Complete, partial, and non-coalescence of sessile droplets on superhydrophobic surfaces*

Multiple droplets coalesce to form larger droplets during dropwise condensation on a water-repellant surface. Here, we report that such multi-droplet coalescence sequences do not necessarily result in the merging of all participating droplets. The non-coalescing droplets can bounce away from the coalescing droplets with significant tangential momentum. Existing studies have reported partial coalescence and bouncing of droplets in the presence of an additional external force field or due to the bulk motion of the droplets. However, the phenomenon reported here is observed without these factors, occurring on deposited or condensed droplets. It is challenging to discern the parameters influencing the outcome of these coalescence processes during condensation due to the small length and time scales involved. Hence, we perform controlled droplet coalescence experiments wherein two sessile droplets are made to coalesce in close vicinity of a third droplet. We observe three distinct phenomena across a wide range of droplet diameters: complete coalescence, coalescence followed by re-separation, and bouncing of the third droplet without coalescence. We discuss how the initial geometrical arrangement and the traveling capillary wave can affect the outcome.