Michael Cates: Bulletproof Custard: Fluids That Stop Flowing When You Push Them Too Hard

When small, hard particles are suspended in a fluid, they make it more resistant to flow. The higher the particle concentration, the higher the viscosity. Add enough particles and fluid stops flowing entirely, becoming a jammed solid - this makes intuitive sense.

Less intuitive and more intriguing are suspensions that flow smoothly if pushed gently, but that suddenly solidify if you push too hard. This behaviour is called Discontinuous Shear Thickening (DST). You can try it yourself by mixing cornstarch with water - in the right proportions, the mixture will flow smoothly when stirred gently, but will refuse to flow at all if stirred too hard.

More than an interesting kitchen trick, DST has important real-world consequences. It can cause catastrophic failure of industrial pumping equipment, but can also have life-saving applications to bulletproof vests.

For many years, the mechanism behind DST was unclear, but we have very recently found a new and stunningly simple explanation based on the idea that the contacts between particles become less lubricated and more frictional as the force between them increases. Although this dependence is typically gradual, when a fluid gets close to the “jamming” point, global instabilities can result in the sudden switching from liquid to solid.

Michael Cates (Lucasian Professor of Mathematics, University of Cambridge) will explain this peculiar form of “bulletproof custard” with a few equations, plenty of diagrams, and even some hands-on demonstrations.