Structure of the system-spanning dynamically jammed region in response to impact of cornstarch and water suspensions

摘要

We experimentally study the impact response of concentrated suspensions ofcornstarch and water. We hypothesize that the dynamically jammed region thatpropagates ahead of the impactor is responsible for the strong stress responseto impact when it spans between solid boundaries. Using surface imaging andparticle tracking at the boundary opposite the impactor, we observed that avisible structure and particle flow at the boundary occur with a delay afterimpact. We show the delay time is about the same time as the the strong stressresponse, confirming that the strong stress response is a consequence of thisdynamically jammed structure spanning between the impactor and a solidboundary. We also elaborate on the structure of the dynamically jammed regiononce it spans from the impactor to a solid boundary. We observed particle flowin the outer part of the dynamically jammed region at the bottom boundary, witha net transverse displacement of up to about 5\% of the impactor displacement,which indicates shear. Direct imaging to the surface of the outer part of thedynamically jammed region reveals a change in surface structure that appearsthe same as the result of dilation in other cornstarch suspensions, and crackslike a solid. This shear and dilation are in contrast to the dynamically jammedregion as it propagates through the bulk, where it is argued to exhibit noshear or decrease in packing fraction. These observations suggest thedynamically jammed structure can temporarily support stress like a soil ordense granular material along a network of frictional contacts between theimpactor and solid boundary.