How Do Van Der Waals Forces Influence Adhesion in Glue and Gecko Setae?

[siddharth5129]

I have researched a little online and apparently Van Der Waal's adhesion forces play a key role in the sticky properties of glue and the bonding between gecko setae and tree surfaces. What I don't understand is why this adhesion doesn't happen between the surfaces of any two solids brought in contact. Even if this does happen to some imperceptible extent , why aren't the surface-surface adhesive forces as strong as the cohesive forces between the molecules of each solid, which should render them very hard to separate.(the surfaces should be broken apart to separate them)

 

[alva3]

Apparently they do: https://www.technologyreview.com/s/417518/small-asteroids-are-held-together-by-van-der-waals-forces/

 

[Andrew Mason]

I have researched a little online and apparently Van Der Waal's adhesion forces play a key role in the sticky properties of glue and the bonding between gecko setae and tree surfaces. What I don't understand is why this adhesion doesn't happen between the surfaces of any two solids brought in contact. Even if this does happen to some imperceptible extent , why aren't the surface-surface adhesive forces as strong as the cohesive forces between the molecules of each solid, which should render them very hard to separate.(the surfaces should be broken apart to separate them)

Surface separation and shape.

Van der Waals forces are weak and operate over distances smaller than about 1 μm so in order to create such bonds, you need a lot of surface contact in which the molecules of both surfaces all very close. That is generally easier to do if the molecules are able to move around during contact. Rigid bodies are not likely to do that because their surfaces are irregular and do not adjust on contact, so there are not nearly enough Van der Waals bonds created.

Reference: https://www.sciencedirect.com/topics/chemistry/van-der-waals-force

AM