Laura Warburton, age 16, of Baltimore, Md., for her question:
CAN YOU EXPLAIN COHESION?
Cohesion is the force that holds a material together. Cohesion results from the attraction that all atoms and molecules have for one another. This attraction decreases greatly as the distance between the molecules of a substance increases.
With only a few exceptions, cohesion can be said to be highest in solids. Liquids have somewhat less cohesion than do solids and gases have much less.
Because of cohesion, effort is required to separate a material in two. This effort is called the work of cohesion. The work required to separate the material is twice its surface tension, because two new surfaces have been created.
Scientists can calculate the tensile or breaking strength of materials if they know the work of cohesion. These calculated strenght sare very high. Actually, however, may materials can be broken relatively easily.
Scientists believe this is so because of small cracks and other imperfections in solid materials. Glass fibers, as an example, have great resistance to breakage when first manufactured, but they develop fine cracks and lose strength rapidly.
High tensile strengths cannot be easily measured in liquids because the molecules flow when force is applied.
Surface tension is a force that causes the surface of liquids to behave in certain ways. It causes a liquid to behave as if a thin, elastic film covered its surface. An example, the surface of water can support needles and razor blades if they are placed there carefully.
Surface tension also causes liquid to rise in a thin tube when the tube is dipped in the liquid. This action is called capillarity. Surface tension is caused by cohesion because the force that causes the molecules of a substance to be attracted to one another is at work.
The molecules of a liquid that are below the surface have molecules pulling on them from all directions. But the molecules on the surface are attracted only by the molecules below and to their sides. The downward and eideward attraction of the molecules creates a constant pull on the surface molecules, causing surface tension.
Adhesion is the property of two unlike substances that causes them to stick together. Adhesion occurs because of the attraction between all molecules and atoms.
Adhesive strength varies, depending on the characteristics of the substances coming together. The adhesion between the surfaces of two solid substances tends to be low, even if they seem perfectly flat and clean. The surfaces are actually rough when viewed through a microscope and touch each other at relatively few places.
The strongest adhesives are applied as thin layers of liquids. As these solutions set, their molecules become immovable. The adhesion can be so good that a strong force will cause the substance, but not the adhesive bond, to break.