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Effect of colloidal weight at different planar interfaces

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.