Is it possible that a droplet of water can bounce from the surface of a volume of water?
Fluid Dynamics – Can a Droplet of Water Bounce Back When It Hits a Water Surface?
fluid dynamicssurface-tension
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Rather than adding more cameras, just add some mirrors.
The problem you have is that you are trying to do tomography with an under sampled system. This is a VERY broad subject - a bit outside of the scope of your question. But mirrors will work. I would recommend that you place them so the images are all in focus - depending on the depth of focus of your camera you may need to play with the "direct" path as well. But for example four mirrors set up as a pair of periscopes would allow one camera to take two images from +- 22.5 degrees. A second setup at 45 degree from that would give two more views for a total it four. This will reduce the degeneracy.
Here is a diagram of what I had in mind:
You can actually buy a device similar to this - see for example https://www.lhup.edu/~dsimanek/3d/stereo/3dgallery5.htm which includes some examples of the pictures taken with the Loreo stereo adapter - it includes a picture not unlike the beamsplitter I drew. Note though that they have the two sets of mirrors offset slightly, and that they are aimed "straight in front". This ensures that the focal planes for the two sides will coincide, but it also means that you may have a hard time getting very close to the subject - although the stagger may help with that. At any rate - I suggest you play around with the setup (using a small bead the size of your drop) until it looks right with a normal camera, before trying to do this with the high speed camera.
It occurred to me that since the drops are quite small, you might be able to simplify the setup with a pair of prisms - the basic idea being that a prism shifts the angle of the light between input and output. It might be tough to get the image you need (magnification, distance, ...) especially since a simple prism bending light through a large angle may have significant chromatic aberration which will make measuring the dimensions hard. Unless, of course, you use a monochromatic light source (or a filter on your camera). If you have a digital color camera you could see whether looking at just one of the R,G,B components of the image would give you a clearer image...
If you include a flat part in your prism, you might be able to increase the number of views to 6, spaced just 15 degrees apart. And since the "straight through" view is then coming through the prism, I believe the optical path lengths will in fact be the same - because basically the prism is a "very crude lens" and lenses do their focusing thing by having the same path length for all rays from object plane to image plane.
These are just some late night thoughts. If you have a buddy in the optics department he can probably come up with far better optical arrangements that achieve the same thing. This is really a bit like making a stereo camera - it's been done as the above link shows; and there is a company that makes attachments for various cameras - they may be able to give you exactly what you need.
PS - although this is a LONG time ago, I took high speed pictures of water drops during my graduate studies - so this is a trip down memory lane for me (and yes I had only one camera and multiple mirrors... - but it was an Imacon that could do 10 Mfps and my 0.4 mm diameter water jets were going at Mach 6...)
The phenomenon you mention is called Noncoalescence, and there are many beautiful experimental examples of it (see for instance [1],[2]).
Indeed, droplets of various liquids may float on the respective surfaces for extended periods of time prior to coalescence. The problem of explaining why this happens has been addressed by Klyuzhin et al. a few years ago, with some attempts to explain it better than had been done in the past.
I quote, from this paper:
When a droplet of liquid falls onto the surface of the same liquid, instant mixing is generally expected. However, even in everyday situations, small droplets can sometimes be observed to float on surfaces without instant mixing, for example, when droplets fall into sinks or during rainfall, when splashes create tiny droplets that may move swiftly across puddle surfaces. Despite such common experience as well as a number of relevant scientific studies, still, the concept of delayed coalescence remains counterintuitive.
A number of hypotheses have been proposed to explain the delayed coalescence. In 1900, Reynolds proposed that droplets can reside on the liquid surface because a thin film of air becomes entrapped underneath the droplet. However, Mahajan later reported that water did not form floating droplets at atmospheric pressure but that droplets could be more easily produced at higher altitudes where air pressure was lower, the opposite of what was expected from Reynolds’s hypothesis. The positive effect of reduced air pressure was more recently confirmed in studies of oil-droplet lifetime. Because diminished air pressure should diminish the thickness of any air cushion, these latter observations have seemed difficult to reconcile with the early air-cushion hypothesis.
They then propose an explanation for this mechanism:
Recent results from this laboratory show an interfacial zone extending down from the surface sometimes by up to hundreds of micrometers or more, enhanced apparently by incident infrared radiation and also by oxygen. The presence of a substantial interfacial layer could work as an effective barrier that prevents instant coalescence.
A possible mechanism based on the presence of this interfacial layer is illustrated in Figure 13. Before droplet and bulk come into contact, both entities are presumed to have significant interfacial layers (1), which prevent immediate coalescence (2). Once they touch, the interfacial layers begin to dissipate (3). When the layers have dissipated sufficiently, coalescence begins, and water from the droplet begins to flow downward (4). As the water evacuates, the droplet diminishes in volume, creating a narrower entity between the droplet and the bulk (5). The pinch-off creates the daughter droplet (6). The process then repeats, perpetuating the cascade.
Best Answer
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.