Surface tension

Surface tension is the elastic tendency of a fluid surface which makes it acquire the least surface area possible. Surface tension allows insects (e.g. water striders), usually denser than water, to float and stride on a water surface.

At liquid-air interfaces, surface tension results from the greater attraction of liquid molecules to each other (due to cohesion) than to the molecules in the air (due to adhesion). The net effect is an inward force at its surface that causes the liquid to behave as if its surface were covered with a stretched elastic membrane. Thus, the surface becomes under tension from the imbalanced forces, which is probably where the term "surface tension" came from. Because of the relatively high attraction of water molecules for each other, water has a higher surface tension (72.8 millinewtons per meter at 20 °C) compared to that of most other liquids. Surface tension is an important factor in the phenomenon of capillarity.

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.

Surface Tension (short story)

"Surface Tension" is a science fiction short story by James Blish originally published in 1952. As collected in Blish's The Seedling Stars, it was revised to incorporate material from his earlier story "Sunken Universe", published in Super Science Stories in 1942.

Plot summary

A human colonization ship crash-lands on a distant planet which is earth-like but whose only landmass is completely covered in shallow puddles of water and mostly microscope life forms. Normal humans could not survive on this planet, so the crew must genetically engineer their descendants into something that can survive. (Blish coined the term pantropy to refer to this concept.) They decide to create a race of microscopic aquatic humanoids to complete their mission and colonize the planet.

The majority of the story concerns one groups of these genetically engineered colonists and their intelligence, curiosity, and evolving technology. In particular, the tiny aquatic humanoids develop a "space ship", or rather "air ship", which enables them to pierce the previously impenetrable surface tension of the water and travel through what is, to them, hostile space—open air—to other worlds in other puddles of water.