Colloids, grains and dense suspensions: under flow and under arrest.

This Theme Issue reports papers presented at a Discussion Meeting intended to bring together those working on colloids (suspended particles small enough to have strong Brownian motion) with those working on non-Brownian systems such as dry granular matter, dense suspensions of larger particles and pastes. A better understanding of flow and arrest in these systems is of potential value to wide areas of chemical engineering, materials processing and the design of soft materials. Such knowledge is relevant to personal-care products, oilboredrilling fluids, wastewater processing and handling of slurries, ceramic pastes and dry powders. Although these topics do not necessarily ignite the imagination of the general public when discussed in abstract terms, many people are fascinated by the behaviour of jammable suspensions such as dense cornstarch or tomato ketchup, and intrigued to learn that these pose mysteries that are yet to be answered by scientists. The similarities between such materials and quicksand, landslides and avalanches serve as a reminder that such topics make a valid claim on publicly funded research budgets. Historically, those studying dense particulate materials have tended to divide according to whether or not a fluid medium surrounds the particles and whether, if a solvent is present, the particles are small enough to be Brownian and hence amenable to an equilibrium statistical mechanical description. Increasing focus on non-equilibrium phenomena, even in colloids, means that these divisions are increasingly indefensible. Moreover, within each area there has evolved strong interdisciplinarity in the sense of involving scientists from physics, chemical engineering and other subject backgrounds. Nonetheless, the Brownian versus granular divide remains substantive. The existence of such a divide is understandable since over several decades quite different experimental techniques have been developed for the various cases, and the theoretical gap between equilibrium statistical mechanics (colloids only) and far non-equilibrium (the rest) remains hard to bridge. However, there is now an experimental convergence on real-space methods that track individual particle motions (whether by microscopy, MRI or other means), allowing large datasets to be interrogated by computer; likewise, conceptual links between equilibrium and granular statistical mechanics are finally being built. Moreover, it is realized that the presence of solvent interfaces (capillary bridges, free surfaces) can create forces that entirely suppress Brownian motion even within the colloidal domain, as can the non-equilibrium phenomenon of glass formation.