Van der Waals attractive force: Why don't the imbalances average out?

[nomadreid]

TL;DR

When a van der Waals force between two objects is attractive (before getting too close), it is due to the variable distribution of electrical charge between the molecules. But why wouldn't this variation average out (+ + /+ -/- +/- -) to give zero?

The summary refers, for example, to the Wikipedia explanation
"The force results from a transient shift in electron density. Specifically, the electron density may temporarily shift more greatly to one side of the nucleus. This generates a transient charge to which a nearby atom can be either attracted or repelled."
https://en.wikipedia.org/wiki/Van_der_Waals_force

(for a distance between atoms of between 0.4 to 0.6 nanometers, as explained in that article)

Also, when I speak of the average (+ + /+ -/- +/- -) , this would include the sum (integral) of all the different strengths according to the angles, distances, etc. But the idea is still the same.

 

[Baluncore]

Consider two close spheres, (±) (±). One becomes momentarily polarised, +()– (±). The other sphere then takes an induced polarisation in the same direction, +()– +()–. The closest points on the spheres have opposite charge so attraction dominates.

 

[nomadreid]

Thank you, Baluncore. I had not considered induction. Question answered!