Abstract
A remarkable property of dense suspensions is that they can transform from liquid-like at rest to solid-like under sudden impact. Previous work showed that this impact-induced solidification involves rapidly moving jamming fronts; however, details of this process have remained unresolved. Here we use high-speed ultrasound imaging to probe non-invasively how the interior of a dense suspension responds to impact. Measuring the speed of sound we demonstrate that the solidification proceeds without a detectable increase in packing fraction, and imaging the evolving flow field we find that the shear intensity is maximized right at the jamming front. Taken together, this provides direct experimental evidence for jamming by shear, rather than densification, as driving the transformation to solid-like behaviour. On the basis of these findings we propose a new model to explain the anisotropy in the propagation speed of the fronts and delineate the onset conditions for dynamic shear jamming in suspensions.
Highlights
A remarkable property of dense suspensions is that they can transform from liquid-like at rest to solid-like under sudden impact
Dense suspensions are complex fluids that can exhibit strong, discontinuous shear thickening, where the viscosity jumps up orders of magnitude when a critical shear stress is exceeded[1,2,3,4]
Under a wide range of dynamic conditions, dense suspensions can undergo a transformation to solid-like behaviour, for example, during sudden impact at their free surface[5,6,7], ahead of quickly sinking objects[8,9], under shear[10] or during rapid extension[11]
Summary
A remarkable property of dense suspensions is that they can transform from liquid-like at rest to solid-like under sudden impact. Measuring the speed of sound we demonstrate that the solidification proceeds without a detectable increase in packing fraction, and imaging the evolving flow field we find that the shear intensity is maximized right at the jamming front Taken together, this provides direct experimental evidence for jamming by shear, rather than densification, as driving the transformation to solid-like behaviour. Detailed investigation of the dynamics during impact has shown how such solidification is associated with a propagating front that converts fluid-like, unjammed suspension into rigidly jammed material in its wake[5,12] This dynamic jamming front moves through the suspension with a speed much greater than the impactor itself. This calls into question the mechanism underlying equation (1), even though there is experimental evidence for the basic outcome, namely that the ratio vf/vp increases dramatically as Df approaches zero[5,12]
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