Why don’t the van der Waals forces work when you push pieces of iron together?

Answer

The Van der Waals forces are a combination of several forces, which manifest themselves at the macro level as cohesion and adhesion.

If you take a block of iron, you can assume that that block is well “filled” inside. There will be virtually no holes in it. If you cut an imaginary interface through the block, the metal on one side of the interface will match up almost perfectly with the metal on the other side of the interface. It is this very good contact that ensures that the material has many places to stick well.

If you were to push 2 pieces of iron together, the effective contact area between the 2 pieces will be surprisingly small. Even a relatively smooth surface is often still a disastrous mountainous landscape on a microscopic scale. When you press such two surfaces together you have some peaks here and there that touch the other side, but for the most part the 2 surfaces do not touch (on a microscopic scale).

However, you can make the surface very smooth. This makes the surfaces reasonably flat, even on a microscopic scale, and enough contact surface is created to still get a reasonable force effect. The 2 metal surfaces will indeed start to stick reasonably well.

An example of the latter are the measuring blocks that are used in metal technology. In English they call this “gauge blocks”, I don’t know the exact Dutch translation. These blocks come in a set with a number of standard sizes. If you would need a reference size of, for example, 5.7 millimeters during the metalworking process, you can glue a number of those blocks together. You can then stick a 5 mm block to a 0.7 millimeter block. These blocks are so super flat and super smooth that they stick together due to (mainly) the Vanderwaals forces.