Dynamics of Janus Micromotors in Radial Fuel Gradients

Precise control of self-propelled micro/nano motors holds the key to future innovations in targeted exploration and environmental remediation. Yet, at the heart of their design lies a fundamental challenge—understanding how these tiny explorers sense and navigate the unseen landscapes of their environments. Our recent work delves into this field, unraveling and quantifying the motion of self-phoretic Janus particles as they respond to a radial fuel gradient. Radial gradients are particularly relevant in numerous active matter systems, often emerging due to the presence of localized fuel sources or sinks. We derive an analytical expression for the particle's velocity and explore how the radial fuel gradient influences its trajectory. In contrast to the simpler case of linear gradients, our findings reveal a rich spectrum of dynamical behaviors, ranging from spiraling towards the source/sink to moving away towards infinity. Interestingly, our findings reveal the emerging of fixed points including trapping in the stationary states and a family of closed orbits around the source/sink. We identify the ratio of the phoretic mobilities between the two sides of the Janus particle and source/sink strength as key parameters in tuning their dynamics. Our results provide a path toward finely tuning the dynamics of Janus micromotors, enhancing their utility in diverse applications such as surface cleaning, microscale mixing, and water purification. In addition, this work suggests a method for quantifying the surface properties of self-phoretic Janus particles, addressing a longstanding experimental challenge in the field.