ASK A SCIENTIST
Question: How come glue does not stick in the bottle?
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. 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. The water forms small droplets on the non-stick surface instead of spreading evenly over 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"). 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.)
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