5 Short range forces
Stickiness is something that we take for granted in the macroscopic world; it's what happens when you put your elbow in some jam, or spill a sugary drink then put a paper on the drying surface. It's also responsible for friction: everyone knows that gripping a sliding rope will produce enough heat to cause severe burns. But where does this heat come from, and why is stickiness important in the microscopic world?
Atomic and molecular bonds come with a wide range of strengths: from very strong ionic and covalent bonds, involving exchange of charge, to very weak bonds between molecules in liquids, which have slight and fluctuating charge imbalances. These weak interactions are often referred to as van der Waals bonds.
So how do such interactions occur? Broadly speaking, they take place between electrically neutral atoms and molecules which, at first sight, might appear to offer no means of interacting electrically. Even for an electrically neutral atom or molecule, if I inspect it closely enough, I will find a discernible charge distribution; in the presence of an electric field, if the charges are mobile, this distribution will tend to become more pronounced.
Imagine bringing two electrically neutral objects together from a distance, each with an internal charge distribution; see Figure 22. These could be atoms or molecules or even larger entities. I'm going to use atoms because the locations and mobility of the charges are easy to visualise. At large distances there would be no attraction between the two. However, as they approach each other the charge distribution from the electrons in one will influence that in the other and an attractive force will emerge.
If the two continue to move together, then the outer electrons will reach a state where they can no longer screen the positive charges of the nuclei from each other and a strong repulsive force will emerge. Somewhere between these two positions is a neutral position where the atoms would ‘prefer’ to rest. The way that the force between the two atoms changes with their separation is shown in Figure 23.
Identify the two points of equilibrium in Figure 23.
Points of equilibrium occur where the force is zero. One is at infinite distance (very large separation, practically speaking) and the other is where the repulsive forces balance the attractive forces and the line crosses the separation axis.
There are two distinct ways in which charge distributions that produce these weak bonds can occur: one is static and the other is time-dependent. I will look at the two cases separately; however, in general bonds occur because of combinations of both effects.