Kitchen physics: water drops falling on oil layer

It is a common occurrence for water drops falling around on your kitchen tables that may be a bit oily. Will that water drop bounce off, or will that spread and stay on the oily surface?

The impact of water droplets on thin layers of immiscible viscous liquids, such as oil films, is commonly encountered across contexts ranging from kitchen activities to industrial processes. In this study, we experimentally investigate the short-term and long-term behavior of water drops spreading on silicone-oil-coated surfaces. In the short term, i.e., a few milliseconds after the drop comes in contact with the oil layer, the drop edge exhibits stick-slip dynamics, characterized by intermittent stops during the spreading. The stick-slip behavior diminishes with increasing spreading energy from impacts. The stick-slip spreading is also absent when the drop (ethanol) density is lower than that of the oil. It implies that ethanol moves over the oil layer without penetrating into the layer.

During this smooth spreading and the early time of the stick-slip spreading, the base radius vs time (t) follows the well-known scaling corresponding to the inertial-capillary regime. Contrary to the expected rupturing of the oil layer upon the water drop impact, we note that the initial spreading is mostly peripheral without noticeably displacing the oil layer.

In the long-term dynamics, regardless of whether spreading occurs upon soft contact or impact, the water drop eventually spreads onto the substrate by dewetting the oil layer. It means that the water drop pushes the oil out of the surface because water loves to come in contact with the surface. The growth of the dewetting hole beneath the drop follows a dynamics⁠ characteristic of the dewetting of thin oil layers. During the slow spreading of the water drop on the substrate, the precursor oil film forms a tiny oil droplet under the water drop.

Our findings provide new insights into the dynamics of water–oil interactions, with implications for both practical applications and fundamental research.