The Gravity of Small Things How Tiny Forces Rule the Insect and Droplet World

Drop a mouse down a thousand-yard mine shaft, and when it reaches the bottom it gets a slight shock and walks away, provided the ground is fairly soft. A rat dropped down the same shaft is killed. A man is broken. A horse splashes.

That is how the biologist J. B. S. Haldane put it in his essay "On Being the Right Size," and it is one of the most quoted lines in the history of biology. The reason behind it is plain geometry. Haldane asked his readers to take an animal and divide its length, breadth, and height each by ten. Its weight falls to a thousandth of what it was. The surface of its body falls only to a hundredth. The small animal is much lighter for its surface area, so the air pushing against that surface counts for far more than gravity pulling on that weight.

This idea is often called the square-cube law, and it goes back to Galileo. Double every linear dimension of an object, and its surface area grows fourfold while its volume and weight grow eightfold. Run it the other way and shrink the object, and weight shrinks much faster than surface. Keep shrinking, and something strange happens. Gravity, the force that organizes our whole lives, begins to lose its grip on the rules. Other forces that were always present but easy to ignore step forward and take command.

This episode is about those forces. One is surface tension, which acts like a skin on water. Another is the adhesion between touching surfaces. A third is viscosity, the thickness of fluids that makes air and water feel like syrup to a small enough creature. Together they let an insect stand on a pond, let a lizard walk across a ceiling, and let a beetle drink from fog. And now engineers are learning to build with them.