Creates bubbles in pure water

Traces of explosion in the garden pond

When a fountain splashes in a pond, larger and smaller air bubbles form on the surface of the water, which drift away and sometimes lead to a surprisingly long existence in a quieter environment. Actually it is already astonishing that they form at all - isn't one just dealing with water in its different forms and states of motion? But if a drop of the well falls into the water from a height, it pushes it away to the sides when it is immersed and literally tears a hole in it. The surface closes again with a short delay above the sinking drop, and a cavity remains (see “The sound of dripping water”, Spectrum February 2019, p. 60). Because of its low density, the trapped air immediately moves upwards and presses a thin layer of water upwards with it. Often the bubble bursts immediately, but occasionally it takes a while.

The lifetime also depends on the exact composition of the liquid. A skin made of pure water tears quite quickly because of the high surface tension. The impurities that are always present in the pond, for example vegetable proteins, give her a certain relaxation. Then the bladder wall lasts longer and only bursts when it falls below a critical thickness due to gravity due to water flowing down. This leads to a kind of mini-explosion, because the bubble is under a slight excess pressure compared to the outer atmosphere. That is why it also buckles the water surface a little downwards. At the moment of pressure equalization, the dent moves up again, shoots a little over the target due to its inertia, snaps back again due to the restoring force of the tensioned surface and so on.

This article is featured in Spectrum of Science June 2019

Small waves ripple differently than big ones

This triggers movement in the water - rings spread across the pond towards the edge. If you take a closer look, those with the shorter wavelengths run ahead of those with the longer ones. This particular sequence can only be observed in water waves that are no more than a few centimeters long. On such small scales, surface tension dominates as the driving force of the oscillation, and so-called capillary waves are created. The shorter the wavelength, the faster they run. On the other hand, if you throw a stone into the water, the influence of gravity predominates. It also determines the ups and downs in the oceans. In the case of such "gravity waves", the longer waves are faster and the corresponding ring systems appear to be sorted in reverse order.

“But what happened to the bladder in the meantime? It has burst into nothingness, where is there still a trace of the majesty with which it was clad? «(Bettina von Arnim, 1785–1859)

When the bubble bursts, the wet skin tears into smaller fragments. These purr into tiny droplets at lightning speed in order to minimize the interface with the air and to release as much energy as possible into the environment (second law of thermodynamics). A few of the droplets created in this way do not immediately combine with the rest of the water, but rather flit a little way over its surface beforehand (see “Surfing Drops of Water”, Spectrum February 2018, p. 60).

The photo below on the left shows what happened shortly after such an explosion. The last oscillations with the greatest wavelength are just moving away from the scene and leaving a smooth surface. Nevertheless, something is reminiscent of the preceding: a wreath of tiny bubbles in the center. The larger bubbles are usually surrounded by several much smaller ones. The former collected the latter, so to speak - also a consequence of the second law of thermodynamics. By sharing a common surface area, large and small bubbles reduce their overall energy. Since the small ones are more durable, they will remain for a while after the explosion. On the right of the picture you can see the rest of an object that had exploded before. The wave-like disturbance caused by this has long since disappeared.

The inherently transparent medium of water makes the processes visible in the first place by reflecting the surroundings in a distorted manner or - in the case of shallow, clear water - breaking and curving the image of the subsurface. A third possible effect comes when the waves focus or defocus the sunlight as light and dark rings on the pond floor. Sometimes all three phenomena occur together and literally compete for the observer's attention. Then the bubbles and waves impressively demonstrate how numerous mechanical and optical effects can work together in a seemingly simple process.