Rapid deformation of concentrated particulate suspensions can lead to striking non-Newtonian behaviors as discontinuous shear thickening (DST), in which the viscosity increases by orders of magnitude, or even the formation of a solid-like shear jammed (SJ) state. Upon removal of stress, the driven non-equilibrium SJ state "melts" back into a free-flowing suspension.
Dense suspensions allow flexibility under normal conditions, yet rapidly solidify and absorb energy under impact (Figure 1). For this reason, these rate-responsive soft materials are attractive for impact mitigation such as "liquid" body armor and needle stick-proof surgical gloves.
As described in more detail below, our research group uses these complex fluids as a platform to uncover new physics in driven nonequilibrium systems. We coalesce insights from physics, chemistry, and engineering to control shear thickening and shear jamming at the particle-scale. Beyond studying steady-state shear thickening, we also develop new metrologies to characterize the transient response of dense suspensions. Currently, we are collaborating with the Rowan and de Pablo groups at the Pritzker Institute for Molecular Engineering (PME) and researchers at NIST to explore how these systems can be used for impact mitigation applications.
We have shown that by engineering particle level details such as particle shape, particle surface chemistry, or particle solvation strength we can control the packing fractions at which one observes continuous shear thickening (CST), DST, and shear jamming. These studies have provided fundamental insights into how microscopic parameters at the particle level can conspire to control the bulk flow behavior of these systems. These insights could lead to programmable suspensions where an arbitrary desired flow behavior can be achieved through engineering particle level details. Through collaboration with the Rowan group we're currently exploring novel solvent and particles chemistries to see how we can further tune the rheological behavior of suspensions. This work is complemented by simulations done by the de Pablo group that help us understand how our bulk rheological measurements are controlled by particle level details.
Steady state rheology is ideal for measuring CST or DST fluids, but if the suspension is close to the jamming packing fraction the system's viscosity can diverge as the suspension enters a shear jammed state. To interrogate this transition to a solid-like state we impact the suspension from the top and image the flow field using ultrasound imaging from the bottom. By measuring the flow field we can see a high shear-rate front propagate throughout the suspension, shown in Fig 3a. This front leaves everything in it's wake in a jammed solid-like state that relaxes back to a liquid after the impact has stopped. Recently, we have also also measured these jamming fronts in 2D experiments with a pool of suspension near a moving wall. There we find qualitatively similar fronts as those observed in ultrasound measurements and we find that the fronts propagate at constant stress, possibly enabling stress controlled rheology for shear jammed systems.