# Forces acting on water that's flowing down from a faucet

I don’t understand the nature of the force that is acting to cause certain shapes to form in water that’s flowing vertically downward from a faucet.

If you experiment a bit with the faucets in your home, I’m pretty sure you’ll be able to duplicate the following illustrative examples (NB - make sure to keep the flow from the faucet laminar, or apparently so. Also make sure there are no gaps in the flow and, what is probably just another way of saying ‘laminar flow’, make sure there’s no obvious turbulence or spraying of the water.)

One example is when the water coming down assumes a corkscrew-like shape. Another is when the water, as it pours out of the faucet downward, seems to bulge out from side to side, then come back towards the centre, then bulge out again etc. This second example sort of looks like two sine waves positioned as if they were reflections of one and other through a vertically placed mirror.

I don’t understand why the water doesn’t do anything but flow (or, rather, drop) vertically in a straight line down from the faucet. After all, what force can be acting on the water after it’s left the faucet? I suppose that surface tension and/or hydrogen bonding of water molecules may play some role, but my question remains (I think)- why is the water ‘twisiting’, moving laterally then medially, etc.?

(btw, I just realized that what I call a ‘faucet’ may be called a ‘tap’ by others. I’m sure you know what I mean, though)

Thanks!

The Coanda effect will cause the deflection of the water as it comes out of the end of the tap depending on the geometry of the tap opening. Since the water is laminar, the water continues in the same direction under its own inertia.

Putting an aerator on the end of the tap breaks the laminar flow and causes turbulence. This leads to a more direct flow of water since the turbulence overwhelms the Coanda effect.

This second example sounds like an example of the Plateau-Rayleigh instability. Basically, the faster the water is falling, the narrower the jet will get (given that the rate of water passing any given point is constant, if the water is falling faster then the stream must get narrower.) However, the water also “wants” to minimize its surface area due to surface tension. So once the water has fallen far enough, it’ll split up into a collection of spherical droplets which have less surface area than the original stream.

If you’re not afraid of mathematics, then this MIT lecture on the subject looks quite informative.

Thanks, mighty interesting. As I feared, though, the math and physics is just a tad beyond me. Just a tad.