A Method to Measure the Diffusion Coefficient in Liquids

Molecular diffusion in liquids is a key process in numerous systems: it is often the reaction rate limiting factor in biological or chemical reaction. Molecular diffusion has been recognized as the ultimate mechanism by which substances concentration get homogenized and, thus, their mixing and dilution occur. Here, we propose a novel method to directly measure the diffusion coefficient D of solutes or suspensions in liquids. Differently from current methods, as Dynamic Light Scattering or Fluorescent Correlation Spectroscopy, our method does not rely on previous knowledge on the fluid or tracer properties, but it is based on directly measuring the concentration spatial profile of a considered tracer with optical techniques within a diffusion chamber. We test this novel method on a sample of mono-dispersed suspension of spherical colloids for which an estimate for D can be made based on Einstein-Stokes relation. We, then, use this technique to measure the diffusion coefficient of a non-spherical tracer. We further quantify mixing of the considered tracers in the confined domain of the diffusion chamber: we show that, since diffusion-limited mixing (quantified in terms of the dilution index) in a confined space happens faster than un-confined domain, the finite size of the diffusion chamber must be taken into account to properly estimate D and the tracer mixing degree.