Wed January 8th 2014
Seminar Why you can walk (run) on water (and cornstarch)
Scott Waitukaitus


A dense mixture of cornstarch and water is a remarkable material with strange properties. If you perturb it lightly it acts like a liquid, but if you disturb it violently solid-like properties emerge. Perhaps it is best known for its ability to support the weight of a person running across its surface, giving the crowd-pleasing impression that one can "walk on water." While this phenomenon has been widely-known for a long time, existing models of non-Newtonian fluids have not been able to fully explain it, in part because they focus on steady state shear rather than compression. Our approach to this problem has been to specifically concentrate on compression and study how the suspension decelerates an impacting object. We show that an impacting object drives the rapid growth of a solid-like region below the impact site. Being coupled to the surrounding fluid by lubrication forces, this creates a large peripheral flow and ultimately leads to the rapid extraction of the impactor's momentum. With a simple jamming picture to describe the solidification and an added mass model approximate the dynamics, we are able to explain the sudden deceleration. Our findings highlight the importance of the non-equilibrium, transient character of dense suspensions near jamming.
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The 10th Complex Motion in Fluids 2021
Max Planck Gesellschaft
Centre for Scientific Computing