PI Public Lecture Series:
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Title: Bulletproof Custard: Fluids That Stop Flowing When You Push Them Too Hard
Abstract: 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.
During his Nov. 2 Perimeter Institute Public Lecture, Michael Cates will explain this peculiar form of “bulletproof custard” with a few equations, plenty of diagrams, and even some hands-on demonstrations.
Biography: Mike Cates is the 19th Lucasian Professor of Mathematics at the University of Cambridge, a chair previously held by Stephen Hawking, Paul Dirac, George Gabriel Stokes, and Isaac Newton. Mike received a PhD in Theoretical Physics from Cambridge in 1985 and became a research fellow and then Lecturer there before moving to Scotland in 1995 to take up the Chair of Natural Philosophy at the University of Edinburgh, a position he held for 20 years. He was elected a Fellow of the Royal Society in 2007 and since then has received the Dirac Medal of the Institute of Physics, alongside the Gold Medal, Wiessenberg Award and Bingham Medal of the British, European and US Societies of Rheology respectively. "Rheology" is the science of how materials flow, and Cates' main research connects the unusual flow behaviors of soft materials (such as wet sand, putty, molten plastic or mayonnaise) to the motion of their underlying constituents (which may be hard particles, flexible chains, or deformable droplets). Recently he has also become interested in systems such as swarms of micro-organisms, in which the interacting constituents are not inert but capable of self-propulsion.