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What do these have in common: gum, cellophane tape, glue, geckos?
Why do some things stick? What is the explanation at the atomic scale?
What explains the ability of something to stick to clean, smooth glass? Here are some possible explanations:
They each grab onto surface roughness.
Each has special molecules that are very sticky.
Static cling—each has atoms with strong charges that are attracted to atoms of glass.
Each has a thin layer of water that sticks to the glass.
They each penetrate a bit into the glass.
Each has tiny suction cups that create a partial vacuum.
The explanation is that all atoms stick with an attractive force, if they get close enough.
BUT
if all atoms stick, then why doesn't everything stick to everything else?
The answer is surprising: most of the time when two objects touch, very few atoms are close enough to stick. As you know, the attractive part of the Lennard-Jones force kicks in only when two atoms are less than a few diameters apart. Surface roughness, contanimants, water, and air get in the way and keep most atoms from getting close enough to attract.
But each of the sticky substances can get close:
Sticky tapes have a coating of material that can flow a bit to contact a rough surface. Try freezing tape so the coating cannot flow--it is useless until it warms up.
Gum is the same--it can flow so lots of atoms contact any surface.
Glues start as a liquid that can flow over a surface and make atom-to-atom contact before they turn solid.
Geckos have special feet with billions of rods that can adjust to any surface roughness.
This activity answers questions about sticky atoms by looking into the forces between atoms and their energy. The answers come from applying Newton's Laws to atoms.
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They all stick to many surfaces.
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