28 December 2015 (Not) Sticking with air pressure.

I love the fact that I never know what new challenge will appear in my inbox each day. My holiday season has been partly taken up by a "battle" between myself and someone who asked an apparently simple question about adhesion. He'd been looking at my Practical Adhesion website and could not find anything discussing the "obvious" fact that an adhesive joint must be partly held together by air pressure. On the outside is all that air, on the inside there is no air, and so, by analogy to the famous Magdeburg Hemisphere experiment, the joint will be relatively strong, even before any "real" adhesion effect kicks in.

I have come across this idea many times, especially when I discuss the "Red Herring" video which shows how you can stick a block of polyethylene to two other blocks of polyethylene using just water. The video is intended to make a very different point, but the adhesion via the drops of water is almost always seen as being due to "atmospheric pressure", or, rather that if the joint started to come apart it would create a vacuum and this is what is holding the joint together.

I admit that my reply to the question about atmospheric pressure was not very good. It was the holiday season, I'd had a glass or two of wine and the atmospheric idea is so obviously wrong that I could easily dismiss it by ... finding some quotes from Professor Kevin Kendall, one of the great names in adhesion, who dismisses the idea in several places in his wonderful book Molecular Adhesion and Its Applications: The Sticky Universe.

I got back a detailed reply showing how I had missed the point several times. I replied to this reply and that made things worse ... and so it went on. The problem was not the person at the other side of the discussion - I was failing to come up with an explanation that convinced myself - I was being intellectually lazy.

So after taking some time to think properly about it I found that there is a neat explanation that convinces myself - and I can't do any better. In adhesion we can talk about things like peel force and also talk about work of adhesion. For simple systems (which is what we are considering here) the two are identical. In adhesion science it is sometimes clearer to argue in terms of force and sometimes in terms of energy. They are exactly equivalent, but in many cases it is rather harder to do calculations in force mode than it is in energy mode (or the other way round). For this problem it is easier to think in energy mode. When the system is stuck together (e.g. polyethylene-water-polyethylene) the energies involved are those of the attraction between the polymer and water surfaces. The attraction is not very strong (so water isn't a great adhesive!), but it is an attraction all the same. When the surfaces are separated they only have the attraction to the low density of air molecules, i.e. almost none at all, and the surfaces are very "unhappy". So the system is at a lower energy (more stable) state when together than when apart. The energy difference is the work (energy) of adhesion.

Note that the atmospheric pressure plays no role in the energy explanation. How could it? It can't. It is completely irrelevant. In fact, the adhesion would be slightly greater in a vacuum because the small polymer/air interaction is better than nothing, so the difference between water and air contacts is less than that between water and vacuum contacts, so the work of adhesion is less.

If we go back to the Magdeburg Hemispheres we can use another energy argument. When you put the spheres together there is no adhesion. You have to do a lot of work to pump out the air to create the vacuum. The harder you work, the stronger the adhesion. So in this case the "work of adhesion" is the same as the "work of pumping." If you watch the Red Herring video, when you put a drop of water onto one of the blocks of polymer and bring it close to the other block it jumps across the air-gap to make contact. You don't have to provide any work to remove the air from the contact - the surface energy attraction is enough. The fact that you are not doing any work equivalent to creating the vacuum for the hemispheres shows that air pressure is of no relevance.

During a talk at one conference I happened to say: "If it's not on YouTube it doesn't exist". This phrase became popular during the rest of the conference and contains a large degree of truth. I'm aware that the "energy" argument will still convince almost no one. So my hope is that I can work with a local school here in Ipswich to create a YouTube video to show how the adhesion between two pieces of plastic due to surface energy is, if anything, stronger in a vacuum. Seeing is believing, no matter how strong the intellectual argument might be.