Effect of Catalyst Distribution on Spherical Bubble Swimmer Trajectories

Spherical colloids decorated with a surface coating of catalytically active material are capable of producing autonomous motion in fluids by decomposing dissolved fuel molecules to generate a gaseous product, resulting in momentum generation by bubble growth and release. Such colloids are attractive as they are relatively simple to manufacture compared to more complex tubular devices and have the potential to be used for applications such as environmental remediation. However, despite this interest, little effort has been devoted to understanding the link between the catalyst distribution at the colloid surface and the resulting propulsive trajectories. Here we address this by producing colloids with well-defined distributions of catalytic activity, which can produce motion without the requirement for the addition of surfactant, and measure and analyze the resulting trajectories. By applying analysis including fractal dimension and persistence length calculations, we show that spatially confining catalytic activity to one side of the colloid results in a significant increase in directionality, which could be beneficial for transport applications. Using a simple stochastic model for bubble propulsion we can reproduce the features of the experimental data and gain insight into the way in which localizing catalytic activity can reduce trajectory randomization. However, despite this route to achieve trajectory control, our analysis makes it clear that bubble-driven swimmers are subject to very rapid randomization of direction compared to phoretic catalytic swimming devices with equivalent geometries.