Have you ever wondered why glue does not stick inside its own tube?
Glue as we know comes out so nice and liquid and then sticks to any surface you apply it to. And then there is insta glue or the crazy glue that sticks seconds after it comes out of its tube. And it always ALWAYS sticks to your fingers and there is a horrible mess.
To answer these questions the VERITAS team travelled all the way to the Andes mountains, crossed the Amazon and came back to Noida and found the answer in the Scientific American ( Just joking).
Lets see how glues and adhesives actually work. There are two forces that are involved here: adhesion and cohesion. Cohesion is when molecules of the same substance bind together. Adhesion is when molecules of two different substance bind together. In glue both the forces are strong. So glue sticks to itself and to other surfaces too. And the binding properties of glue are because of both these properties. The glue binds to any surface and then binds togther with its own kind to form a strong bond.
Glue molecules are giant plastic polymers. Now there are two kinds of glues :
1) Normal ones( less sticky, used in offices etc): This glue has lots of giant plastic molecules floating about in water. So when you apply it to a surface and dry it then all the water evaporates. All that is left is the sticky glue molecules that sticks. So the point is that it contains water so you have to dry it to remove water.
This kind of glue does not stick inside the tube because it cannot dry inside the closed tube. However if you keep the tube open then the mouth of the tube dries and the lil bit of glue near the mouth will dry . And you will have to puncture it again.
2) Insta glue, crazy glue, super glue( very sticky, used to really STICK): This kind of glue is the one that sticks IMMEDAITELY on contact with a surface. This glue is usually made of lots of small molecules of ethyl-2-cyanoacrylate. THis molecule is a small one. But on coming in contact with water it IMMEDIATELY joins together to form a HUGE, LONG chain. i.e it polymerises on contact with water. Now all surfaces and air contain some moisture. So as soon as it comes out of the tube it polymerises to form a hard brittle substance, sticking anything it finds on the way. Inside the tube there is no moisture so no sticking.
Another reason which does not let glues stick to their tubes is that the insides of the tubes have a single charge on them and glues work best on polar substances( both charges + and – separated by a distance) .