Drop Rest Phenomena on Moving Liquid/Liquid Interface*

When a drop approaches the surface of its homophase through another immiscible liquid, it may rest on the interface to allow the trapped film of the other phase to drain out. In this work, the rest phenomena of drops on moving liquid/liquid interfaces prior to coalescence were studied. The experiments were carried out in a flow channel (5 cm × 5 cm square section and 1 m length) with a layer of aqueous phase at the bottom and an organic phase on the top. The channel had two openings at the inlet and the outlet with 5 mm height to enable the flow of the aqueous phase at the channel bottom. To vary the properties of the fluids, glycerol solutions at different concentrations (0%, 25%, 45%) were used as the aqueous phase while 5 cSt silicone oil and Exxsol D80 oil were used as the organic phase. The rest times were found to increase significantly with increasing interface speed; this increase was more prominent with the less viscous oil. The velocity fields near the interface and the structure of the thin film between the drop and the interface were studied with high-speed imaging to better understand the mechanism of film drainage.