2
$\begingroup$

From Wikipedia Capillary Action

Thus for a 4 m diameter glass tube in lab conditions given above (radius 2 m), the water would rise an unnoticeable 0.007 mm. However, for a 4 cm diameter tube (radius 2 cm), the water would rise 0.7 mm, and for a 0.4 mm diameter tube (radius 0.2 mm ), the water would rise 70 mm.

My question is: if we were to sit a collection of such narrow walled tubes around the edge of a bowl of water, and bend the ends of the tubes over in towards the center of the container, what would stop a continual water drip back into the water from the tubes?

The narrower the tube, the greater the capillary action and it is possibly the introduction of the bend that would restrict further water flow and that may be the solution to this "paradox".

Obviously, a perpetual motion device is impossible, but I wonder, at what point does "something" prevent the drips returning to the bowl, or what other mechanism prevents contravention of thermodynamic laws.

$\endgroup$

2 Answers 2

3
$\begingroup$

The dripping would occur as you describe it, if the drip ends of your tube or tubes is low enough to let gravity overcome the capillary action. That is what prevents a perpetuum mobile: Just as the water from the container is sucked into the tubes, water from the opposite end is also sucked in. To remove it, you have to expend energy, the same (in steady state) as you gained from the liquid's attraction to the capillaries.

$\endgroup$
2
  • 3
    $\begingroup$ Or, to say it in a single sentence: Capillary action doesn't draw water up, it draws water in. $\endgroup$ Commented Apr 3, 2017 at 18:13
  • $\begingroup$ Couldn't you change the material from glass to something different at the right position by the right amount (interlaced materials) to use the motion of water to "jump" onto the next material and then drip down? $\endgroup$
    – DarkTrick
    Commented Jan 14, 2021 at 16:58
2
$\begingroup$

From Wikipedia Perpetual Motion:

A capillary action based water pump functions using small ambient temperature gradients and vapour pressure differences. With the "Capillary Bowl", it was thought that the capillary action would keep the water flowing in the tube, but since the cohesion force that draws the liquid up the tube in the first place holds the droplet from releasing into the bowl, the flow is not perpetual.

From rationalwiki.org Perpetual motion:

(...) However, the flask would fail to fill regardless as water tension (related to the same force that causes capillary action) would prevent the flow emerging from the capillary at the end.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.