System-spanning dynamically jammed region in response to impact of cornstarch and water suspensions

TL;DR

This study experimentally investigates how a dynamically jammed region forms and propagates in cornstarch-water suspensions upon impact, revealing its role in the suspension's strong stress response and structural behavior.

Contribution

It provides detailed experimental evidence linking the formation of a spanning dynamically jammed region to impact stress response in cornstarch suspensions, highlighting its structure and deformation behavior.

Findings

Dynamically jammed region propagates ahead of impactor.

Strong stress response correlates with the region spanning boundaries.

Surface imaging shows dilation and brittle fracture features.

Abstract

We experimentally characterize the impact response of concentrated suspensions of cornstarch and water. We hypothesize that the dynamically jammed region that propagates ahead of the impactor is responsible for the strong stress response to impact when it spans between solid boundaries. Using surface imaging and particle tracking at the boundary opposite the impactor, we observed that a visible structure and particle flow at the boundary occur with a delay after impact. We show the delay time is about the same time as the the strong stress response, confirming that the strong stress response results from deformation of the dynamically jammed structure once it spans between the impactor and a solid boundary. A characterization of this strong stress response is reported in a companion paper (arXiv:1407.0719). We also elaborate on the structure of the dynamically jammed region once it spans from the impactor to a solid boundary. We observed particle flow in the outer part of the dynamically jammed region at the bottom boundary, with a net transverse displacement of up to about 5\% of the impactor displacement, indicating shear at the boundary. Direct imaging of the surface of the outer part of the dynamically jammed region reveals a change in surface structure that appears the same as the result of dilation in other cornstarch suspensions. Imaging also reveals cracks, like a brittle solid. These observations suggest the dynamically jammed structure can temporarily support stress according to an effective modulus, like a soil or dense granular material, along a network of frictional contacts between the impactor and solid boundary.