Quantifying and understanding the tilt of a Pt Janus active colloid near solid walls

Active colloids powered by self-generated gradients are influenced by nearby solid boundaries, leading to their reorientation. In this study, the tilt angles (the angle between where an active colloid moves and where it faces) were measured to be 13.3° and -33.9° for 5 μm polystyrene microspheres half coated with 10 nm Pt caps moving in 5% H₂O₂ along the bottom and top glass wall, respectively, indicating that the colloids moved with their PS (forward) caps tilted slightly toward the wall. The speeds and tilt angles of Pt Janus colloids increased consistently with increasing H₂O₂ concentration (0.5 to 10 v/v%) and Pt cap thickness (5 to 50 nm). We propose that the tilt results from a balance between gravitational torque (caused by the Pt cap's weight) and chemical activity-induced torque (from self-generated chemical gradients), qualitatively supported by finite element simulations based on self-electrophoresis. Our findings are useful for understanding how chemically active colloids move in, and interact with, their environment.