In terms of length scale, bacteria belong to the colloidal domain. On the other hand, they are alive, and do things that (until recently) synthetic colloids do not do at all - they consume free energy from their environments to engage in growth and division, as well as self-propelled motion (swimming). (Self-propelled synthetic colloids are now available.) Bacteria are therefore active colloids - suspensions of particles in which the individual particles are intrinsically out of thermal equilibrium. In this lecture, I will review a number of recent experiments studying bacteria as active colloids, and show that growth as well as motility lead to new kinds of behaviour. In the first half of the lecture, I will introduce a growing colony of Escherichia coli as an unstable active liquid crystal of rods, and show that concepts from liquid crystal physics can be used to make sense of some aspects of experimental observations. In the second half of the lecture, I will review the behaviour of swimming bacteria, focussing on how flagellated E. coli swim in complex fluids (polymer solutions and gels), and the behaviour of swimming cells confined inside water-in-oil emulsion droplets. Time-permitting, I will also compare the behaviour of swimming E. coli and synthetic swimmers (Janus colloids) in an 'obstacle course' in the form of a crystalline array of large, immobile colloidal particles.
w.poon@ed.ac.uk
