Dense Suspensions

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.

Figure 1. Impact of a marble drives an initially free-flowing cornstarch suspension into a shear jammed solid-like state.

Controlling Steady State Rheology at the Particle Scale

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.

Qin Xu et al. "Stress Fluctuations and Shear Thickening in Dense Granular Suspensions" J. Rheol. 64, 321 (2020)
Nicole M. James et al. "Tuning Interparticle Hydrogen Bonding in Shear-Jamming Suspensions: Kinetic Effects and Consequences for Tribology and Rheology," J. Phys. Chem. Lett. 10, 1663–1668 (2019)
Nicole M. James, Huayue Xue, Medha Goyal, Heinrich M. Jaeger, "Controlling Shear Jamming in Dense Suspensions via the Particle Aspect Ratio," Soft Matter, 15, 3649-3654 (2019)
Nicole James, Endao Han, Ricardo A. Lopez de la Cruz, Justin Jureller, and Heinrich M. Jaeger, "Interparticle hydrogen bonding can elicit shear jamming in dense suspensions," Nature Materials 17, 965–970 (2018)
Qin Xu, Sayantan Majumdar, Eric Brown and Heinrich M. Jaeger, “Shear thickening in highly viscous granular suspensions”, Europhysics Letters 107, 68004 (2014)
Eric Brown and Heinrich M. Jaeger, “Shear thickening in concentrated suspensions: phenomenology, mechanisms, and relations to jamming”, Reports on Progress in Physics 77, 046602 (2014).
Figure 2. (a) Flow state diagram for suspension systems with different levels of hydrogen bonding between PMMA particles. As hydrogen bonding is inhibited between particles the frictional jamming fraction moves to higher values. (b) Artist depiction of how particle surfaces interact through hydrogen bonding. (c) Depiction of how two carboxylated PMMA particles hydrogen bonding together can hinder tangential motion on the left. On the right is how that hydrogen bonding is blocked by urea functionalizing the carboxylated surface.

Beyond the Steady State

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.

Endao Han et al. "Stress Controlled Rheology of Dense Suspensions Using Transient Flows" Phys. Rev. Lett., 123, 248002 (2019)
Endao Han et al. "Dynamic Jamming of Dense Suspensions Under Titled Impact" Phys. Rev. Fluids, 4, 063304 (2019)
Endao Han et al. "Shear fronts in shear-thickening suspensions," Phys. Rev. Fluids 3, 073301 (2018)
Sayantan Majumdar, Ivo R. Peters, Endao Han, and Heinrich M. Jaeger, “Dynamic shear jamming under extension in dense granular suspensions”, Phys. Rev. E 95, 012603 (2017)
Ivo R. Peters, Sayantan Majumdar, and Heinrich M. Jaeger, “Direct observation of dynamic shear jamming in dense suspensions”, Nature 532, 214–217 (2016)
Endao Han, Ivo Peters, Heinrch M. Jaeger, "High-speed ultrasound imaging in dense suspensions reveals impact-activated solidification due to dynamic shear jamming" Nature Communications 7, 12243 (2016)
Scott Waitukaitis and Heinrich M. Jaeger, “Impact-activated solidification of dense suspensions via dynamic jamming fronts”, Nature 487, 205-209, (2012)
Figure 3. (a) Video taken using ultrasound methods that shows the propagation of a jamming front (region between bright red and white) as it races through the suspension leaving a solid-like sample in it's wake.
Figure 3. (b) 3D depiction of jamming front at tilted impact angle showing similar to the 2D jamming fronts shown in Fig 3 (a).