Convection-driven mixing inside microdroplets

Droplet generators represent a fast-growing area of microfluidics, since each volume of the dispersed phase functions as an individual microreactor, therefore they offer high throughput and reduced sample consumption. By injecting the dispersed phases containing the reactants, the mixing can be tuned separately, thus a higher control over reactions inside the droplets is achievable. The numerical investigation of such multi-phase and multi-component systems proposes various challenges, since the concentrations of species also depend on the distribution of phases.

In this article, we investigate the concentration distribution of the species in microfluidic droplet generators both experimentally and theoretically. Besides the characterization of the distribution of the dispersed and continuous phases, we have also studied the effect of injection rates on mixing inside the droplets. The size of the droplets decreases on increasing the flow rate of the oil phase and increases on increasing the flow rate of water phase compared to the oil phase. The numerical analysis based on solving the Navier–Stokes equation with volume-of-fluid method including a diffusive component in the aqueous phase corroborated the experimental findings. Furthermore, by determining the contribution of various transport processes to mixing of reactants inside the dispersed phases, we have shown that convection is dominant inside the droplets for all parameters used.