Investigating Diffusing on Highly Curved Water–Oil Interface Using Three-Dimensional Single Particle Tracking

Abstract

Diffusion on highly curved surfaces is important to many industrial and biological processes. Despite the progress made in theoretical studies, how diffusion is affected by the curvature is unclear due to experimental challenges. Here, we measured the trajectories of polystyrene nanoparticles diffusing on highly curved water-silicone oil interface, where the oil droplet diameter ranges from several μm to as small as ∼400 nm. To analyze the diffusion coefficients on curved surface, an analytical solution developed by Castro-Villarreal containing an infinite series can be used. Through Monte Carlo simulations, we simplified the Castro-Villarreal equation and defined the conditions that satisfy corresponding approximations. For the experiments, unexpectedly, we found that the diffusion slows down significantly when the oil droplet becomes smaller. Possible reasons were discussed, and a diffusion-induced droplet deformation and interface fluctuation model is consistent with the experimental results. This study reveals an unexpected decrease of particle diffusion on small oil droplet surface and sheds new light on understanding diffusion on highly curved interface.

Brownian Diffusion of Individual Janus Nanoparticles at Water/Oil Interfaces

“…For example, it was shown that such particles took untenably long to reach equilibrium after adsorption to a liquid interface. 14 , 15 Furthermore, the translational and rotational diffusions of adsorbed particles were often found to be slower than the predictions of the Stokes–Einstein relation and hydrodynamic theories, 16 − 25 whereas the underlying mechanisms were not captured by the simulations. 26 , 27 Some reasons for the slow diffusion of micron-sized particles have been suggested such as the ubiquitous surface “defects” that induce multiple metastable pinning of the three-phase contact line, 28 − 30 producing an extra reverse random force to slow diffusion.…”

Ultrasensitive Electrochemistry by Radical Annihilation Amplification in a Solid–Liquid Microgap

“…Visualizing the contact dynamics of micro-and nanodroplets is difficult. 39 Previously, our group has developed methods using electrogenerated chemiluminescence to visualize phase boundaries for large (10 s of μm) droplets. 28 The wetting of micro-and nanodroplets onto various hydrophilic and hydrophobic substrates will be the topic of future investigation.…”

Colloidal Soft Matter Physics

“…The aforementioned ideas and proposals to elucidate the particle transport in non-Euclidean spaces have been partially and recently corroborated in experiments of polystyrene nanoparticles diffusing on highly curved water-silicone oil interfaces [128]. However, further experimental and computational studies are needed in order to better understand the rich diffusion mechanisms that emerge in colloidal dispersions when the curvature plays an important role.…”