Determination of particle diffusion in a confined fluid layer by dynamic light scattering

This work aims to develop a fundamental understanding of the influence of geometric confinement on particle diffusion by its experimental characterization in a fluid layer confined under well-defined conditions using dynamic light scattering (DLS). The confinement was realized using a new wedge-shaped sample cell which allowed for the systematic variation of the confinement size, i.e., the wall distance L and the surface characteristics of the confining walls. Applying different scattering geometries in DLS experiments, comprehensive investigations of particle diffusion considering momentum transfer of particle number density fluctuations with respect to the confining walls were performed. For this, particle diffusivities D were determined by studying confined aqueous dispersions of spherical colloidal silica particles with nominal diameters d P of (300 and 100) nm at L between (5.3 and 104) µm. A surprising slowing down of particle diffusion parallel to the walls with decreasing L was identified. In contrast, D characterizing particle diffusion perpendicular to the walls agreed with that of the dispersions in free media for all L , which demonstrates that the momentum transfer of particles perpendicular to the walls is influenced only in the near-contact regime at particle-wall distances similar to d P . Studying the influence of d P , it was demonstrated that the confinement-induced slowing-down of D is governed by the relative confinement L / d P rather than by absolute dimensions. Comparing hydrophilic and hydrophobic characteristics of the confining walls, it was demonstrated that increased hydrodynamic slip at hydrophobic surfaces results in a less pronounced slowing-down of particle diffusion parallel to the walls. • Particle diffusion under well-defined geometric confinement studied using DLS • Wedge-shaped sample cell allows systematic variation of confinement characteristics • Particle number density fluctuations with defined momentum transfer were probed • Slowing down of diffusion under confinement is related to hydrodynamic interactions • Surface characteristics of confining walls influence diffusion under confinement