Surface Forces-Some Clarification Needed

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The Pauli exclusion principle is what leads to the attractive force in the first place, and electrostatic repulsion plays a role in the repulsive forces at short distances.In summary, the van der Waals forces (Debye, Keesom, and London) are not always attractive, with the London force being the only exception. This is due to higher order effects and the Pauli exclusion principle, as well as electrostatic repulsion at short distances.
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Caveman11
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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
 
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Caveman11 said:
-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.
 

FAQ: Surface Forces-Some Clarification Needed

What are surface forces?

Surface forces refer to the interactions between two surfaces that are in contact with each other. These forces can be either attractive or repulsive and are caused by various physical phenomena such as van der Waals forces, electrostatic forces, and capillary forces.

Why is clarification needed on surface forces?

There are often misconceptions and confusion surrounding surface forces, particularly in regards to how they affect the behavior of materials at a microscopic level. Clarification is needed to ensure a better understanding of these forces and their implications in various fields of science and engineering.

How do surface forces affect the behavior of materials?

Surface forces play a crucial role in determining the properties and behavior of materials, such as adhesion, friction, and wetting. They can also influence the stability and structure of biological systems, as well as the performance of devices and technologies.

What are some examples of surface forces in everyday life?

Surface forces are present in many everyday situations, such as when two objects stick together, when water droplets form on a surface, or when a gecko can walk on walls and ceilings. They also play a critical role in the production and processing of various consumer products, such as adhesives, coatings, and cosmetics.

How can we measure and control surface forces?

Surface forces can be measured using specialized techniques such as atomic force microscopy and surface plasmon resonance. They can also be controlled through various methods, including surface modifications, coatings, and external forces such as magnetic or electric fields.

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