Physico-chemical and dynamic study of oil-drop removal from bare and coated stainless-steel surfaces

We have studied the removal of sessile oil drops from stainless-steel surfaces under the action of water flow. A shear-flow cell is used to compare bare and polysiloxane-coated stainless-steel surfaces. We consider a rectangular channel where initially deposited drops are subjected to drag, gravity and pressure gradient forces. Our results indicate that a drop detachment mode is observed for the bare steel, whereas a sliding mode is observed for the coated steel. The removal of large drops, which requires low critical shear flows, is essentially dominated by the combined action of the lift and gravity forces. However, for small drops with a large critical shear flow, the capillary forces are the key factor. The detachment was also studied with surfactants added to water. It was found that the detachment mode exhibits a 'depinning effect', which results in drops sliding. Due to low pressure near the triple line, an accumulation of the surfactant induces surface tension gradients along the interface (Marangoni effect), which, in turn, facilitates depinning of the contact line. These results underline the crucial role of the capillary forces governed by the physico-chemical nature of stainless steels, a key factor for understanding the cleanability processes of these materials.