  Advection-diffusion coupling can enhance particle and solute dispersion by
orders of magnitude as compared to pure diffusion, with a steady state being
reached for confined flow regions such as a nanopore or blood vessel. Here, by
using evanescent wave microscopy, we measure for the first time the full
dynamics of Taylor dispersion, highlighting the crucial role of the initial
concentration profile. We make time-dependent, nanometrically-resolved particle
dispersion measurements varying nanoparticle size, velocity gradient, and
viscosity in sub-micrometric near-surface flows. Such resolution permits a
measure of the full dynamical approach and crossover into the steady state,
revealing a family of master curves. Remarkably, our results show that the
dynamics depend sensitively on the initial spatial distribution of the
nanoparticles. These observations are in quantitative agreement with existing
analytical models and numerical simulations performed herein. We anticipate
that our study will be a first step toward observing and modelling more complex
situations at the nanoscale, such as target finding and chemical reactions in
nanoconfined flows, dynamical adsorption and capture problems, as well as
nanoscale drug delivery systems.
