In an active system, elements draw energy from their surroundings and convert it to mechanical motion. Numerous examples exist in nature; from macroscale schools of fish to microscale bacterial colonies. Recently, attention has been drawn to a synthetic analog of these biological systems comprised of custom microfabricated colloids (1-10um in size) suspended in a bulk fluid, which can be activated in a variety of ways including surface chemical reactions and the application of external fields. Due to their small size, the particles move in the Stokes regime and in accordance with the scallop theorem, require some form of symmetry breaking to attain net motion. Most commonly, this is achieved through coating an inert microsphere with a partial metallic coating; the choice of which depends on the intended propulsion mechanism. For example, active colloids driven by asymmetric chemical reactions are coated in a catalyst such as Pt while electric field driven particles will be coated in a non-reactive conductor such as Au.

In this talk, we will examine two different ways in which modelling the metallic coating can enhance our understanding of experimental measurements of active colloid mobility. We begin with looking at electric field driven systems and demonstrate that the equilibrium orientation -and by extension the velocity – of active colloids is a function of the electric field properties (frequency, voltage) and the gravitational torque arising from the “bottom heaviness” of metallic coating. Secondly, we turn to catalytic active colloids, and demonstrate that mathematical models of the relationship between the size of the coating and active colloid mobility can offer insight into the nature of the chemical reactions occurring at the surface. The results of this work can be used to reduce discrepancy between theoretical models and experiment and optimize the design of active colloidal systems.