Editing Surface tension (section)

===Liquid in a vertical tube===
{{Main|Capillary action}}
[[Image:HgBarometer.gif|thumb|upright=0.6|Diagram of a [[mercury (element)|mercury]] [[barometer]]]]

An old style [[mercury (element)|mercury]] [[barometer]] consists of a vertical glass tube about 1&nbsp;cm in diameter partially filled with mercury, and with a vacuum (called [[Evangelista Torricelli|Torricelli]]'s vacuum) in the unfilled volume (see diagram to the right). Notice that the mercury level at the center of the tube is higher than at the edges, making the upper surface of the mercury dome-shaped. The center of mass of the entire column of mercury would be slightly lower if the top surface of the mercury were flat over the entire cross-section of the tube. But the dome-shaped top gives slightly less surface area to the entire mass of mercury. Again the two effects combine to minimize the total potential energy. Such a surface shape is known as a convex meniscus.

We consider the surface area of the entire mass of mercury, including the part of the surface that is in contact with the glass, because mercury does not adhere to glass at all. So the surface tension of the mercury acts over its entire surface area, including where it is in contact with the glass. If instead of glass, the tube was made out of copper, the situation would be very different. Mercury aggressively adheres to copper. So in a copper tube, the level of mercury at the center of the tube will be lower than at the edges (that is, it would be a concave meniscus). In a situation where the liquid adheres to the walls of its container, we consider the part of the fluid's surface area that is in contact with the container to have ''negative'' surface tension. The fluid then works to maximize the contact surface area. So in this case increasing the area in contact with the container decreases rather than increases the potential energy. That decrease is enough to compensate for the increased potential energy associated with lifting the fluid near the walls of the container.

[[Image:CapillaryAction.svg|thumb|upright=0.6|Illustration of capillary rise and fall. Red=contact angle less than 90°; blue=contact angle greater than 90°]]
<!-- [[Image:Dscn3156-daisy-water 1200x900.jpg|thumb|Surface tension prevents this flower from sinking]] -->

If a tube is sufficiently narrow and the liquid adhesion to its walls is sufficiently strong, surface tension can draw liquid up the tube in a phenomenon known as [[capillary action]]. The height to which the column is lifted is given by [[Jurin's law]]:<ref name="s_z"/>

<math display="block">h = \frac {2\gamma_\mathrm{la} \cos\theta}{\rho g r}</math>

where

* {{mvar|h}} is the height the liquid is lifted,
* {{math|''γ''<sub>la</sub>}} is the liquid–air surface tension,
* {{mvar|ρ}} is the density of the liquid,
* {{mvar|r}} is the radius of the capillary,
* {{mvar|g}} is the acceleration due to gravity,
* {{mvar|θ}} is the angle of contact described above. If {{mvar|θ}} is greater than 90°, as with mercury in a glass container, the liquid will be depressed rather than lifted.
{{Clear}}