Active Colloids: Novel Mesophases and Devices
Swimmers and self-propelled particles are physical models for the collective behaviour and motility of a wide variety of living systems, such as bacteria colonies, bird flocks and fish schools. Such artificial active materials are amenable to physical models which reveal the microscopic mechanisms underlying the collective behaviour. Here we study colloids in a DC electric field. Our quasi-two-dimensional system of electrically-driven particles exhibits a rich and exotic phase behaviour. At low field strengths, electrohydrodynamic flows lead to self-organisation into crystallites with hexagonal order. Upon self-propulsion of the particles due to Quincke rotation, we find an ordered phase of active matter in which the motile crystallites constantly change shape and collide with one another. At higher field strengths, this "dissolves" to an active gas. We parameterise a particulate simulation model which reproduces the experimentally observed phases and, at higher field strengths predicts an activity-driven demixing to band-like structures.
Active matter also provides a route towards novel devices. We have already demonstrated torque control at the colloidal scale using optical tweezers to drive a colloidal "gearwheel". Our experiments with the Quincke rollers suggests that this circular motion can be accelerated by some 10,000 times. Harnessing active matter has huge untapped potential, and that the electric field-driven Quincke rollers are an ideal model system for prototyping active machines at small lengthscales.