Sweeping by Sessile Drop Coalescence*

Condensation is a ubiquitous heat transfer process that manifests as either dropwise or filmwise, depending on the wettability of the condensing surface. In practice, the condensation mode is often filmwise owing to easier control. However, dropwise condensation has gained recent attention due to its higher heat transfer coefficient. During dropwise condensation onto a cooled surface, vapor condenses as drops. Maximal heat transfer is favored by condensing onto fresh surface since smaller drops have a higher heat flux per unit footprint. An important mechanism of fresh surface generation is the sweeping up of nearby drops by a coalescence event. In this talk, we report on sweeping induced by sessile drop coalescence. Our focus is on how static contact angle influences the area swept by the dynamic motions of the coalescence process. Experiments are used in conjunction with numerical simulations to show how different surface types affect coalescence sweeping for a solid-water-air system. Larger swept areas are observed as static contact angle increases, which is explained in terms of surface energy conversion to kinetic energy.