Buoyancy-driven co-flow of miscible solutions in a microchannel

Buoyancy-driven flows are ubiquitous and have been studied extensively in traditional lock-exchange configurations. Within the past decade, with the rise of lab-on-a-chip devices, buoyancy-driven co-flows of miscible solutions, with similar viscosities, have been studied. Here, we experimentally study the buoyancy-driven co-flow of two miscible liquids, with a viscosity contrast, in a flow-focusing microchannel. Poly(ethylene glycol) (PEG) solution is introduced as the inner stream, and water is flowed as the outer stream. By performing confocal microscopy at different positions in the microchannel, we observe the evolution of the moving fronts of the two fluids on the walls of the device. Water, the less dense fluid, spreads across the top wall of the channel as the fluid moves downstream in the channel, while the denser fluid, the PEG solution, spreads across the bottom wall of the device. We quantify the motion of the moving front of both fluids, by tuning the flowrate of each phase and the density difference between the two fluids (which results in changes in the viscosity ratio between the two fluids). We further quantify the normalized width of the moving fronts, using an appropriate two-phase flow description, based on the ratio of the convective to the buoyancy time scales within this system. We observe the raw data from the experiments collapse onto a line. We anticipate that buoyancy-driven co-flows within microchannels may have important implications for design and operation of lab-on-a-chip devices.