Today I had a chance to see, close up, a yellowjacket swoop down and then hover to get a drink from the waters of our condo development's swimming pool. Unfortunately it made the mistake of doing it into a trough between shallow waves, resulting in its failing to rise as fast as the side of an approaching swell was rising. Then the creature was no longer hovering but instead was in the water, trapped by surface tension.

Surface tension is also responsible for the beautiful sheen on the gentle waves in the swimming pool -- the water appears to have a tight-fitting skin. And indeed it does, again due to surface tension. The surface of the water is quite literally attracted to the body of water beneath. With no counteracting attractive force from the air above, a thin region of high viscosity is formed -- and the yellowjacket is pulled down into the water and trapped there.

What's the source of this force?

It has to do with the fact that a molecule of water is polar -- weakly charged positively at the oxygen end, weakly charged negatively at the two hydrogens that bend away from the oxygen like a deeply curved banana. This polar effect is not the ionic bonds between the hydrogens and the oxygen -- the net charge across the molecule is precisely zero and therefore cannot by itself be a source of attraction between water molecules. (The zero net charge issue is another story for another day)

Rather, what we are talking about here is the distribution of the net charge -- a greater than nominal electron density around the hydrogens, a less than nominal electron density around the oxygen. So while the oxygen does in fact become weakly positive, the hydrogens actually become less positive than nominal. There is a small but computable and measurable tendency for the hydrogens of water molecule A to stick to the oxygen of water molecule B.

And now for a not-so-obvious fact: To a chemist (I was trained as one), water is a neutral aqueous solution of solvated protons. The protons have a tendency to drift away from their parent oxygens and stick briefly to a different oxygen. Because protons are positively charged, this phenomenon causes the approaching proton to tend to pull electrons away from the receiving oxygen and toward the proton.

The result is a proton surrounded (solvated) by a very weak cloud of negative electric charge, and an oxygen ion that has had its negative charge reduced slightly. This causes a bias in the distribution of the electric charges, resulting in a net attractive force between what we think of as ionically bound molecules of water.

But at the surface there is nothing to counter the net attractive force between the water molecules immediately beneath the surface -- and the skin of surface tension forms. Anything sticking down in this skin -- like the dangling legs of the unfortunate yellowjacket -- will be attracted to the underlying water, dragging the creature down into the water with such force that it is unable to escape, its struggles serving only to get itself wet, deepening its plight.

The yellowjacket's only way out of the situation is for a force majeure -- my hand -- to sweep it out of the pool and onto the adjacent concrete, where the water will evaporate, once again allowing the creature to fly.

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The example just cited is one of three categories of force known collectively as Van der Waals forces, named after a Dutch scientist who identified two of the three categories.

Van der Waals forces are residual forces, far far weaker than the parent forces giving rise to them ... and what we call the strong nuclear force is actually the equivalent of Van der Waals attraction between quarks, resulting in a tendency of neutrons and protons to stick together even though there is no ionic bond between them. This should give you some idea of the strength of the quantum chromodynamic force between quarks -- unbelievably strong, far far stronger than the unbelievably strong nuclear force they give rise to.

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Footnote: Not only is there no ionic bond between the protons and neutrons of a nucleus, the protons are repelling one another very strongly because they are all positively charged.

You can get an idea of the strength of the electric force as follows: Place a piece of paper on the floor. Now run a comb through your hair a few times and hold it just above the paper. The comb will then lift the paper, that small amount of charge overcoming the gravitational pull of the entire earth.

And now imagine the strong nuclear force, as relatively more powerful than the electric force as the electric force is than gravity.