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

We have developed a microfluidic tool to measure the diffusion coefficient D of solutes in an aqueous solution by following the temporal relaxation of an initially steep concentration gradient in a microchannel. Our chip exploits multilayer soft lithography and the opening of a pneumatic microvalve to trigger the interdiffusion of pure water and the solution initially separated in the channel by the valve, the so-called free interface diffusion technique. Another microvalve at a distance from the diffusion zone closes the channel and thus suppresses convection. Using this chip, we have measured diffusion coefficients of solutes in water with a broad size range, from small molecules to polymers and colloids, with values in the range D is an element of [10(-13)-10(-9)] m(2)/s. The same experiments but with added colloidal tracers also revealed diffusio-phoresis and diffusio-osmosis phenomena due to the presence of the solute concentration gradient. We nevertheless show that these interfacial-driven transport phenomena do not affect the measurements of the solute diffusion coefficients in the explored concentration range. Published under an exclusive license by AIP Publishing.

Automated summary

We have developed a microfluidic tool to measure the diffusion coefficient of solutes in an aqueous solution. By triggering the interdiffusion between pure water and the initially separated solution, our chip can track the temporal relaxation of the concentration gradient to measure the diffusion coefficient. Our experiments show that the interfacial-driven transport phenomena do not affect the measurement results.