Just curious. When you agitate water in a tub with your hand, you will notice the formation of some transient surface bubbles which pop shortly. Also they do not form or last without the continuos agitation. Given these observations, can someone explain why or how they are formed,because they are definitely not one of the regular soap bubbles we encounter in physics texts (or are they)?
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$\begingroup$ could be soap residue? $\endgroup$– JasenCommented Jan 8, 2017 at 8:32
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$\begingroup$ I doubt that, because it happens with any water. Even a glass of filtered water stirred vigorously with a spoon will develop these temporary surface bubbles. Give it a try yourself. You can even shake a transparent half filled bottle of water and they will appear. $\endgroup$– LelouchCommented Jan 8, 2017 at 10:30
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2 Answers
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Rather than agitate I suggest that you allow water to trickle onto a large water surface from a tap. Within a certain range of heights you can get such droplets to form. Although some are bubbles (containing air) which are partly submerged, the ones of interest are droplets which hover or sometimes "skim" over the surface. The latter last longest, up to about 2 seconds.
This phenomenon is discussed in Persisting water droplets on water surfaces (2010) where it is called delayed coalescence. The paper suggests that the droplets oscillate on the surface and form a bridge with the bulk, pumping out water in several (up to 6) steps. It contains a brief review of previous work, eg by Rayleigh and Reynolds, and an extensive list of references.
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.
The authors reject an earlier (2005) explanation cited in Noncoalescing and bouncing droplets, that an air cushion forms between the droplet and the bulk. Instead they suggest the existence of transient "interfacial layers" which they consider to be more consistent with their observations. However, in the earlier work the droplets emerge from the bulk liquid by vibrating it (similar to your agitation) whereas in the later work drops fall onto the bulk surface (and probably cause some oscillation). So it is possible that both mechanisms are correct.
answered Jan 9, 2017 at 3:23
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$\begingroup$ Thanks for the papers. I will surely check them out. This is interesting. $\endgroup$– LelouchCommented Jan 9, 2017 at 4:01
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Those are noncoalescing droplets. We had Paul Neitzel give a seminar on the subject last fall here. His review article on "Noncoalescing and Nonwetting Behavior of Fluids" is in the 2002 issue of Annual Review of Fluid Mechanics Vol. 34: pp. 267-289. He showed a number of simulations as well as first-principles arguments for the air cushion that forms due to effects associated with surface tension gradients in the droplets.
