# Can a droplet of water bounce back when it hits a water surface?

Is it possible that a droplet of water can bounce from the surface of a volume of water?

• see for yourself May 15 '16 at 12:06
• Wow! Amazing video! And that surely answers my question! May 15 '16 at 12:16

if you watch extreme slow motion film of a single water droplet falling into other water, usually a completely flat horizontal surface, you see many circular ripples being created, most of the droplet being absorbed by the body of water, but a tiny ( about 5% of original drop ) droplet being formed, and returning back, vertically upwards.

How much of this is the falling droplet I am unsure of, it may me mostly from the target pool.

I suggest a further experiment, where the falling droplet contains an intense dark dye, falling into completely pure water, filming with a high frequency stroboscope or fluorescent exciter, to further determine this.

• How come the last, most little droplet, goes higher than the preceding one? Does the greater mass of the preceding one takes care of that? May 15 '16 at 14:43
• @descheleschilder momentum conservation , less mass higher velocity. (Think of bouncing ball kinematics, momentum conserved between huge mass of wall and tiny mass of ball , carried on liquid) May 15 '16 at 18:00
• One would have to use equations to really see what is happening, the elasticity of the fluid . Qualitatively, a large droplet has large momentum, it transfers it to the liquid and a bounce of the same size droplet would have the same height minus absorption. For the small droplet to go much higher it means it got a larger part of the momentum than its mass by equal distribution. The large mass coalesced and the tiny bit left got all the momentum by momentum conservation ( instead of zillions of surface water molecules getting a tiny bit vertical motion as a wave). May 16 '16 at 7:05
• @descheleschilder Observation is the first thing. You've seen that the droplet does bounce higher than the original droplet fell from - that's a given thing, confirmed by observation. You know that the momentum left (or regained) to the smaller droplet is high enough to make the velocity higher than the impact velocity of the original droplet. No point discussing those - they're simple facts. Now, explaining how exactly the mechanics work that this is the case is a bit trickier, and will likely require more observations - time to get that high-speed camera! :D May 16 '16 at 8:05
• The video: youtube.com/watch?v=W7DpP311NxU might interest people in this comment chain. It basically deals with a droplet of water with red dye falling, instead of pure water. It is quite clear that most of the water that composes the first bounce droplet is from the original droplet. Unfortunately, the video is cut before the other bounces take place... May 19 '16 at 17:52

Yes, as you can see in this video. As you can see, the droplet will hit the surface, partially coalesce (merge) with the bulk, re-emerge as a smaller droplet, bounce 1-3 times, partially coalesce again, re-emerge again as an even smaller droplet and so on. This process is known as coalescence cascade. You can find another video here. Eventually, the coalescence will be total and the droplet won't re-emerge anymore.

This if the surface is still: but if it is vibrating, another really interesting phenomenon can be observed (video). As you can see, in this case the droplets don't coalesce with the bulk, and will stay intact while "jiggling" on the surface until the vibration is switched off.

• So little droplets can roll (or slide) over the water too, according to the second video, where the water is vibrating. Can they also when the water is not vibrating? On a still surface? May 16 '16 at 4:19
• I think the answer is yes, but only for a short period of time. As a matter of fact, you can see at the end of the video that when the vibration is turned off the droplets won't immediately disappear, but will continue to "jiggle" for a short time. But they will eventually coalesce: if you want to prevent this, you must keep the vibration turned on. May 16 '16 at 7:36
• @valerio92Can you possibly think of any reason why the droplets (after a short time) stop staying on the surface when the vibration is turned off? Or asked the other way round, why they jiggle around so long on the wavy surface when the water vibrates? May 23 '16 at 12:52
• @descheleschilder I found the explanation in this article: journals.aps.org/prl/abstract/10.1103/PhysRevLett.94.177801. Quoting: "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 this bouncing regime, the air separating the drop from the film is constantly renewed: there is no aging of the system. As a result, the lifetime of these drops appears unlimited." So it looks like the air layer is the key. May 23 '16 at 21:46
• PS Actually I also found out that someone criticized the air layer explanation and proposed another mechanism: ncbi.nlm.nih.gov/pmc/articles/PMC3208511 May 23 '16 at 21:59