Force and Mass Dynamics in Non-Newtonian Suspensions

TL;DR

This study investigates the boundary forces and shock dynamics in dense cornstarch suspensions under impact, revealing complex pressure wave behavior and the relationship between impact speed, packing fraction, and shock propagation.

Contribution

It provides the first quantitative measurements of impact-driven boundary forces and shockfront dynamics in non-Newtonian cornstarch suspensions.

Findings

Mass shocks occur during impact and are correlated with the intruder.

A second, faster pressure front propagates to the boundaries.

Pressure wave speeds are much slower than ultrasound speeds, indicating complex response.

Abstract

Above a certain solid fraction, dense granular suspensions in water exhibit non-Newtonian behavior, including impact-activated solidification. Although it has been suggested that solidification depends on boundary interactions, quantitative experiments on the boundary forces have not been reported. Using high-speed video, tracer particles, and photoelastic boundaries, we determine the impactor kinematics and the magnitude and timings of impactor-driven events in the body and at the boundaries of cornstarch suspensions. We observe mass shocks in the suspension during impact. The shockfront dynamics are strongly correlated to those of the intruder. However, the total momentum associated with this shock never approaches the initial impactor momentum. We also observe a faster second front, associated with the propagation of pressure to the boundaries of the suspension. The two fronts depend differently on the initial impactor speed, $v_0$, and the suspension packing fraction. The speed of the pressure wave is at least an order of magnitude smaller than (linear) ultrasound speeds obtained for much higher frequencies, pointing to complex amplitude and frequency response of cornstarch suspensions to compressive strains.