Rotational Analysis of Spherical, Optically Anisotropic Janus Particles by Dynamic Microscopy

Abstract

We analyze the rotational dynamics of spherical colloidal Janus particles made from silica (SiO2) with a hemispherical gold/palladium (Au/Pd) cap. Since the refractive index difference between the surrounding fluid and a two-faced, optically anisotropic Janus microsphere is a function of the particle's orientation, it is possible to observe its rotational dynamics with bright-field optical microscopy. We investigate rotational diffusion and constant rotation of single Janus microspheres which are partially tethered to a solid surface so they are free to rotate but show little or no translational motion. Also, since the metal cap is a powerful catalyst in the breakdown of hydrogen peroxide, H2O2, the particles can be activated chemically. In this case, we analyze the motion of coupled Janus dimers which undergo a stable rotary motion about a mutual center. The analysis of both experimental and simulation data, which are microscopy and computer-generated videos, respectively, is based upon individual particle tracking and differential dynamic microscopy (DDM). DDM, which typically requires ensemble averages to extract meaningful information for colloidal dynamics, can be effective in certain situations for systems consisting of single entities. In particular, when translational motion is suppressed, both rotational diffusion and constant rotation can be probed.

Simultaneous characterization of rotational and translational diffusion of optically anisotropic particles by optical microscopy

“…In Ref. [18] the authors have shown that traditional DDM can be used to recover the rotational diffusion coefficient of single optically anisotropic Janus particles but, owing to the peculiar nature of the sample, the study was performed without optical polarizers. This was made possible by the use of large particles (above the diffraction limit) made of two different materials, which become visible because of differential absorption.…”

Differential dynamic microscopy microrheology of soft materials: A tracking-free determination of the frequency-dependent loss and storage moduli

“…These data are treated via an image processing algorithm [22] or equivalent versions of it [23] that combines image differences and spatial Fourier transformations to obtain as a result the intermediate scattering function f (q,t) that is typically probed in DLS experiments as a function of the scattering wave vector q and time t [24]. Since its introduction, DDM has been profitably used and also extended by several groups [25][26][27][28][29][30][31][32][33][34] for a variety of applications [35]. In particular, DDM has been recently proven to be an effective tool to measure also the rotational dynamics of anisotropic colloidal particles in solution.…”

Rotational motion of dimers of Janus particles

“…For example, circular motion of L-shaped asymmetric microswimmers on the substrate of a thin film and near channel boundaries has been reported [14]. Very recently, stable rotation was observed for a dimer system of chemically active Janus particles [15].…”