Coalescence of vibrated droplets.

Droplet coalescence is at the heart of many everyday phenomena, such as the formation of dew, clouds or rain, and it needs to be carefully controlled for many industrial processes, such as water recovery, spray coating or targeted drug delivery.

We have recently demonstrated the benefits of substrate vibration in generating multiple coalescences of the collection of droplets that forms a condensation pattern. By impacting a solid bead on the back of a fogged surface, we have shown that the number of droplets can be greatly reduced. We have shown that efficiency (drop number reduction %DNR) begins to increase with average drop size, starting from a minimum drop size. This threshold size decreases with increasing acceleration. Moreover, for a given size, efficiency increases with acceleration. By defining an effective Bond number β∗, calculated with maximum substrate acceleration in place of gravity, we have demonstrated the universal nature of this phenomenon. Indeed, the different curves of %DNR as a function of this effective Bond number then fall onto a master curve. Hence, we show that the impact accelerates the natural aging of the breath figure. This could provide a new solution to increase the efficiency of dew recovery processes.

I will then present new experiments on the coalescence dynamics between two sessile water droplets submitted to vertical vibrations. We show that the coalescence can be instantaneous, delayed or totally avoided. We measure the neck growth dynamics as function of drops approach speed and substrate wetting properties.