2022 Patrick Warren

Patrick Warren

Patrick Warren is a world-renowned soft matter theorist who, perhaps unusually, has spent nearly his entire career in industry, working at Unilever R&D continuously for three decades. During this period, Warren produced nearly 100 publications arising from the practical problems that he was solving for the company. Impressively, many of these also have high scientific impact (h-index = 39, > 10,000 citations). In other words, industrial problems that Warren works on often turn into scientific gold! We cannot, of course, know whether there is also industrial gold for his employer; but his longevity with Unilever suggests so. Such ‘dual impact’ is reminiscent of P-G. de Gennes, except that Warren contributed ‘from the other side’. His uniqueness is exemplified by 5 publications that came out since 2017, which span four areas in which Warren is well known: dissipative particle dynamics (DPD), fluid mechanics, electrostatics and polymer physics. Warren is a world authority on DPD, a popular simulation method invented to bridge the gap between the atomistic and mesoscopic scales. Patrick first put the metod onto a firm theoretical footing in 1995 with Español (a paper with > 2000 citations), and has deployed it with effectiveness ever since. Most recently, Warren and his collabo- rators have pioneered a ‘molecular Lego’ approach to parameterize surfactants for DPD simulations that is potentially applicable to a wide range of molecules . Paper [1] conclusively demonstrates that key properties such as the cmc can be quantitatively predicted for a class of common surfactants, the alkyl sulfates. This result is a tour de force: it renders high-throughput in silico predictive formulation of surfactant products within reach of everyday users, something that has been dreamed about for decades but never before realized: A scientific achievement of high order, because it could not have been accomplished without a deep understanding of the underlying statistical mechanics. 

Fluid dynamics is a second area in which Warren excels. Recently, he has deployed this expertise in a series of outstanding papers published with Howard Stone on the peculiarly colloidal-scale phenomenon of phoretic transport whereby particles move in response to gradients. Paper [2] clearly arose from an industrial context, and sheds light on a longstanding problem in laundry: how can dirt be cleansed from pores apparently too small to allow significant fluid flow? Warren’s theoretical analysis – verified by Stone’s experiments – provides a startling new insight: local gradients in surfactant concentration can drive diffusiophoretic motion of dirt particles that leads to their continual removal from fine pores. Moreover, if the surfactants are charged, then coupling to electrophoresis should allow this effect to be enhanced by the addition of salt. A time-honoured practice in laundry has at last received scientific justification. Warren’s recent work on phoretic transport also covers the important case of colloid- polymer mixtures. He has had a longstanding associated with this area, with his first big ‘hit’ being a 1992 paper on the equilibrium phase behaviour of mixtures of hard spheres and non-adsorbing ideal polymers (> 850 citations). In paper [3], he returns to such mixtures and considers their dynamics. In particular, he and his collaborator Richard Sear show that diffusiophoresis may drive stratification in such mixtures when they dry. This result clearly has important implications for a range of applications such as paints. Crucially for fundamental science, the work highlights the importance of considering the solvent explicitly. Studies using ‘implicit solvent models’ in which hydrodynamics is necessarily absent will miss or at least mistreat a range of phenomena. The elucidation of charge effects in narrow-pore laundry demonstrates a third area in which Warren is a world authority: the electrostatics of soft matter. Warren has a longstanding reputation for repeatedly ‘getting it right’ in an area where many pitfalls await the unwary due to the long-ranged nature of Coulombic forces. Recently, Warren has not only used his electrostatics expertise to explain the effect of salt in laundry, but has also made an important contribution to measurement technique. Paper [4] demon- strates a novel method for accessing the zeta potential based on solute gradients that is cheap to set up and easy to use. The importance of this quantity across the board in col- loid and interface science means that this work will have significant and long-term im- pact for almost everyone who does experiments in this area. Few theorists can aspire to direct such impact in the laboratory. 

A final area in which Warren is a world expert is polymer physics. Here, his deep under- standing of the underlying statistical mechanics has enabled him not only to solve a number of important problems in ‘polymer physics proper’, but also to borrow polymer physics techniques to tackle novel problems concerning ensembles of objects that share certain features of polymers. His work on how hair-hair interaction controls the shape of ponytails, which won him the 2012 Ig Nobel Prize, is one good example. In a more recent Paper [5], Warren and his collaborators develop the thinking of the earlier pony-tail paper further to explain an apparently trivial phenomenon: that woven fabric does not fall apart. The discovery that an ensemble of overlapping fibers with frictional contacts may undergo a generic percolation transition beyond which they can efficiently transmit tension will have very wide impact, e.g. in the understanding of paper. The creative use of linear programming techniques in arriving at this conclusion must be con- sidered a tour de force in the theoretical analysis of soft matter problems. In sum, the 5 papers associated with this nomination show that Patrick Warren is an outstanding theorist at the peak of his power tackling problems that are simultaneously of industrial and fundamental scientific interest. These papers will undoubtedly continue to set the direction for each of the fields represented for a considerable period of time to come. 

[1] Micelle Formation in Alkyl Sulfate Surfactants Using Dissipative Particle Dynamics, R. L. Anderson, D. J. Bray, A. Del Regno, M. A. Seaton, A. S. Ferrante, and P. B. Warren, J. Chem. Theory Comput. 14, 2633-2643, 2018.
[2] Cleaning by Surfactant Gradients: Particulate Removal from Porous Materials and the Significance of Rinsing in Laundry Detergency, S. Shin, P. B. Warren, and H. A. Stone, Phys. Rev. Applied 9, art. no. 034012, 2018.
[3] Diffusiophoresis in nonadsorbing polymer solutions: The Asakura-Oosawa model and stratification in drying films, R. P. Sear and P. B. Warren, Phys. Rev. E 96, art. no. 062602, 2017.
[4] Low-Cost Zeta Potentiometry Using Solute Gradients, S. Shin, J. T. Ault, J. Feng, P. B. Warren and H. A. Stone, Adv. Mater. 29, art. no. 1701516, 2017.
[5] Why Clothes Don’t Fall Apart: Tension Transmission in Staple Yarns, P. B. Warren, R. C. Ball, and R. E. Goldstein, Phys. Rev. Lett. 120, art, no, 158001, 2018.