Everyone has glued something together at one point or another and many of us would have seen wonderful little geckos climbing up walls and across ceilings with seemingly no trouble. But how?
Both of these phenomena can be explained using some simple chemistry.
Why does anything stick together?
If you think about this, it’s actually a good point. Most of the matter we see around us (the chair you’re sitting on, the device you’re reading this on, etc..) isn’t just crumbling or becoming a gas so something is holding it together.
The simple answer is the one given by the amazing physicist Richard Feynman, “Forces”. Forces hold everything together and so that’s what this post will be discussing today.
Forces that make sure solid or liquid things stay together are called cohesive forces and forces that hold two different substances together are called adhesive forces. As you’ve no doubt guessed, adhesive forces are responsible for adhesives, like glue. However, an adhesive or ‘sticky’ substance isn’t going to be any good unless it also sticks to itself well using cohesive forces.
Think of the proverbial chain only being as strong as its weakest link- if a glue secures adhesively really, really well to the two materials it’s gluing, but doesn’t stick cohesively to itself, then the two substances will come apart with a bit of glue on each part.
So what causes the “forces”?
Both adhesive and cohesive forces arise from the same set of intermolecular forces. As the name suggests, these are forces between molecules that hold them together. They come in three types: dispersion forces, dipole-dipole forces and hydrogen boding.
- Dispersion forces: Every single molecule has these and they’re the major cause of bonding and holding stuff together for many substances. They’re caused because all the electrons around atoms and compounds are continually moving which means that sometimes one side of a molecule has almost all the electrons, giving it a positive side and a negative side for just a split second. These are called temporary dipoles and they last just long enough to attract to another temporary dipole or cause one in an adjacent molecules by repelling like charge (+ and +, – and -) and attracting unlike charge (+ to – or – to +). The bigger the molecule is, the stronger these forces are holding the substance together because bigger molecules have more electrons to randomly flit around. This results in both cohesive forces and adhesive forces as the molecules can form these temporary attractions with molecules of the same type or with other molecules.
- Dipole-dipole forces: These are generally stronger than dispersion forces and a little more complicated to explain. The dispersion forces arise when temporary dipoles arise, whereas dipole-dipole forces arise when permanent dipoles exist in the molecule. This happens when one of the atoms is more electronegative than the other, literally, more electron-wanting. This electronegative atom attracts the other atom’s electrons closer to itself, making it more negative and the other atom more positive all the time. This means that a negative and positive end of the molecule are formed and can attract other positive and negative ends towards themselves to form strong bonds.
- Hydrogen bonds: These only occur in special situations where a dipole is formed with hydrogen as the more positive atom bonded to a very strongly electronegative atom (nitrogen, oxygen or fluorine) that almost completely “steals” hydrogen’s sole electron. Pretty much, it’s a really strong type of dipole-dipole bond and the typical example is that of water, which is good at cohering to itself and is still a liquid at room temperatures, unlike most similar substances.
So how do the forces work in glue?
As we said earlier, a glue needs to have good cohesive and adhesive forces so that it sticks to itself really well and sticks to the stuff you’re gluing together. If we look at the forces above, we can see that a good glue might have big molecules to cause a lot of dispersion forces to cohesively hold the glue particles together and also create more of those temporary dipoles to attach to the substance. This good glue might also have a dipole to help it attract to other glue molecules; however, this probably wouldn’t help it stick to surfaces as well unless the surfaces also had dipoles.
This is called adsorption and means that the only thing holding your grand craft project together is microscopic electron interactions flitting across molecules in split-seconds. These glues work really well mechanically by also “oozing” into all the little rough points along the surface, giving even more points to bond on the sides of all the miniature cracks and scratches.
There’s also some really strong glues that work by chemisorption. This means that the the glue molecules actually chemically bond to the substances that they’re gluing together. These occur with some plastics and are extremely strong glues as they’re effectively forming new chemical compounds (and bonds that hold the atoms together in compounds are stronger again than the ones holding the different compounds together). They still have to be very cohesive and hold themselves together and you’d really struggle to seperate the substances intact afterwards!
But then, why doesn’t glue stick to itself?
It’s great that these glues stick so well, but what stops them from gluing together their entire tube into a brick-solid mass? There are a couple solutions. One is that the glue has a non-sticky chemical called a solvent in the tube as well which evaporates in the air (and gives that distinctive gluey smell) as soon as the glue is out so it can harden. Some glues also have to react chemically with oxygen or surface moisture to harden (superglue is one of these) and do so very quickly, but you’ll run into problems if you leave the lid off!
And what about geckos?
Remember the dispersion forces? Well, geckos manage their incredible sticking ceiling feats because their feet have millions of tiny hairs (setae) on the bottom. Because these hairs are so tiny, they can get really, really close to the surface that the gecko is walking on. Just like before, this results in millions and millions of microscopic temporary dipoles forming all the time that, working all together, provide enough force to overcome gravity and allow the gecko to hang upside down.
Isn’t that a cool fun fact and a brilliant example of chemistry explaining our world once more!