Structure and dynamics of a layer of sedimented microspheres near a horizontal planar wall*

Structure and dynamics of a sedimented layer of silica microspheres is investigated using computer simulations and confocal-microscopy measurements. The system is characterized by the particle area fraction $\phi_s$ and the dimensionless sedimentation parameter $l_0=k_BT/(mgd)$, where $k_BT$ is the thermal energy, $m$ is the buoyancy-corrected particle mass, $g$ is the gravitational acceleration, and $d$ is the particle diameter. The range $0<\phi_s<0.62$ and $l_0\approx 1.6$ is explored in our experiments. The near-wall particle distribution exhibits a layered structure, with the second layer developing at $\phi_s\approx0.4$. Particle distribution is well described by a phenomenological model that involves equilibration of a quasi-two dimensional chemical potential. The effective self-diffusivity of the first and second particle layer has been determined. We find that the suspension microstructure is significantly affected by particle polydispersity, whereas the self-diffusivity is only moderately affected.