Identifying the Orientation of Active Janus Colloids in Dilute Limit

An active Janus colloid consists of a passive face and an active face that can transfer surrounding energy into its motion. The structure and dynamics of the system are deeply related to the position and orientation of each colloid. Despite the extensive studies based on the positional information of individual colloids, the effect of the orientational information is less explored experimentally due to the challenge in orientation identification at the single particle level. Here, a new method of identifying the orientation of Janus colloids is proposed, which combines the traditional Hough circle transform and a novel refining technique based on the axial correlation function. The center position of the whole Janus colloid and the active part were identified, followed by the refinement of the later one. The orientation of each Janus colloid in a dilute solution was overlaid onto the original image and the clear improvement in the orientational identification was demonstrated.