  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.
