Engulfment of a drop on solids coated by thin and thick fluid films

When an aqueous drop contacts an immiscible oil film, it displays complex interfacial dynamics. When the spreading factor is positive, upon contact, the oil spreads onto the drop’s liquid–air interface, first forming a liquid bridge whose curvature drives an apparent drop spreading motion and later engulfs the drop. We study this flow using both three-phase lattice Boltzmann simulations based on the conservative phase field model, and experiments. Inertially and viscously limited dynamics are explored using the Ohnesorge number Oh and the ratio between the film height H and the initial drop radius R. Both regimes show that the radial growth of the liquid bridge r is fairly insensitive to the film height H,and scales with time T as r∼T^1/2 for Oh≪1,and as r∼T^2/5 for Oh ≫ 1. For Oh ≫ 1, we show experimentally that this immiscible liquid bridge growth is analogous with the miscible drop–film coalescence case. Contrary to the growth of the liquid bridge, however, we find that the late-time engulfment dynamics and final interface profiles are significantly affected by the ratio H/R.