Influence of cap weight on the motion of a Janus particle very near a wall

Abstract

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...

Thermotaxis of Janus particles

“…This effect is presumably mostly due to the hydrodynamic flows induced by the heterogeneous heating in the narrow fluid layers between the particle and the glass cover slides [80][81][82]. For simplicity, the following analysis assumes perfect in-plane alignment, thereby neglecting weak perturbations due to rotational Brownian motion and the weak bottom-heaviness of the Janus particle [83]. Within the free-space approximation, (2) and (3), the in-plane alignment is maintained by assuming that both the heat source and the Janus particle are perfectly centered between the cover slides.…”

Developing Scattering Morphology Resolved Total Internal Reflection Microscopy (SMR-TIRM) for Orientation Detection of Colloidal Ellipsoids

“…Moreover, this orientation information would be critical to tracking dynamics and calculating the potential energy landscape sampled by an anisotropic particle. Previous work has shown that observations of both position and orientation can be analyzed to assemble potential energy landscapes 30 , thereby providing a framework for assembling such a landscape for an anisotropic particle. One particular example in which an interaction landscape for an anisotropic particle would be useful is for the design of diagnostic tools for determining the disease state of a red blood cell, which is anisotropic, as it interacts with a boundary.…”

A Practical Guide to Analyzing and Reporting the Movement of Nanoscale Swimmers

“…the electrohydrodynamic effect, or EHD 124,125 ) level, and also cause a nanoswimmer to float slightly above the substrate. 126,127 Experimentalists should be aware of these wall effects and measure the surface zeta potential, wettability, and roughness of the substrate used, among other relevant properties. The most common substrate used is a glass slide (or cover slip), the surface properties of which depend on the sample history and adsorption of molecules in the laboratory environment.…”