Wilson Poon

Wilson Poon

The University of Edinburgh

Awarded Bingham Medal 2022
The University of Edinburgh weblink

By combining rheological and microstructural measurements on carefully characterized model systems, Wilson Poon has transformed our understanding of the flow of colloidal gels, attractive and repulsive colloidal glasses, and non-Brownian suspensions of frictional particles. He has thereby elucidated universal principles underpinning the formulation of many industrial products.

It is a pleasure to sketch the achievements of Wilson Poon, this year’s Laureate for the Bingham Medal.

Wilson Poon was born in Hong Kong, and was educated there to the age of 16, learning to do mental arithmetic in Chinese (a habit that, he says, remains). He was especially fond of reading science books, including a slim volume on ‘Atoms’ which became heavily annotated and dog-eared. Before long he was teaching himself calculus. Finding it easier to read and think about science in English, he persuaded his parents that he should study physics at university in the UK, prefaced with two final high-school years there. He departed Hong Kong for Rugby School in 1978. A precocious scholar even then, the school gave him his first teaching opportunity – giving a class to younger pupils on a subject of his choice (group theory).

Wilson studied Natural Sciences at Cambridge University, graduating with first class honours in Physics and Theoretical Physics in 1984. He was especially influenced by the problem-based pedagogy of his tutors, including biophysicist Aaron Klug and astrophysicist Peter Scheuer. He remained in Cambridge for his PhD work on phase transitions in minerals, before moving to his first academic post, a lectureship in Portsmouth Polytechnic in 1989. A year later he moved to the Department of Physics at the University of Edinburgh where he has remained ever since, being promoted to a Personal Chair in 1999 and to the Chair of Natural Philosophy in 2016. The latter chair has existed since 1708 and was famously not awarded to James Clerk Maxwell when he applied for it in 1860, being given instead to Peter Guthrie Tait. (Maxwell had to be content with the Cavendish Professorship in Cambridge instead.) Tait was a scientist of great distinction but clearly no rival to Maxwell; he won the chair on the grounds that, quite unlike Maxwell, he was an inspiring teacher. Poon likewise embodies the strong Scottish tradition that leadership in research and in pedagogy go hand in hand.

Wilson’s early research career focused on Raman spectroscopy, fluorescence and birefringence studies of molecular materials in complex stuctural phases. In Edinburgh, however, he encountered Peter Pusey (recently appointed to a chair there), already renowned as one of the outstanding colloid physicists of his generation, and a pioneer in the use of light scattering to elucidate structure and dynamics of colloids. Thus began a scientific collaboration that lasted over two decades: Pusey’s insistence on leaving no stone unturned proved a productive counterweight to Poon’s creative flair. Over time, however, Wilson came to share Peter’s eye for detail, with no dimming of his own natural tendency to innovate. By the time Pusey retired, Poon had not only assumed the mantle as head of the experimental soft matter group in Edinburgh, but also emerged as a scientific powerhouse in his own right. In particular he was among the first to recognize, and develop, the power of direct microscopy in colloidal materials. He showed this capable of resolving individual particle trajectories even in systems under flow, enabling for the first time the connection to be made between continuum physics (phase equilibria and rheology) and fully resolved microstructural data. This goal had eluded the scattering methods that had held sway for the preceding half century.

Poon’s early work with Pusey on the phase behaviour of colloid-polymer mixtures remains widely influential today. His subsequent career included numerous important contributions to the rheology of Brownian colloidal suspensions, while in the past 5 years, Wilson has made pivotal contributions to an emerging understanding of dense non- Brownian suspensions. These two bodies of work on rheology are discussed further below. Alongside them, Poon has made a series of important contributions to biophysics and to the dynamics of selfpropelled colloids.

Contributions to rheological science

Poon’s first paper in colloid rheology put the entire field onto a firmer footing. The ‘standard model’ in the field (since Einstein) addresses a suspension of Brownian hard spheres, and how its reduced viscosity (η) varies with volume fraction (ϕ), up to the point at which the system crystallises at ϕ = 0.49. Precise measurement of both quantities is crucial; its absence had led to published results for η(0.49) varying by more than an order of magnitude. Poon’s careful measurements using a bespoke Zimm rheometer established the ‘gold standard’ data set for the reference hard-sphere system.

Wilson then turned to the exploration of kinetically arrested states in colloidpolymer mixtures. His studies in the 1990s of ‘delayed sedimentation’ in low-ϕ gel states were ahead of their time, and underpinned a subsequent resurgence of interest in the mechanisms of such phenomena. This work combined macroscopic flow observation of gel collapse with microstructural probes using light scattering.

There followed a seminal body of work on high-ϕ glassy states. Leading a team of experimentalists, simulators and theorists, Poon announced in 2002 the experimental existence of two distinct glassy states in colloidal suspensions (attractive and repulsive glasses). This was exactly as predicted by mode-coupling theory (MCT) and established this rather mathematical theory as the predictive method of choice for colloid rheology at high ϕ. This work on glassy states framed the agenda for a decade’s research (by Poon and many others) on experimental colloidrheology. Poon himself soon discovered that repulsive hard-sphere glasses yield in a single step, while attractive colloidal glasses yield in two distinct steps. These and his other observations on well-characterised model systems have now been replicated in a multitude of academic and industrial contexts.

