Why don't solids always stick together?

  • #1
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Summary:

Why don't two solid surfaces that touch always stick? It seems to me that should happen through van der Waals forces at least, if not ionic bonds (for ionic solids), covalent bonds (for covalent lattices with unpaired electrons on the surface), or metallic bonds (for metal surfaces).

Main Question or Discussion Point

Hi all,

Something I've been wondering - why don't two solid surfaces always stick together when touching each other?

As far as I'm aware there are five basic types of solids:

  • Atomic solids: Frozen noble gases containing single atoms held together by London dispersion forces.
  • Molecular solids: e.g. water ice, sulfur, etc. These are made up of molecules held together by London dispersion forces, dipole-dipole interactions, or hydrogen bonds.
  • Giant covalent lattices: e.g. diamond, silica. These are made up of atoms held together by covalent bonds in a giant lattice.
  • Ionic lattices: e.g. sodium chloride. These are made of ions held together by ionic bonds in a giant lattice.
  • Metallic lattices: e.g. metallic elements and alloys. These are made up of the "cores" (nuclei and inner electrons) of electropositive elements surrounded by a "sea" of delocalised electrons, and are held together by the electrostatic attraction of the positive cores for the negative "sea".
(I'm aware that's an oversimplification and many substances are intermediate - for example, semiconductors mostly fit the "giant covalent lattice" model but have some delocalised electrons, graphite has 2D sheets of carbon atoms held together by covalent bonds but the sheets are held together by London dispersion forces, and highly polar diatomic molecules or polarised ions can lead to something intermediate between a molecular and an ionic solid, but still...)

With that understanding it seems to me that holding two solid surfaces together would cause them to stick? I've listed my reasons why I think they should, along with possible reasons for why they don't, below. Could someone please clarify what really happens?

  • Atomic and molecular solids: It seems that if you hold two of these lattices close together, London dispersion forces would bind the two surfaces together and fuse these lattices into one bigger one.

    Possible resolution - these solids grow during the freezing, solidification or precipitation process one atom/molecule at a time. The weak London dispersion forces are strong enough to bind individual atoms and molecules to the lattice, but not strong enough to bind massive crystals together containing billions of atoms or molecules.

  • Giant covalent solids: When a covalent solid like diamond or glass is broken, covalent bonds are broken. That means you'll have plenty of unpaired electrons in the exposed surface valence shells of these solids. If you hold the broken surfaces close together again, the unpaired electrons will pair up again and covalent bonds will re-form, sticking the pieces back together.

    Possible resolution - the geometry of the surfaces makes it unfeasible or not entropically favourable for enough covalent bonds to re-form and stick the surfaces together. On top of that, if we're doing this on Earth then the unpaired electrons will probably combine with oxygen or something else in the air (but wouldn't that mean a diamond should slowly disappear into carbon dioxide?) and "plug the gaps" in the valence shells.

    This one feels like a bit of a cop-out though. If the covalent bonds formed in the first place when the solid began to grow, why would it be unfavourable for them to stick back together again? And why doesn't a diamond disappear as carbon dioxide if there's a whole bunch of carbon atoms with unfilled valence shells on its surface sticking out into the oxygen-rich air?

  • Ionic solids: When the cations from one surface line up with the anions on another, they'll attract and stick the lattices together.

    Possible resolution - the lattices have to be lined up exactly right to stick the surfaces together - the tiniest nudge will line up cations with cations and will blow the lattices apart.

    I know ionic solids placed in a saturated solution can gain more ions, so they're clearly able to attract individual ions one at a time, just not another whole lattice - I think my resolution makes the most sense with this one perhaps?

  • Metallic solids: When two metal surfaces are stuck together their delocalised electron "seas" merge and the atom "cores" will settle into place on top of each other, fusing the two metals into one.

    Possible resolution: All I can think of for why this doesn't happen is layers of metal oxide forming on the surface and covering the metallic lattice up with an ionic one. But why wouldn't this happen in space, or on the surface of a planet without an oxidising atmosphere?
I hope someone can clarify all this for me! Are my resolutions anywhere near the right answer, or have I missed something big and obvious here?

Thanks!
 
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Answers and Replies

  • #4
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Not a complete answer, but there was an experiment done: cleave a microscale Cu bar in ultrahigh vacuum. When you bring the halves back together, they do fuse back into a solid bar. So in many cases, it is really just surface contaminants. Unfortunately I can't find the reference for this again.

Edit: I think I was remembering the Feynman Lectures 12-2.
 
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  • #6
Baluncore
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... why don't two solid surfaces always stick together when touching each other?
They do not always stick because there is often a liquid or a gas contaminating the surfaces that forms a weaker temporary shear layer. That allows them to move apart when the force of stiction is overcome.
 

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