Ask A Scientist
How come glue does not stick in the bottle?
Asked by: Patrick Kane
School: Seton Catholic Central High School
Hobbies/Interests: Soccer, computers and studying
Career Interest: Priest
Answer from Jim Clum
Professor Emeritus, Watson School of Engineering and Applied Science
Research area: Materials Engineering Interests/hobbies: Travel; volunteer Wisconsin public broadcasting & UW Arboretum; gardening
It is not always easy to see since most of the glue in the bottle pours out, but if you look very closely you will see that a thin layer of glue does stick to the walls of the container. You can see the same behavior when pouring honey from the container. There is a thin layer of honey still in the container after it has been emptied.
This phenomenon of \'sticking to the bottle\' walls is called "wetting". When a liquid like the white glue used for scrapbooks is poured out of the bottle and spreads on the scrapbook page we say the glue has "wet" the page. The opposite of "wetting" is seen when drops of a liquid form individual droplets on a surface. This is what we see when we pour a small amount of water onto the surface of a non-stick frying pan. The water forms small droplets on the non-stick surface instead of spreading evenly over the surface.
The process of "wetting" is controlled by the way the atoms or molecules of the surface of a solid material interact with the atoms or molecules of the surface of the liquid, which is poured onto the surface. Those exposed atoms on the solid and liquid surfaces are said to possess a certain amount of "surface energy". If the surface energy of the solid when covered by the air in the atmosphere is greater than the solid\'s surface energy when it is covered by the liquid then the liquid will "wet" the surface.
Let’s use some examples from real life to illustrate this process. The surface energy of a typical non-stick fry pan material exposed to air is about 15 units of surface energy. The surface energy of liquid water exposed to air is about 75 energy units (by the way when we talk about liquids we call the surface energy “surface tension”). So by pouring water onto the non-stick pan surface it makes the new air/water layer greater in energy than the old non-stick/air surface and nature prohibits that spreading from happening so the water forms little droplets instead of spreading. However,if we do the same experiment with water and an iron fry pan (surface energy in air about 1800 energy units) now we can lower the energy of the new combination by substituting water/air for the iron/air and we see the water spread on the iron surface.
So, what does this mean for the glue in the bottle? It means that to be ’good’ glue the glue must be a relatively low surface energy substance so that it will easily “wet” or spread on almost any surface. (Except, of course, surfaces made of a ‘non-stick’ material.) That means the glue really does stick to the walls of the glue bottle – even if it is only a thin layer.
By the way to get the last bit of the coating of honey out of the bottle it is best to fill the bottle with warm water which will dissolve the honey and that sweetened honey/water solution can be poured out of the bottle leaving a clean interior.