Diffusion of micrometer-sized particles in confined spaces

Quantifying the diffusion of particles in small confined spaces is important in fields like microfluidics. When confinement approaches the size of the particle, wall-enhanced drag coefficients and electrostatic interactions with the confining walls have a significant impact on particle diffusion. Theoretical research and simulations have evolved our understanding of these systems over the last two decades. However, experimental results are needed to verify and clarify that work. In this talk, we present results from an experimental system that measures the parallel and perpendicular diffusion of a micrometer sphere confined between two parallel walls. The confinement is gradually varied from ~1 µm to ~25 µm and measured in-situ using optical interference. Using a high-speed camera and particle tracking techniques, the diffusion of a micrometer-sized particle is measured. The location of the particle between the two walls is restricted by a weak optical trap, allowing for the measuring of diffusion coefficients as a function of particle position. Experimental findings are compared with recent theoretical and simulation results.