Understanding Universal Reorientation Behavior of Active Janus Colloids.

The dynamics of Janus particles near a wall, including velocity, MSD, propulsion mechanisms, and direction, are an ongoing subject of research. However, the orientation of these particles while swimming near a solid boundary remains largely unexplored. In particular, a systematic analysis of the factors influencing their behavior across a broad range of materials and fuels is missing. Here, we present and explore the intriguing observation that the cap orientation is constant across a wide range of material-fuel combinations. Our findings showed a stable orientation at an angle (θ) of around 90°, indicated by the vector from the catalytic "pole" to the inert pole, with respect to the wall normal. To understand this behavior, we apply a continuum model of neutral self-diffusiophoresis near an osmotic-responsive planar wall. This model predicts sliding states θ ≳ 90° (≲90°) for inert-forward (cap-forward) particles. The experimentally confirmed angles of around 90° correspond to these sliding states and persist even as the active materials of the Janus particles and the fuels, and thereby the respective b values in the model, are changed. Moreover, an analytical model reveals a critical role for the hydrodynamic force quadrupole in biasing particle tilt toward the wall. An approximate angle of ≲90° is also observed for "cap-forward" particles under certain conditions on the osmotic responsiveness of the wall.