Janus particles have anisotropy in surface chemistry or composition that will effect dynamics and interactions with neighboring surfaces. One specific type of Janus particle is that consisting of a native micrometer-scale particle with a cap of gold, platinum, or another metal deposited with a typical thicknesses of ∼10 nm. A key characteristic of metal-capped Janus particles prepared with glancing angle deposition is the cap thickness. The nominal thickness is usually assumed to be uniform across the cap for modeling or interpretation of data, but the vapor deposition fabrication process likely does not produce such a cap because of the particle's curvature. These nonuniformities in the cap thickness may have a profound impact on Janus particle dynamics at equilibrium and in response to external fields. Herein, we summarize an experimental technique that utilizes focused ion beam slicing, image analysis, and results for the direct and local measure of cap thickness for 5 μm polystyrene spheres with a gold cap of nominal thicknesses of 10 or 20 nm. We found the cap varied in thickness continuously along the perimeter of the particle and also that the deposition rate, varying between 0.5 and 2.0 Å/s, did not significantly alter the way in which the thickness varied. These data support the hypothesis that cap thickness of a Janus sphere will vary across the gold surface contour, while demonstrating a feasible route for direct measurement of Janus particle cap thickness.

This article describes the simulated Brownian motion of a sphere comprising hemispheres of unequal zeta potential (i.e., "Janus" particle) very near a wall. The simulation tool was developed and used to assist in the methodology development for applying Total Internal Reflection Microscopy (TIRM) to anisotropic particles. Simulations of the trajectory of a Janus sphere with cap density matching that of the base particle very near a boundary were used to construct 3D potential energy landscapes that were subsequently used to infer particle and solution properties, as would be done in a TIRM measurement. Results showed that the potential energy landscape of a Janus sphere has a transition region at the location of the boundary between the two Janus halves, which depended on the relative zeta potential magnitude. The potential energy landscape was fit to accurately obtain the zeta potential of each hemisphere, particle size, minimum potential energy position and electrolyte concentration, or Debye length. We also determined the appropriate orientation bin size and regimes over which the potential energy landscape should be fit to obtain system properties. Our simulations showed that an experiment may require more than 10 observations to obtain a suitable potential energy landscape as a consequence of the multivariable nature of observations for an anisotropic particle. These results illustrate important considerations for conducting TIRM for anisotropic particles.

The dynamics of anisotropic nano-to microscale 'colloidal' particles in confined environments, either near neighboring particles or boundaries, is relevant to a wide range of applications. We utilized Brownian dynamics simulations to predict the translational and rotational fluctuations of a Janus sphere with a cap of non-matching density. The presence of the cap significantly impacted the rotational dynamics of the particle as a consequence of gravitational torque at experimentally relevant conditions. Gravitational torque dominated stochastic torque for a particle > 1 m in diameter and with a 20 nm thick gold cap. Janus particles at these conditions sampled mostly cap-down or 'quenched' orientations. Although the results summarized herein showed that particles of smaller diameter (< 1 m) with a thin gold coating (< 5 nm) behave similar to an isotropic particle, small increases in either particle diameter or coating thickness drastically quenched the polar rotation of the particle. Histogram landscapes of the separation distance from the boundary and orientation observations of particles with larger diameters or thicker gold coatings were mostly populated with quenched configurations. Finally, the histogram landscapes were inverted to obtain the potential energy landscapes, providing a path for experimental data to be interpreted. 3 I. INTRODUCTION. Colloidal particles dispersed in a liquid interact via surface forces that play a critical role in dictating the properties and performance of complex fluid. Over the past decade, the dynamics and interactions of anisotropic colloidal particles have gained attention [1] because of potential applications in various fields such as optical displays [2], magnetorheological system [3], controlling interfacial microstructure [4], self-assembly [5,6], microfluidic devices [7], tuning interparticle interactions [8,9], and biomaterials or drug delivery [10].Supporting these efforts have been a variety of new techniques for the synthesis of anisotropic colloidal particles [5,[10][11][12][13][14][15][16][17][18][19][20][21]. Newly developed fabrication techniques provided the ability to tune the shape and the surface properties of these materials. Janus particles are one class of anisotropic colloid, typically with some property difference in the hemispherical domain. Each hemispherical domain of a Janus particle may have its own surface chemistry, shape, or other properties [22].Predicting the dynamics of anisotropic colloids is important for applications in real systems, for example during processing when complex fluids are often not at equilibrium [23,24].Various parameters influence the dynamics of anisotropic colloids [25][26][27][28]. Particle confinement will strongly impact the hydrodynamic interactions between the colloid and boundary, thereby strongly influencing particle mobility. Brownian motion and conservative (i.e. path independent) forces, such as electrostatic double layer repulsion and gravity, will also impact the dynamics of a confined spherical Janus part...

Micrometer scale colloidal particles experiencing ∼kT scale interactions and suspended in a fluid are relevant to a broad spectrum of applications. Often, colloidal particles are anisotropic, either by design or by nature. Yet, there are few techniques by which ∼kT scale interactions of anisotropic particles can be measured. Herein, we present the initial development of scattering morphology resolved total internal reflection microscopy (SMR-TIRM). The hypothesis of this work is that the morphology of light scattered by an anisotropic particle from an evanescent wave is a sensitive function of particle orientation. This hypothesis was tested with experiments and simulations mapping the scattered light from colloidal ellipsoids at systemically varied orientations. Scattering morphologies were first fitted with a two-dimensional (2D) Gaussian surface. The fitted morphology was parameterized by the morphology’s orientation angle M ϕ and aspect ratio M AR. Data from both experiments and simulations show M ϕ to be a function of the particle azimuthal angle, while M AR was a sensitive function of the polar angle. This analysis shows that both azimuthal and polar angles of a colloidal ellipsoid could be resolved from scattering morphology as well or better than using bright-field microscopy. The integrated scattering intensity, which will be used for determining the separation distance, was also found to be a sensitive function of particle orientation. A procedure for interpreting these confounding effects was developed that in principle would uniquely determine the separation distance, the azimuthal angle, and the polar angle. Tracking these three quantities is necessary for calculating the potential energy landscape sampled by a colloidal ellipsoid.

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