if you set a dish filled with water onto a loudspeaker or other vibrator and make vibration, When it oscillates, it is possible to hold small droplets on its surface for a long time. I mean, when you put a droplet of water on vibrating water surface, it doesn't coalesce with the water and bounce on the surface of water.( see this video https://youtu.be/KZ5ZLPWasrM )
As we know, drop reaches its minimal potential when coalescing with water ( that's why drops coalesce)
what exactly cause this? why doesn't the drop coalesce with the water?
Answer
This phenomenon is described in the paper:
Couder, Y., et al. "From bouncing to floating: Noncoalescence of drops on a fluid bath." Physical review letters 94.17 (2005): 177801. (PDF)
To quote the opening paragraph:
Intuition expects the merging of two volumes of the same fluid when they come into contact with each other. This is also expected by physics. As the two free surfaces come close, a van der Waals attractive force tends to bring them into contact. As soon as this contact is effective, surface tension leads to a minimization of the interface area and to merging. This coalescence is usually slightly delayed by the need for air to flow out of the intermediate region.
In other words, coalescence involves the escape of interstitial air which takes a finite amount of time. So if you 'kick' the drop away from the surface in less time than it takes for this process to complete, then the drop won't coalesce.
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