A capacitive antenna for measurement of the dispersion relation of capillary waves on water

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Abstract
A noninvasive sensor has been developed to monitor the complex wave number of capillary waves on water (λ≊1 cm). It has been successfully employed to monitor surface tension and wave damping of varying surface conditions with surface tension resolution δγ≊0.25 mN m−1 and imaginary wave‐number resolution δki≊0.0055 cm−1. The principle of detection is based on the capacitance between an rf driven metallic plate in proximity to the water surface as modulated locally by the presence of surface water waves. Results are found to be in good agreement with a companion surface slope detection method. This new method is simpler, more portable, and measures the complex wave number directly whereas most conventional devices either do not measure both components of the wave number or rely on shallow or restricted water surface containers or measure only static surface tension and elastic modulus.