Séminaire de Joshua McGraw: Pre-asymptotic and nanoscale Taylor dispersion

Transport of Brownian particles in shear flows is influenced by random thermal motions and advection along the flow. This advection-diffusion coupling can indeed enhance particle and solute dispersion by orders of magnitude as compared to pure diffusion, with a steady state reached for bounded flow regions such as a nanopore or blood vessel. In this presentation we show how, evanescent wave microscopy can be used to study these so-called Taylor dispersion phenomena at nanoscales.
In the first part [1], we measure the pre-asymptotic dynamics of Taylor dispersion, highlighting the crucial role of the initial concentration profile. We make time-dependent, nanometrically resolved particle dispersion measurements permitting a measure of the full dynamical approach and crossover to the steady state.
In a second part, we reveal how the finiteness and nanoscale of the particles and the observation window can lead to reductions in the expected dispersion as compared to purely point-like particles and passively reflecting walls. Concerning the latter, we particularly demonstrate the influence of a charged, planar wall on one side of the microchannel, and a particle-consuming boundary at the other. All of our observations are captured by theoretical modelling, and our results will impact the performance of any device implicating nanoscale transport of diffusive objects.

[1] Vilquin, Bertin et al., Physical Review Fluids (2021)