The non-linear physics of driven colloids and bacteria.

This project consisted of two parts. In the first part dealing with concentrated colloidal dispersions, our research has rewritten the text books on how these systems deform and flow when external forces are applied. The flow of concentrated suspensions feature in the majority of industrial processes and products, but the understanding of the basic physics is still at a rudimentary stage. By using a new instrument developed and commercialised under previous EPSRC grants, we were able to show that the flow of even the simplest colloids possible - a suspension of hard spheres of the same size - show many features that either contradict textbook expectations, or are not expected at all. Thus, for instance, they show 'shear banding' - breaking into bands that flow at different rates; no existing theory predicted this behaviour. By working closely with theorists in Edinburgh, we were able to develop a model to account for our experimental observations. In the same part of the research, we were able to invent a new kind of soft solid by dispersing a high concentration of particles into liquid crystal (of the kind used commonly in displays). In a second part of the project, we initiated a new research programme into the physics of motile bacteria, considered as self-propelled colloidal particles. We invented a new, high-throughput method for measuring the distribution of swimming speeds in such bacterial suspensions. We were also able to coax motile bacteria to assemble spontaneously into microscopic structures that spin like the rotors in motors. Simple theory can account for the relationship between the spinning speed and the number of bacteria making up the structures. This work potentially opens up a novel way to assemble functional micro- and nano structures.