2
$\begingroup$

Capillary rise is due to surface tension of water and the angle of contact it makes inside the capillary i.e. since is forms a concave meniscus, the pressure above the meniscus is more than it is down the meniscus and so to equalise the pressure, the water rises in the capillary. But what about the water contained around the capillary. Since surface tension with act there too, there will be a meniscus formed . Like a half cut torus.(shown in picture, the dark color shows depth), so pressure above that surface will be greater than inside the torii meniscus too. Questions -

Will the water rise? If so, will a vacuum, even in smallest possible amount,be created at the bottom of the surface?

If the outside water doesn't rise,why shouldn't it?

If water rises ,will the water inside the capillary and outside capillary be in such equilibrium that there is no vacuum created at the bottom of the floor?

enter image description here

|cite|improve this question|||||
$\endgroup$
1
$\begingroup$

There is one fundamental misunderstanding in your reasoning. The water level in the beaker is completely flat for the majority of the width of the beaker, it is only curled upwards near the edge because of surface forces. The distance over which the glass can exert a force on the surface is rather limited. This distance is called capillary length and depends on the surface tension, density and $g$.

Only when the width of a beaker is small enough (i.e. a capillary), the surface remains non-flat, and hence you will observe capillary action.

Hence, the water in the beaker will not rise, but rather go down. The amount the surface level decreases in the beaker can be calculated from a simple mass balance.

|cite|improve this answer|||||
$\endgroup$
  • $\begingroup$ Does that mean if the beaker is quite small such that it's dimensions are twice of dimensions of the capillary, water in the capillary and the beaker , both, will rise? and vacuum will be created? I'm just talking hypothetical here. $\endgroup$ – Quark Dec 29 '15 at 14:10
  • 1
    $\begingroup$ @Quark That is a bit of a different, but interesting question. If beaker is capillary size, but still larger, it's water level will still diminish. You will see that the water level in the small capillary will rise, but not as much. Try making a diagram, consider hydrostatic pressure, and the pressure drops across both menisci. Take into account that ambient pressure is the same for both. (Another interesting experiment is when you close either of the capillary). $\endgroup$ – Bernhard Dec 29 '15 at 14:30
  • $\begingroup$ That's what i meant by asking the third question that if they both will rise but such that they are in equilibrium, if they will be in equilibrium, it will appear tha water level has decreased, right? $\endgroup$ – Quark Dec 29 '15 at 14:34
  • $\begingroup$ You will not get a vacuum, so the one with the strongest capillary force will rise, the other one will go down. $\endgroup$ – Bernhard Dec 29 '15 at 14:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.