2009 Dirac medal
Professor Michael Cates
University of Edinburgh
For pioneering work in the theoretical physics of soft materials, particularly in relation to their flow behaviour.
Michael Cates is distinguished for his research into the statistical mechanics of soft materials, such as polymers, emulsions, colloids, foams and liquid crystals. These show mesoscopic structure which can lead to startling flow behaviour: viscoelasticity, jamming, and flow-induced phase transitions.
Professor Cates has played a leading role in the creation of statistical-mechanics models to explain such phenomena. Early in his career he developed a model for entangled polymer-like objects that can break and recombine reversibly, thereby explaining the flow of behaviour of a class of viscoelastic surfactant solutions, used in products ranging from shampoos to oil-bore fluids. He then co-invented models for the isotropic ‘sponge phase’ of surfactants, identifying its structure and explaining its reversible conversion into a liquid crystal under shear. With Milner and Witten, Cates created a theory of polymer chains coating a surface by attachment at one end- the ‘polymer brush’.
Since the mid 1990s, Cates’ major interest has been in modelling the flow of soft materials that are not in equilibrium even at rest. His work has caused many such materials (shaving foam, mayonnaise, wet corn starch) to become viewed as ‘soft glasses’, with distinctive properties stemming from their structural arrest. Recently, with Fuchs and others, Cates developed first-principles treatments of the flow of dense colloidal suspensions, building on modern theories of glass transition. Another recent achievement has been to accurately simulate the demixing of binary fluids, and then study its arrest by colloidal particles. This has led directly to the creation of new functional materials (‘bijels’) in the laboratory.
His work exemplifies how cutting-edge theoretical physics can have impact, not only on other disciplines such as chemical engineering and materials science, but on readily observable domestic phenomena such as the failure of tomato ketchup to come out of the bottle.