nenineni-bububu said: What make glue sticky?
This is on the face of it a pretty simple question, but the chemistry behind it is actually a little complicated. It’s also complicated further by the fact that different glues will work in different ways.
As one example, superglue contains the chemicals from the cyanoacrylate family, one of which, methyl cyanoacrylate, is shown below. This chemical rapidly polymerises (forms long chains) with other molecules of itself when it comes into contact with moisture - even the moisture in the air is enough to start this process. The polymerisation bonds the joined surfaces together. So, when you get superglue on your skin, the ‘stickiness’ is caused by the polymerisation, set off by the moisture in your skin.
Other types of glue can stick things together in different ways. Even an object that feels smooth will have a very rough surface on a molecular level, and liquid glue can seep into microscopic cracks in an object’s surface. ‘Mechanical bonding’ sticks the two objects together as the glue hardens within these crevices.
Intermolecular forces also play a part in the ‘stickiness’ of glue, in particular Van der Waals forces. Electrons in molecules are mobile, and at any point in time there could potentially be more electrons at one end of the molecule than at the other. This leads to what we call a ‘temporary dipole’ - meaning the molecule has one slightly positively charged end, and one slightly negatively charged end. Because electrons in molecules are constantly moving, temporary dipoles are constantly being created.
If molecules with temporary dipoles get close enough to other molecules, they can create temporary dipoles in those molecules too. These are known as ‘induced dipoles’. In order for this to occur though, the molecules have to be very close together, no more than a few angstroms. An angstrom is a unit of measurement equal to 0.00000001cm. This is why glue being wet is important - so it can spread and flow to ensure this close contact. So, molecules in the adhesive can induce temporary dipoles in the molecules of the surface it is sticking to, increasing the strength of mechanical bonding.
This is as much as I’ve been able to dig up on the subject. If anyone has anything else to add, I’d be very interested to hear it!
References & Further Reading