickened state still flows and thus cannot be fully jammed. Furthermore, while suspensions are incompressible, the onset of rigidity in the standard jamming scenario requires an increase in particle density. Finally, while shear thickening occurs in the steady state, impact-induced solidification is transient.

As a result, it has remained unsettled how these dense suspension phenomena are related and how they are connected to jamming. Here we resolve this by systematically exploring both the steady-state and transient regimes with the same experimental system. We demonstrate that a fully jammed, solid-like state can be reached without compression and instead purely by shear, as recently proposed for dry granular systems. In contrast to dry granular materials, however, this state is created by transient shear-jamming fronts, which we track for the first time directly. We also show that shear stress, rather than shear rate, is the key control parameter. From these findings we map out a state diagram with particle density and shear stress as variables. Discontinuous shear thickening is newly identified with a marginally jammed regime just below the onset of full, solid-like jamming. This state diagram provides a new, unifying framework, compatible with prior experimental and simulation results on dense suspensions, that connects steady-state and transient behavior in terms of a dynamic shear-jamming process.

1. •Ivo R. Peters, Sayantan Majumdar, and Heinrich M. Jaeger, “Direct observation of dynamic shear jamming in dense suspensions”, Nature (in press, 2016). link
   

In the above state diagram, we plot the shear stress as a function of packing fraction. Blue symbols correspond to shear thinning or Newtonian behavior, red crosses denote weak (continuous) shear thickening, DST denotes discontinuous shear thickening, SJ is the shear jammed regime, and J indicates for the isotropically jammed regime.

The two movies below show the general behavior observed in these experiments. The suspension (here cornstarch in water) is contained in a cylindrical vessel. An inner metal cylinder is submerged into the suspension and connected to a rheometer that rotates it at fixed speed or fixed applied stress. Note that with this type of Couette geometry the jammed state is created purely by shear, not by compression (as in the impact experiments). Once the stress applied from the inner (rotating) cylinder becoms sufficiently large (see state diagram above), the suspension transforms from a fluid-like, flowing state into a rigidly shear-jammed state.  This transformation occurs via a jamming front that rapidly propagates radially outward (see Movie 2). The rigidity of the shear jammed state is seen by the fact that the inner cylinder slips and that a small ball bearing dropped onto the suspension does not sink in (it can even bounce!). Note also that shear jamming is completely reversible: once the applied stress is reduced sufficiently, the suspension return to a fluid state and the ball bearing starts to sink.