What is Surface Tension?

Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as paper clips and insects (e.g. water striders) to float on a water surface without becoming even partly submerged.

Surface tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent, but when referring to energy per unit of area, it is common to use the term surface energy, which is a more general term in the sense that it applies also to solids.

Cohesive forces are those that hold the body of a liquid together with minimum surface area, like a water droplet, and adhesive forces are those that try to make a body of a liquid spread out. So if the cohesive forces are stronger than the adhesive forces, the body of water will maintain its shape, but if the opposite is true than the liquid will be spread out, maximizing its surface area. Any substance that you can add to a liquid that allows a liquid to increase its surface area is called a wetting agent.

Plastic is low surface energy and water is very high, so it has a tendency not to wet the surface, but instead form water droplets. This also effects the phenomenon of capillary action which allows a liquid to flow against other forces such as gravity. Think of water wicking up in a sponge. This may play a roll in the the ability for water to flow between the inner and outer welds in a Poly Tank.