# Can anyone explain what actually surface tension is?

## Main Question or Discussion Point

can anyone explain what actually surface tension is??

I am confused about surface tension.
Everywhere it is written that due to unbalance of forces at surface the molecules at boundary tend to move inwards and thus surface tends to take minimum area as possible. This is ok but it is also written that an imaginary elastic skin is created. Now why such a skin will exist due to inward forces acting on boundary mloecules??
Also on wikipedia it was written that due to surface tension surface of liquid tend to attract other surfaces. Why this would happen if boundary molecules have net inward forces on them??

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I am confused about surface tension.
Everywhere it is written that due to unbalance of forces at surface the molecules at boundary tend to move inwards and thus surface tends to take minimum area as possible. This is ok but it is also written that an imaginary elastic skin is created. Now why such a skin will exist due to inward forces acting on boundary mloecules??
Also on wikipedia it was written that due to surface tension surface of liquid tend to attract other surfaces. Why this would happen if boundary molecules have net inward forces on them??
I will use water and nonpolar substances and give this a stab...

I think this is due to the fact that the boundary with air is not only not attractive, as air contains nonpolar molecules (thus more likely to have an unattrative electron configuration), but also because air is a gas and mobile. On other surfaces such as plastic or glass (also nonpolar), water does bead up (attract to itself), but clearly rises in a thin glass capillary tube (even plastic straws) which is interesting and will adhere. Maybe it is because water molecules can induce an attractive electron configuration in the immobile glass and plastic molecules that the water sits right next to.

Interesting...

Mapes
Homework Helper
Gold Member

Here's how I view it, and it might be helpful to you: solids and liquids are solids and liquids because it's energetically favorable for the atoms to bond together (otherwise they'd be gases). And if it's energetically favorable to have a certain coordination number (i.e., number of atoms attached to each atom), then it's less favorable for an atom to sit on a surface with some bonds unsatisfied. Finally, systems tend to evolve to the lowest possible energy level. The consequence is that, dependent on kinetic limitations and in the absence of all other effects, isolated condensed matter tends to form spheres in order to minimize surface area.

Now for the question of liquids in contact with surfaces: the behavior all depends on which bond is "worse" (i.e., higher energy, less favorable): liquid-to-air or liquid-to-surface. If the liquid-to-air bond is less favorable than the liquid-to-solid bond, the liquid will wet the surface and climb capillaries. If it's the reverse (e.g., mercury on glass, or water on an oily surface), the liquid will bead up.

So the model of "inward forces" isn't as useful a concept, in my opinion, as a consideration and comparison of surface energies.

Andy Resnick

Surface tension (and interfacial energy in general) is a property of a *surface*, which is usually modeled as a Gibbsian dividing surface (as opposed to, for example, Korteweg's 3-D description of an interfacial region or a molecular model, as Ono and Kondo). The interface is allowed to have physical properties independent of the bulk, and physical parameters (density, etc.) are allowed to be discontinuous across the interface.

Interfacial energy is simply the energy content of a surface- energy is required to generate surface area, energy minimization results in area minimization (i.e. spherical drops in the absence of gravity, more complex shapes in the presence of gravity). Molecular models of interfacial energy are immature (IMO) and not particularly useful.

i have doubts:

1.) In case of capillary action, it was written somewhere that glass surface should attract the fluid boundary particles only, but surface tension holds the water weight in between the boundary also which has risen. That's why water rises and not only water at boundary of glass. How can you explain this?

2.) How will you explain the needle floating on water. Everywhere it is written that due to surface tension a force acts on needle which supports it.

The reasons written in my above doubts state that surface tension is merely the strong bonding of nearby atoms at the boundary surface. What is reason for this strong bonding.

Mapes
Homework Helper
Gold Member

i have doubts:

1.) In case of capillary action, it was written somewhere that glass surface should attract the fluid boundary particles only, but surface tension holds the water weight in between the boundary also which has risen. That's why water rises and not only water at boundary of glass. How can you explain this?

2.) How will you explain the needle floating on water. Everywhere it is written that due to surface tension a force acts on needle which supports it.
Both cases can be explained by local minimization of total energy (potential energy + surface energy of liquid-air interface + surface energy liquid-solid interface).

In the case of capillary action for a wetting liquid, there are energy penalties for (1) large curvature, (2) lifting of the water, and (3) liquid-air surface area relative to liquid-solid surface area. The way that the liquid climbs up the surface is a compromise of all of these penalties.

In the case of a floating needle, the liquid does not wet the needle (i.e., there is an energy penalty for liquid-solid surface area relative to liquid-air surface area, which is the opposite of the first case). If the needle were to begin to sink, the total energy of the system would increase, which is unlikely.

It might be helpful to sketch a few scenarios (e.g., needle atop water, needle halfway immersed in water, needle under water) and consider the relative magnitudes of these energy penalties.