= Capillary wave
{wiki}
A <capillary wave> is a <wave> along a <fluid> interface whose restoring stress is predominantly supplied by <surface tension>. A curved <free surface> requires a <pressure> jump by the <Young–Laplace equation>; increasing its corrugation also costs <surface energy>. In the ideal limit, inertia carries the interface through its flat position and the resulting <oscillation> propagates. When <gravity> also contributes appreciably, use the combined <gravity-capillary wave> model. Within the <capillary wave> regime, shortening a ripple makes it travel faster. https://davidtong.org/pdfs/teaching/fluid-mechanics/fluids3.pdf[David Tong, Fluid Mechanics, §4.1.3: the capillary restoring pressure]{ref}
The <laser-diffraction measurement of capillary waves> below turns this mechanism into a test: set the <frequency>, measure the <wavelength> independently with <light>, and ask whether the <dispersion relation> follows. The theory is useful both for predicting how ripple <wave packets> separate and for inferring <surface tension> from their motion.
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