Film drainage between a moving droplet and liquid layer

In a pipe flow with crude oil and water, a dense packed layer of water droplets in oil can form at the interface which can strongly impact pressure losses, especially when oil viscosity increases. The water in the dense packed layer reduces the amount of water in the free water layer and therefore the ratio between oil-wetted and water-wetted perimeter (hence wall friction). It is experimentally observed that a flowing dense packed layer is more stable than a static one. A dense packed layer is rather complex, so we simplify the problem by looking at a single droplet in contact with a liquid layer. Experiments have showed that the coalescence between a moving droplet and a liquid layer is delayed with respect to a static droplet and liquid layer. The purpose of this work is to explain this behavior by DNS of the aforementioned multiphase configuration. Two effects are in play: The first one is a flow-induced lift force exerted on the droplet in the direction away from the liquid layer. The second one is a lubrication force, induced by the motion of the droplet itself, pushing the droplet away from the liquid layer (Reynolds' lubrication theory), similarly to a slider bearing. An attempt to quantify and understand the physical mechanism with the strongest impact is then made.