Droplets coalescences induced by an impact on a breath figure.

A breath figure describes the droplet pattern formed when a vapor condenses onto a surface. When vapor is constantly provided, the average droplets radius increase. This evolution has been actively studied in the last decades, to develop anti-dew surfaces and dew recovery systems.

We present here the effect of a mechanical impact on a solid substrate supporting a breath figure. A falling projectile impacts the top of the plate, the droplets being on its bottom part. We changed the drop height of the projectile and we observed at different locations the evolution of the breath figure (the droplets size distribution) with time and with a parameter we introduced: the droplet number reduction. We show that, for a given mean radius of the droplets, when the acceleration of the substrate exceeds a threshold, the final number of droplets starts to decrease and keeps on decreasing as acceleration is increased. We interpret this result knowing that droplets vibrate, their contact line unpin above a threshold in acceleration, which make them contact and coalescing with neighbours giving birth to a liquids networks, eventually fully connected. For a better understanding, we then focus on the coalescence dynamics between only two water droplets partially wetting under vibration.