Interaction between droplets and co-flow interface in a microchannel: Droplet migration and interfacial deformation

We investigate hydrodynamic interactions between droplets and the interface of two co-flowing immiscible streams in a microchannel. We find that small droplets of confinement ratio (drop size to suspending stream width), $\ensuremath{\beta}<0.5$ migrate towards the co-flow interface while larger droplets of $0.5<\ensuremath{\beta}<1$ unexpectedly drift away from the interface. Large droplets with $\ensuremath{\beta}>1$ cause deformation waves in an initially stable and flat interface that propagate downstream akin to traveling peristaltic waves. The deformation wave amplitude associated with a droplet grows spatially downstream but remains the same for a train of droplets crossing a fixed location, indicating convective instability.