Swelling of Diffusive Fluid Threads in Microchannels

The dynamics of viscous fluid threads concurrently flowing with miscible solvents is experimentally investigated in square microchannels. Diffusive fluid threads are found to significantly swell at low flow velocities due to large specific interfacial area and hydrodynamic lubrication. An approach based on bounded function analysis of confined thread diameter is developed to model diffusive behavior of viscosity-differing fluids at the small scale. This work shows the determination of a critical flow rate associated with each fluid pair and the use of dynamic similarity to calculate diffusion coefficients between oils and organic solvents. The thread divergence is estimated based on the growth of diffusion layers and related to diffusion-induced buckling instabilities of viscous threads in parallel flows.