During this period Poon’s team invented confocal rheo-imaging, a combination of hardware and software tools that allow the mapping of thousands of time-dependent particle coordinates while rheological measurements are being made. Such approaches form part of a wider transformation in the way rheology is done, of which Poon is one of the key architects. The ability to make precise microstructural measurements under flow, alone or simultaneously with measurements of bulk rheological stresses, helped establish the modern approach whereby the links between microstructural evolution and rheology are nailed down experimentally rather than left to the theorist’s imagination. This raises the bar for successful theories – a development not always welcomed by theorists, but greatly benefitting rheological science.

For the past decade or so, Poon has focussed his attention on the rheology of dense non-Brownian suspensions. Such suspensions are widely viewed as capricious and unpredictable; their rheology appears very sensitive to small changes in materials, conditions and experimental protocols. This is unfortunate, since such supra-Brownian ‘granular suspensions’ are ubiquitous in industrial settings.

Poon has made pivotal experimental contributions to a recent paradigm shift in how these suspensions are viewed. This is based on the realization that frictional forces, acting at direct solid-solid contacts between particles, are crucial determinants of the rheology. This leads for example to a new and compelling explanation of shear thickening, whereby sufficient stress overcomes short-scale repulsive forces, allowing 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. Within a window of density it can also lead to jamming whereby the suppression of sliding prevents bulk flow altogether (resulting in fracture or granulation instead).

In 2015 Poon used well characterised suspensions to validate a theory for friction-driven shear thickening due to Wyart and Cates, and showed moreover that practically any realistic stress will cause friction to dominate in most granular suspensions of industrial interest. With colleagues from Cornell, he then demonstrated the key role of direct contact forces, including friction, using an elegant modern version of a shear-reversal protocol first used by Acrivos and others in the 1980s. That contact forces including friction dominate granular suspension rheology is now broadly accepted, in large part due to Wilson’s efforts.

Since then, progress has been rapid. Poon’s 2019 experiments probe the origins of oscillatory flow instabilities and explain them with an extension of the same model. His subsequent study of extrusion in shear-thickening suspensions explains the ubiquitous industrial phenomenon of dewatering, showing its onset to be predictable using shear rheology data alone. In 2018, Poon’s group published a model that generalised further the ‘constraint rheology’ theory, in which particles may form adhesive contacts alongside frictional ones. This allows almost all classes of rheology so ar reported for non-Brownian suspensions to be explained within a single framework. The industrial value of this framework was established in a 2019 paper on the physics of chocolate conching, opening the way to a more rational approach to formulation of granular suspensions in an industrial context based on ‘contact engineering’.

Interacting with Wilson: a personal note

Anyone who has heard Wilson Poon lecture at a conference or summer school will be aware of his infectiously energetic style of delivery. The same characteristic has enthused students, postdocs, and senior collaborators for many years. On the other hand, he is happy to play devil’s advocate, which sometimes can lead to ruffled feathers – years ago I heard him ask a seminar speaker “but isn’t that a complete waste of time?” (followed by a good reason to think it might be). Wilson has mellowed since, but in any case his adoption of such positions is never entrenched. His issuing of such a challenge does not represent a dismissal, but an invitation to advance science through unfettered discussion of the key ideas.

During the 20 years I myself spent in Edinburgh, such discussion with Wilson was a constantly productive feature of my own scientific life. One crucial element in our collaborations has been to find where there is genuine overlap between what is calculable and what is measurable – for only here can hypotheses be tested, and theories stand or fall. Another has been to jointly shape the scientific agenda, not just identifying the questions that matter most, but also ensuring that experiments remain maximally informative, and theory does not stray into idle speculation.

In his role as the head of an experimental lab, Wilson Poon is unusual in several ways, including the ability to successfully delegate work when appropriate. For instance, most organisers of extended theory-based workshops find it hard to attract senior experimentalists to leave their lab teams unattended for more than a few days. Not so Wilson, who was happy to leave experiments in the capable hands of colleagues during his 6-month immersion in 2004 at the Isaac Newton Institute in Cambridge to explore emerging ideas of biological physics.

Beyond science

Wilson is a lover of music and an accomplished amateur pianist. He is an avid reader across diverse areas of the arts and humanities, as well as science, and owns a substantial library. He is a lay preacher (sermons are available on his website!) and has played a major role in Edinburgh in exploring the connections between science and religion. He is married to Dr Heidi Poon, a distinguished lawyer and latterly a Judge, specializing in UK taxation law. They have two children – Rebecca, now doing a PhD in biophysics at the University of Exeter, and Aidan, who has just completed a Masters in Mechanical Engineering at the University of Warwick.

In conclusion

Wilson Poon’s experiments on the rheology of suspensions, first in the Brownian and then in the non-Brownian domain, have conspicuously and consistently shaped the field over the last two decades. He has introduced major new methods for connecting rheology to microstructure, and his work on frictional suspensions has very direct implications for industrial practice. Wilson Poon is a worthy winner of the SOR’s Bingham Medal.