Compound Bubble-Drop Trains In Microfluidics

We describe the formation and flow of monodisperse compound bubble-drop trains in a simple microfluidic channel geometry. Gas and aqueous liquid are delivered in co-flow at a T-junction, where a cross flowing oil alternately pinches off bubbles and drops. The bubbles and drops move downstream at different speeds (dictated by fluid properties), eventually coming into conformal contact and forming well defined compound bubble-drops trains. Increasing the liquid capillary number by reducing oil-water interfacial tension or by increasing oil viscosity changes the shape of the interface between the fluids in the compound bubble-drop, and eventually alters the relative speeds enough to change the order of the fluids in the bubble-drop. Such trains of compound fluid objects may have several potential applications, from studying the dynamics of three phase foams to devices for combinatorial chemistry with gas-liquid chemical reactions.