Although a fundamental physical parameter, surface tension is difficult to measure. Common tensiometry inaccuracy comes from failure to control air-liquid-solid contact conditions, or account for liquid meniscus geometry and buoyancy corrections. This paper describes an in situ tensiometry technique, based on withdrawal of a thin-walled tube from the liquid interface, that enforces a known air-liquid-solid contact condition. This technique can be pursued at any level of experimental hygiene. Experimental results for filtered tap water, an alcohol-water solution, and a surfactant-water solution show that results repeatable to three significant digits are obtained with modest effort for a variety of geometrical parameters.

Naturally occurring soluble-surfactant slicks in¯uence the properties of water waves. This paper describes results from wave tank experiments involving a soluble surfactant, and linear and nonlinear gravitycapillary waves. Instantaneous surface de¯ections were measured using optical techniques to determine the damping, phase speed, and the frequency content of the waves for wavemaker frequencies from 4 to 22 Hz. Measured linear-wave phase speed and damping agree well with existing theory at surfactant concentrations away from that leading to maximum damping. Under conditions leading to nonlinear waves, an as-yet-unexplained subharmonic wave with one-sixth the wavemaker frequency was found only with soluble surfactant present.

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