Navigating Gravity: Competing Effects Result in Opposing Taxis for Different Janus Swimmers

Microorganisms often employ tactic behaviors as a survival strategy, enabling them to move in response to different stimuli. One fascinating example is the use of gravity for their orientation, where gravity provides a directional cue for swimming. Separately, when microswimmers move near solid surfaces, hydrodynamic interactions with the wall can alter their trajectories and reorientation dynamics. This study investigates the combined effects of gravitational torque and wall-induced hydrodynamics on the orientation and movement of Janus colloids coated with either platinum or copper, swimming toward their inert side (Pt) or the catalytic half (Cu), respectively. Both particle types exhibit alignment with the gravitational direction on inclined substrates due to bottom-heaviness; however, Cu@SiO₂ particles, with lighter metal caps, show a significantly narrower orientation distribution compared to Pt@SiO₂ particles. This enhanced alignment is attributed to the distinct hydrodynamic coupling of catalytic-forward type swimmers near boundaries. Using these differences, a simple yet effective method for separating active particles based on their gravitactic behavior is presented. Furthermore, we demonstrate that active and passive particles can develop a lateral distribution analogous to that of a Galton board, highlighting potential applications in educational tools and microfluidic-based computational systems.