Effect of colloidal weight at different planar interfaces

TL;DR

This study investigates how colloidal weight influences particle dynamics near various interfaces, revealing different behaviors at water-glass, water-air, and polymer solution-air interfaces, supported by experiments and simulations.

Contribution

It provides new experimental insights into colloidal motion near interfaces under varying weights and introduces a Brownian dynamics model to explain the observed behaviors.

Findings

Diffusion decreases with increasing weight at water-glass interfaces.

Negligible weight effect at pure water-air interfaces.

In polymer solutions, particle diffusivity behaves similarly to solid interfaces.

Abstract

In many physical and biological systems, particles and microorganisms move in the proximity of an interface. Understanding the dynamics of a particle suspended close to an interface is not only important conceptually but is crucial for practical applications ranging from the treatment of waste waters to industrial applications of self-assemblies. In this work, we experimentally investigate the effects of colloidal weight on its dynamics while moving in the close proximity of a variety of liquid-solid and liquid-air interfaces. Using an upward magnetic force, we change the effective weight of a superparamagnetic colloid. At water-glass interfaces, we observe the expected decrease of the diffusion coefficients with increasing effective weight. At liquid-air interfaces, while for a pure water-air interface there is a negligible dependency between the diffusivity and the effective weight, we find that in semi-dilute polymer solution at the solution-air interface the diffusivity of the particle shows similar behavior as those at liquid-solid interfaces. We implement a Brownian dynamics simulation to support our results and show how the interplay between hydrodynamic interactions and electrostatic interface repulsion explains the experimental results.