Surface Forces-Some Clarification Needed

[Caveman11]

Hi all,

I'm currently doing some reading into surface forces and am confused about the following:

-Are the van der Waals forces (Debye,Keesom and London) always attractive? I thought they were until I read that the London force is not always.

-When approaching a surface for example say with an SPM tip and a sample surface the potential that is experienced is given by the Lennard-Jones potential. However at very short distances the force becomes repulsive. Is this due to the Pauli exclusion principle and electrostatic repulsion of the atom's outer electrons?

Any answers you have are greatly appreciated.

Nick

 

[alxm]

-Are the van der Waals forces (Debye,Keesom and London) always attractive? I thought they were until I read that the London force is not always.

Only the London force, and only to higher orders (Axilrod–Teller potential) with three-body effects. (Same goes for the Casimir effect, which is essentially the same phenomenon)

The other two are, more or less by definition, described by classical electrostatics, whereas the London dispersion force is a purely (nonrelativistic) quantum effect.

Is this due to the Pauli exclusion principle and electrostatic repulsion of the atom's outer electrons?

Both.