I have observed that if you pour small globs of water from a considerable height (let's say more than 20 meters), the globs will break up. This is not surprising at all since the amount of water dropped was probably too much for a stable drop. In my experience (which took place many years ago while sipping water from a plastic bottle at the window of a tall building) the break-up appeared to happen at the same distance from the point of release (again, not surprisingly) and always consisted in the formation of 3 smaller drops in a (still not surprisingly) symmetric pattern.

What surprised me is that the pattern had a constant orientation, that is the three trajectories at 120° from each other were always oriented in the same way with respect to the building. If I remember correctly (many years have passed since 'the experiment'), one drop was always directed away perpendicular to the building wall, the other two diverged at 120 degrees towards it (eye estimate and symmetry prejudice).

I have since resisted the urge of breaking in in tall buildings with different geographic orientations (N-S, sun exposition) to drop water from their highest windows - try to explain that to the police... - so...

...can someone please explain why the orientation is always the same? Is it due to the thermal gradient near the walls/windows, the earth magnetic field, the separation of charges in the drops? Or was it just sheer coincidence? (I was not alone, and three people with two different bottles repeated the experiment many times always with the same result)

To cliarify: when I say "three smaller drops" I mean that from the distance the main glob appeared to 'explode' in three main globs of water. There could have been smaller drops produced, but the separation in three was clear and unmistakable.

EDIT: Well, fan my brow, I tried to replicate "the experiment" and I did not succeed. I did not go in the same building but went up to the 5th ant 8th floor of a condo. As far as I can tell, no breakup occurred, at all. The differences with respect to what I seem to remember from the original 'experiment' were:

  • the buildings are very different: the original one was a university building with classrooms (it means that each floor is higher than in a residential building) and I could have been higher than in my last try at the 8th floor.

  • I dropped water from the window of the original building (possibly with all other windows below me closed), while I dropped water from a balcony in my last try.

  • The drops in my last try were very small to begin with. Could this depend heavily on the temperature? I was not able to produce the relatively large globs I remember. The split up in three I witnessed back then was extremely clear, and even if my eyesight was considerably better back then, I doubt I could have been able to see the splitting with drops smaller than one third of the one in my last try.

Either the building geometry and/or the environmental conditions are at play, or there was something unique in the original building that caused the break-up in the same way (some exhaust, or an experimental water-drop-splitting machine). It appears that this winter I'll have to go at the same window of the same building. This might rule out the first part.

When I asked this question I expected an answer on the line of "But of course, everybody knows that water splits in the same way...". I should have investigated further back then :-(, at least to rule out heat pumps and air conditioners (or maybe even the heat from a classroom full of students).

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    $\begingroup$ Sorry but I'm highly skeptical of these claims. Non-laminar flow almost invariably is subject to the laws of chaotic (strange attractor) forces. $\endgroup$ – Carl Witthoft Sep 13 '14 at 13:42
  • $\begingroup$ That is why I found it surprising. I guess I'll have to try to repeat the experiment in at least another building. Could an air draft directed perpendicular to the building (an air conditioner, maybe? I did not have a clear view of the side of the building below me, but it appeared to be windows like the one I was at) cause this behavior? $\endgroup$ – Peltio Sep 13 '14 at 13:49

I can only make the following hypothesis. It explains why the drop explodes and why the explosion is symmetric. It does not explain why it happens always in the same plane though.

As the water falls along the building, the airflow around it is not uniform because of the presence of the building on one side. Then it is natural that the water bubble starts to rotate along a horizontal axis parallel to the building wall. The rotation will very probably speed up as the drop falls faster until it reaches a critical speed where the surface tension is not able to hold it together any more and it explodes. The explosion pattern will then be determined by the rotation geometry which is the same for all the drops.

This is more a guess than an answer. To test it you would have to go into the hydrodynamics of a rotating water drop and estimate the different forces acting on the surface of the drops. By equating these to the centripetal force you can estimate the critical rotation velocity. The fact that you have three droplets with a 120° angle in between them indicates that your rotating drop is not a sphere so that this analysis might turn out to be difficult.

Then another study of the hydrodynamics of an object falling along a wall should give you an estimation of the external forces on the drop and tell you a which rate (if any) the rotation speeds up. Putting everything together you should be able to estimate the height of the explosion (and publish an interesting paper).


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