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We have developed a microfluidic tool to measure the diffusion coefficient$D$ of solutes in an aqueous solution, by following the temporal relaxation ofan initially steep concentration gradient in a microchannel. Our chip exploitsmultilayer soft lithography and the opening of a pneumatic microvalve totrigger the interdiffusion of pure water and the solution initially separatedin the channel by the valve, the so-called free interface diffusion technique.Another microvalve at a distance from the diffusion zone closes the channel andthus suppresses convection. Using this chip, we have measured diffusioncoefficients of solutes in water with a broad size range, from small moleculesto polymers and colloids, with values in the range $D \in [10^{-13}-10^{-9}]$~m$^2$/s. The same experiments but with added colloidal tracers alsorevealed diffusio-phoresis and diffusio-osmosis phenomena due to the presenceof the solute concentration gradient. We nevertheless show that theseinterfacial-driven transport phenomena do not affect the measurements of thesolute diffusion coefficients in the explored concentration range.

Microfluidic free interface diffusion: measurement of diffusion coefficients and evidence of interfacial-driven transport phenomena