Wilson Poon is a world-leading soft matter and biological physicist who has made seminal contributions across many areas of colloid and interface science, including earlier work on the phase behaviour of colloid-polymer mixtures, which remains widely influential today, and more recent work on bacteria as active colloids, some of which was presented as a plenary lecture at the 2014 ECIS Conference in Cyprus. This nomination relates to his work in the last 5 years on the rheology of suspensions, which has helped bring about a paradigm change in the subject.
While the flow of suspensions of Brownian particles is relatively well understood, many aspects of the rheology of suspensions of larger particles (say, > 5 μm), especially at high concentrations (say, volume fraction φ > 50%), have continued to remain puzzling. Such suspensions are well known to industrial practitioners as being capricious. Their rheology in the laboratory is very sensitive to small changes in materials, conditions and experimental protocols, and is therefore highly irreproducible. At the same time, the ‘holy grail’ of predicting their behaviour in complex geometries, e.g., die extrusion, has remained elusive. This is unfortunate, because such ‘granular suspensions’ are ubiquitous, partly because suspensions in which the particles are derived from ball milling typically do have size distributions peaked in the granular range.
One clue to the basic physics dominating the rheology of granular suspensions is the frequent observation of shear thickening, especially when interparticle attraction is weak or absent. Until recently, shear thickening has been ascribed to the formation of so-called ‘hydroclusters’ due to lubrication forces. Starting from 2014, simulation and theory by different groups have independently converged onto the importance of interparticle contacts. In this new picture, applied stress beyond a certain threshold overcomes whatever stabilising mechanism exists between the particles, and leads to the formation of frictional contacts that stop neighbouring particles from sliding past each other. Such contacts necessitate extra local motions to achieve a given bulk deformation, which leads to higher dissipation and increased viscosity.
Interparticle contacts in a liquid medium seemed initially to be surprising, because diverging lubrication forces prevent such contacts between perfectly smooth spheres. In 2015, Poon published two seminal experimental papers to demonstrate that this objection does not apply to real suspensions. The first [1] uses very well characterised suspensions of sterically-stabilised polymethylmethacrylate spheres in hydrocarbon solvents to validate the predictions of a phenomenological theory for friction-driven shear thickening and jamming due to Wyart and Cates (WC). It also explains why such phenomenology is ubiquitous: the threshold stress for frictional contacts scales as the inverse particle size squared, so that practically any realistic stress will induce thickening in most industrially-relevant granular suspensions.
A second paper [2], published with a Cornell group, demonstrates unambiguously the key role of frictional forces, which, unlike hydrodynamic interactions, vanishes instantaneously when external shear is reversed. That frictional forces dominate the granular suspension rheology is now broadly accepted by the community, and Poon’s two papers deserve a significant part of the credit for bringing about this revolution.
Since then, a string of papers has followed to push this new paradigm forward, extending its range and demonstrating it applicability to real-life systems and geometries of immediate industrial relevance. First, it has been known for some time that near shear jamming, granular suspension flows are unstable. A 2019 paper [3]
investigates the simplest possible instability: spontaneous oscillations. Elegant experiments designed to reveal the origins of such oscillations produced clean data that are explained using by a simple extension of the WC model into the time domain. The novel data analysis also led to the measurement for the first time of the relaxation time of frictional contacts.
This was followed by a second paper in 2019 [4] studying the extrusion of a shear- thickening suspension of repulsive granular particles through a narrow constriction. Dewatering – the concentration of the suspension emerging from the constriction is lower than that remaining in the barrel – is ubiquitous in this common industrial process. Again, a simple phenomenological model was devised to predict the onset of dewatering based on shear rheology data alone. This is a major advance, showing for the first time that laboratory rheology data can be used to elucidate behaviour in a complex geometry.
The four papers cited so far describe the behaviour of granular suspensions with repulsive particles. In 2018, Poon’s group published a theoretical paper [5] proposing a generalisation of the WC model into a ‘constraint rheology’ framework, which can then be used to make sense of the rheology of granular suspensions in which particles may form adhesive contacts that restrict interparticle rotation. The interplay between sliding (frictional) and rotational (adhesive) constraints under increasing applied stress leads to a ‘zoo’ of possible flow curves, so that almost all classes of observed behaviour so far reported in the literature emerge from a single framework.
The applicability of this framework was directly tested in a recent paper [6] explaining the physics of ‘conching’, which is the first step of industrial chocolate manufacturing. Pointing out close similarities of this process with concrete mixing, Poon’s group were able to show that the process uses mechanical agitation to increase the volume fraction at which friction-induced shear jamming occurs, and that, unexpectedly, a major role of added surfactants was to modify interparticle friction. This work is likely to become a classic, because it opens up the way to a rational approach to designing and modifying the flow of industrially-relevant granular suspensions with frictional and adhesive interparticle contacts. The latter also opens up a new era in interfacial science on the micro-scale, conclusively demonstrating that surfactants can act as friction modifiers in suspension flow, whereas traditional understanding simply points to a role in reducing interparticle adhesion.
Taken together, these papers published in the period 2015-2019 represent an outstanding corpus of experimental and work and theoretical insights that simultaneously advance fundamental colloid and interface science and demonstrate the immediate relevance to practical processes and products. Poon is therefore an appropriate recipient of a prize recognising outstanding original work sponsored jointly by the ECIS and a major company involved with colloidal and interfacial technologies.
The nomination is made for the following publication(s) that has appeared during the last 5 years
1. Towards a Unified Description of the Rheology of Hard Sphere Suspensions, B M. Guy et al, PRL 115, 088304 (2015)
2. Hydrodynamic and Contact Contributions to Continuous Shear Thickening in Colloidal Suspensions, N. Y. C. Lin et al PRL 115 228304 (2015)
3. Competing Timescales Lead to Oscillations in Shear-Thickening Suspensions, J. A. Richards et al, PRL 123 038004 (2018)
4. Liquid Migration in Shear Thickening Suspensions Flowing through Constrictions, R. E. O’Neill et al, PRL 123 128002 (2019)
5. Constraint-Based Approach to Granular Dispersion Rheology, B. M. Guy et al PRL 121, 128001 (2018)
6. Conching Chocolate is a Prototypical Transition from Frictionally Jammed Solid to Flowable Suspension with Maximal Solid Content, E. Blanco et al, PNAS 116, 10303-10308 (2019)