The impact of an oil droplet on an oil layer on water

We present a study of droplet impingement on a two-layer liquid, specifically an oil droplet impinging on a layer of oil on water. In our experiments, the diameter and impact velocity of the droplet and the thickness of the oil layer were varied, and the maximum depth of the crater and the maximum height of the Worthington jet were measured. When the thickness of the oil layer was less than times the droplet diameter, the depth of the crater depended on the thickness of the oil layer. Otherwise, the two-layer liquid behaved like a single layer. This observation is rationalized by considering the oil-water interface, whose deformation is negligible when the oil layer is thick but becomes significant when the oil layer is thinner. We define an effective Weber number for the two-layer liquid and show that the height of the Worthington jet is proportional to this effective Weber number.