Chapter 2 — Nucleation
How the liquid finds a hole to break through.
The previous chapter ended with a three-orders-of-magnitude gap between the theoretical tensile strength of water ( atm) and the measured tensile strength of a routine sample ( atm). This chapter closes the gap.
The resolution is that measured tensile strength almost never reflects the bulk cohesive limit. It reflects the threshold of whatever nucleation site — preexisting weakness, dissolved gas, particulate impurity, surface-trapped gas pocket — happens to be weakest in the sample. We develop the relevant physics in four lessons:
- 2.1 Homogeneous nucleation theory — the Gibbs free-energy barrier for a vapour bubble appearing by thermal fluctuation alone. Reproduces the atm theoretical limit and confirms it is irrelevant for any practical sample.
- 2.2 Heterogeneous nucleation and the Harvey crevice model — how a gas pocket trapped in a hydrophobic surface crevice serves as a long-lived, ready-made nucleation site at modest tensions.
- 2.3 Nucleation site populations — the distribution of preexisting nuclei in a liquid sample, the role of dissolved gas in stabilising nuclei against dissolution, and what is meant by cavitation susceptibility.
- 2.4 Nucleation in flowing liquids — the cavitation inception number that engineers use to characterise where in a flow cavitation begins.
By the end of the chapter the puzzle of the pathetic measured strength will be resolved into a coherent picture of the cavitation threshold of a real sample, and we will be ready to follow what happens after a bubble appears — the subject of Chapter 3 — the Rayleigh–Plesset equation.