Many living cells and organisms are active: they can move in a certain direction using a chemical fuel. The characteristics of systems that contain active objects is fundamentally different from passive materials. In dense active matter systems, for example, the out-of-equilibrium nature of the system and the self-organisation of the constituents give rise to intriguing collective dynamics: collective swirling motion, macroscopic fluid flows and giant density fluctuations. Such dynamics has been observed in schools of fish, flocks of birds, films of bacteria and myosin motors on actin filaments. However, the physical mechanisms underlying these collective effects in dense active matter systems remain poorly understood.
An important bottleneck in current efforts to unravel these physical effects and to study collective dynamics in active matter is the lack of reliable experimental model systems for dense active matter. To overcome this limitation, we developed a well-defined experimental model system of active colloids and studied this at the single-particle level. The active particles are enclosed in a thin optical cell with electrodes at the top and bottom to control the activity of the particles by means of an oscillating electric field. Analysis of the activity, orientation and interactions of every particle in these active systems will help gain more insight in the physical mechanisms that underlie collective effects in large groups of active objects, such as schools of fish and flocks of birds.
