The non-linear physics of driven colloids and bacteria

Statistical mechanics is the branch of physics dealing with how the collective properties of a large number of entities depend ib how they interact with each other. The hardest problems in the subject are those that deal with systems driven off equilibrium by some external force. I propose a programme of experimental research to probe two kinds of driven systems: the deformation and flow of dense colloidal suspensions, and collections of bacteria.The interacting entities in a concentrated colloidal suspension are microscopic particles suspended in a liquid. The flow properties of such suspensions are both a fascination for fundamental science, and important for applications. One of the many everyday examples is the way tooth paste behaves like a liquid when being squeezed out of its tube, but acts like a solid while sitting on the tooth brush. I will use a range of new experimental tools to study the deformation and flow of very well defined, 'model' suspensions. In particular, it is now possible to use advanced optical microscopy to follow the trajectories of individual particles in a suspension in flow. The data obtained will give us an unprecedentedly detailed picture of how the strange flow properties of dense suspensions are related to their constituent particles.There is today emerging a new area of statistical mechanics devoted to the study of 'agents' - complex entities interacting with each other, resulting in novel collective behaviour. The entities can be mobile phones on a network, or stock brokers on a trading floor, or a collection of bacteria. Individual bacteria are about the same size as inert colloidal particles (a thousandth of a millimeter). The main difference is that bacteria are active - they can propel themselves through the surrounding liquid. They also 'signal' to each other by secreting and 'decoding' a range of chemical 'messages'. A large number of bacteria can therefore show novel collective behaviour. Thus, e.g., they can 'swarm' on a surface (much like birds do in air). My research will address a range of bacterial collective behaviour, such as how they clump together to form 'biofilms' - complex two-dimensional bacterial 'cities', and how biofilms deal with 'cheaters', individual mutants who take advantage of their neighbours. I will also draw on the analogy with colloids and compare how suspensions of bacteria differ from suspensions of inert particles. Emerging results from the theory of interacting agents will also be tested experimentally using bacteria as 'models'.