Chapter 1 — The liquid state

What a liquid is, and why it can in principle hold tension.

Cavitation is the appearance of a vapour-filled hole in a liquid that is under tension. To understand why such a hole appears at the tensions it does — and not at the much larger tensions the liquid could in principle withstand — we need a careful picture of the liquid state itself.

The sound book treated water as an effectively incompressible continuum with a single bulk modulus and a single speed of sound. That picture is adequate for everything sound does inside the linear regime. It is not adequate for what we want to do here. A liquid pulled hard enough will eventually fail, and the manner of its failure depends on the molecular-scale structure of the liquid and on a population of preexisting defects in it that the continuum picture does not see.

Three lessons:

• 1.1 What a liquid is — the microscopic picture: molecules in soft contact, the Lennard-Jones potential, why a liquid is neither a dense gas nor a soft solid.
• 1.2 Tensile strength in principle — the theoretical cohesive limit of pure water (about $-10^3$ atm).
• 1.3 Why the measured strength is pathetic — what real labs actually measure, and the nucleation puzzle that motivates the next chapter.