Michael Cates

Michael Cates

2016 Bingham Medalist

University of Cambridge

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I am delighted to profile Michael Cates, who has been named the 2016 Bingham Medalist of The Society of Rheology. Mike is the 19th Lucasian Professor of Mathematics at the University of Cambridge, UK. He is also a Fellow of the Royal Society (FRS) and Fellow of the Royal Society of Edinburgh (FRSE). Prior to July 2015, he held the Chair of Natural Philosophy (1708) and was the Royal Society Research Professor at the School of Physics and Astronomy, University of Edinburgh. He completed his Ph.D. studies in 1985 under Sir Sam Edwards from Cambridge University with a thesis on The Statistical Mechanics of Complex Polymers. He won the Gold Medal of the British Society of Rheology in 2009 and the Weissenberg Award of the European Society of Rheology in 2013, which are the highest honors in the field of rheology from the United Kingdom and the European rheology societies, respectively. He has co-authored over 300 publications, many of them of high impact in the field of rheology. There are few who have contributed to the science of rheology as profoundly as has Mike Cates, as detailed briefly below.

Thread-like micelles. Cates is especially well known among rheologists for his work on the dynamics and rheology of threadlike micelles, wherein he combined reptation theory with a model of breakage and re-joining of long micelles. The initial work, published in 1987, immediately and clearly explained the mystery of the nearly single-relaxation-time behaviour of some of these solutions. In the original paper and subsequent work, a range of conditions and phenomena were covered, including fast and slow breakage, fluctuation effects and Rouse modes, and nonlinear effects as well, all with great elegance and enduring impact. Reading his very first papers on the subject, it is astonishing to realize how much of the physics, including the multiple dynamical regimes, were laid out correctly. Thus, some 25 years later, the work remains the standard theory for the rheology of thread-like micelles, serving the same role for surfactant solutions that the Doi-Edwards theory serves for entangled polymers. Surfactant solutions remain of great intellectual and practical interest: they are widely used in shampoos, body washes, and other cleansers and personal care products.

Soft Glassy Materials. Nearly equally well known is Cates’ work on the dynamics and rheology of what have come to be known as “soft glassy materials,” which typically have a virtually inaccessible longest relaxation time. In 1997, with Sollich and others, Cates proposed a simple generic model to explain their low frequency dynamics, including ‘aging’ as they slowly relax toward equilibrium. The model demonstrated that anomalous properties of emulsions, foams, and the like, could be made explicable by adopting the viewpoint of glass physics, thus connecting the dynamics of pastes and gels with those of glasses. Cates, with Sollich and Fielding, showed that only minor modifications of the original model are needed to give new insights into the interplay between aging and shear-banding. This work showed great imagination and courage in tackling a most profound and difficult aspect of aging materials, namely the lack of a well-defined linear relaxation spectrum. In addition, he and his coworkers have also contributed greatly to the theory of glassy materials more generally, including recent work describing the nonlinear rheology of both “simple” glasses and polymeric glasses.

Colloidal glass rheology. In 2002, Cates helped lead a key Edinburgh collaboration showing that mode coupling theory (MCT) is the method of choice to describe hard-sphere colloids with short-range attractive interaction. Their subsequent simulations have confirmed this, while also highlighting where MCT fails. With Fuchs, also in 2002, Cates used MCT to obtain the first quantitative theory for the yield and shear-melting of colloidal glasses. It has spawned successful ‘schematic’ models of shear thickening and, with Brader, Cates later generalized it to give a full nonlinear constitutive equation for colloids close to the glass transition subjected to arbitrary flow histories. The model admits a simple schematic representation that exhibits the first statistical-mechanical derivation of a von-Mises like yield surface for a plastic material. This work on colloidal glass rheology also inspired a promising effort to understand polymer glasses, and fed into a new understanding of shear banding in hard-sphere colloids. Most recently, Cates has focussed on addressing the shear-thickening of very dense suspensions of large particles in which Brownian motion does not dominate, building on an emerging realization (thanks to Denn, Morris, Pouliquen, Lemaitre and others) that the flow of very dense suspensions is dominated by friction at interparticle contacts. In 2005, Cates showed that, unlike earlier (hydrodynamic) models, an MCT-based schematic approach to shear thickening accounts for the bizarre bistability of droplets of dense colloidal suspension: when prodded with a spatula, these can convert into a metastable jammed state, which returns to a flowable droplet if vibrated.

Other important work. Among Cates’ many fundamental discoveries, work that impacts rheology includes his lattice Boltzmann simulations on colloids in binary solvents, where he found an entirely new type of gel (the bicontinuous interfacially jammed emulsion gel or ‘bijel’) in which the interface between two bicontinuous fluids is arrested by a monolayer of colloidal particles. This structure was subsequently created by an experimental team in Edinburgh, is the subject of two patents, and forms the basis of a continuing programme in the design of new materials for Li-Ion batteries and other applications. In addition, with Tailleur, Cates predicted the MIPS (Motility-Induced Phase Separation) paradigm, whereby active colloids with purely repulsive interactions can phase separate into dense and dilute coexisting phases, so long as their effective propulsion speed is a sufficiently strongly decreasing function of density. This idea proved useful in biological contexts and then came into its own in explaining the very strong phase-separation tendencies seen experimentally in self-propelled, autophoretic Janus colloids. This work, which addresses separation into two phases that are both isotropic, complements efforts by Cates and coworkers and others to understand the statistical physics of self-propelled rods in phases with long-range orientational order. Finally, In 1994 (with Bouchaud and others) Cates created a successful model for avalanche dynamics in sandpiles, explaining the observed hysteresis of the average slope. From 1995 on, he and his collaborators developed a new continuum approach for finding the stress in static granular packings.

Beyond science. Mike has broad interests in music, the arts, and outdoor recreation. During his 20 years in Scotland he got to know the songs of Robert Burns, some of which he occasionally performs, and completed the Munros (which means that he has climbed all 282 of Scotland's hills over 3000ft high). Cambridge has fewer hillwalking options; but having moved back there in 2015 to take up the Lucasian Professorship, he will probably now last several more years without knee surgery! Mike grew up in the south of England, one of six children; his extended family includes 4 nieces and 8 nephews. He is supported in life by his partner, Henry Jabbour, a visual artist born in Lebanon who specializes in figurative painting.

Mike Cates is well known for crystal clear writing and speaking, deep physical insight, and mastery of mathematics. He exhibits a keen wit, broad interests, an excellent sense of humour, skilled leadership, devoted mentorship, and gracious hospitality. His receipt of the Bingham Medal is well earned and will add to its luster.