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

TL;DR

This study uses Brownian dynamics simulations to explore how the weight of a cap on Janus particles influences their rotational and translational behavior near walls, highlighting the dominance of gravitational torque in larger particles.

Contribution

It demonstrates how cap weight affects Janus particle orientation and motion near boundaries, providing insights into gravitational effects on anisotropic colloids.

Findings

Gravitational torque dominates for particles > 1 μm with thick gold caps.

Smaller particles or thinner coatings behave similarly to isotropic particles.

Increased particle size or coating thickness quenches polar rotation.

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 micrometer 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 micrometer) 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.