In-flow measurement of molecular diffusion coefficients using differential dynamic microscopy

Understanding diffusive mass transport in fluid flows is fundamental in several fields, ranging from virus particles in chromatographic test kits to nutrients in ocean currents. However, experimentally resolving diffusion processes in flowing fluids is challenging due to local sample deformation caused by advective displacements under varying shear rates. Here we demonstrate that differential dynamic microscopy, combined with image velocimetry, provides a robust method to quantitatively measure molecular diffusion coefficients across a laminar flow profile within a rectangular microfluidic channel. By using subresolution tracer particles homogeneously distributed along the flow profile, we capture the effects of differing shear rates across the flow profile and measure the local diffusion coefficient by scanning through different focal planes with a microscope. While demonstrated with water, this approach can be easily adapted to more complex fluids with shear-dependent viscosity, offering broad applicability for studying diffusion in dynamic environments.