Janus particle-based microprobes: Determination of object orientation

Employing information extracted from their translational dynamics, colloidal particles have served extensively as micro-sized probes in in situ analytical techniques such as particle image velocimetry and microrheology. Janus particles, partially shadowed by metal caps appear directional when imaged with an optical microscope, and hence detectable rotational dynamics are introduced into an otherwise spherically symmetric system. Current tracking methods require high-resolution imaging, are time consuming, or focus on bulk light intensity variations. Here, we introduce two image analysis methods that identify the actual body center of gold-capped Janus particles (AuJPs), as well as distinguish the particle orientation within the space. Method I tracks the optical centroid, determines the particle's in-plane tilting angle, and locates its actual body center (i.e., the Optical Centroid-Tilting Angle-Geometry, OCTAG procedure). Method: II tracks the diffraction ring center and the moon-phase center revealing in-plane orientation (i.e., the Ring-Phase Dual Optical Centroids, RPDOC procedure). With both methods we are able to successfully identify actual body centers and orientations of AuJPs with a resolution of 0.5 pixels and an angular uncertainty of sub-10. It is thus possible to probe the local medium response on sub-micrometer length scales. The data analysis is applied to track dilute suspensions of AuJPs undergoing Brownian motion in a viscous glycerol-water mixture of volume ratio 1 or 2. Mean-square displacement (MSD) and angular mean-square displacement (AMSD) extracted from the experimental measurements are related to medium viscosities according to the Stokes Einstein equation and the Perrin rotational diffusion, respectively. The viscosities calculated based on the diffusion coefficients determined from analysis of the translational and rotational dynamics of the microprobes differ by less than 6%. The 6% deviation observed is attributed to wall and gravitational effects. Results indicate that Janus particles can serve as novel probes in microrheology, and their asymmetric surface properties may potentially be used to investigate particle-medium interactions. (C) 2016 Elsevier B.V. All rights reserved.