The calming effect of oil on water has been known since ancient times. Benjamin Franklin was the first to investigate the effect, but the underlying mechanism for this striking phenomenon remains elusive. We used a miniature laser interferometer to measure the amplitude of surface waves to a resolution of , making it possible to determine the effect of an oil monolayer on the attenuation of capillary waves and the surface dilational modulus of the monolayer. We present attenuation data on pure water, water covered by olive oil, water covered by a fatty acid, and a water-acetone mixture for comparison. From the attenuation data at frequencies between 251 and , we conclude that the calming effect of oil on surface waves is principally due to the dissipation of wave energy caused by the Gibbs surface elasticity of the monolayer, with only a secondary contribution from the reduction in surface tension. Our data also indicate that the surface-dilational viscosity of the oil monolayer is negligible and plays an insignificant role in calming the waves.
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Franklin may be referring to the passage in Ref. 6 where the action of oil on waves is mentioned: (Ref. 6) “Again everybody is aware that all springs are colder in summer than in winter, as well as the following miracles of nature: that bronze and lead sink when in mass form, but float when flattened out into sheets; that heavy bodies are more easily moved in water; that sea is warmer in winter and saltier in summer; that all sea water is made smooth by oil, and so divers sprinkle oil on their faces because it calms the rough element and carries light down with them.”
Benjamin Franklin did not mention the kind of oil he used. However, the area covered by a teaspoonful of oil gives a footprint per molecule which strongly suggests that he used olive oil (Ref. 9).
As modern students of hydrodynamics are quick to note, this conclusion is false, except as far as Franklin acknowledged the reduced aerodynamics of a wrinkled surface. The initial waves are generated by pressure variations on the water’s surface (Ref. 13), and the oil layer actually drags a thin section of water with it as it spreads over the surface, coincidentally helping to cleanse the surface of contaminants (Ref. 14).
By this time, Franklin was a Fellow of the Royal Society. He had been elected a Fellow shortly after he was awarded the prestigious Copley Medal in 1754 for his work on electricity.
Horace Lamb observed that, even though the molecules might be enough to cover the surface in the quiescent state, the increase in surface area when waves are present could result in a lessened density of molecules across the surface and perhaps even exposed patches of water (Ref. 13). Because the olive oil is usually applied to waves in their most agitated state, the latter explanation seems unlikely.
Although the surface tension of the oil monolayer might seem high, this value represents the sum of the tensions in the oil-air and water-oil interfaces, which is what matters for wave formation. This value was obtained using the DuNuoy ring method and compares well with perturbative estimates from the dispersion relation.
