Mixing in stratified fluids induced by bubble swarms

The mixing dynamics induced by bubble swarms rising across a sharp stably-stratified density interface are studied experimentally. In the middle of a column, an interface separates two Newtonian-miscible-liquids: fresh water and brine. The bubble swarm is injected from the bottom with a bank of small capillaries. When the bubbles cross the interface, they drag denser fluid into the upper lighter fluid and then some denser fluid returns to the lower layer (Diaz-Damacillo et al., 2015). This process induces mixing. We record the bubble with a high speed camera and track the temporal evolution of the fluids conductivity with a \textit{Conduino} (Carminati and Luzzatto-Fegiz, 2017). We obtain the mixing coefficient , ${D_b}$, by fitting the concentration profiles to a simplified advection-diffusion equation, $\partial c / \partial t = D_b \nabla^2 c$. Experiments are conducted for a range of gas volume fractions and density contrasts between the fluids. Finally, we discuss the implications of our results for lake and ocean mixing.