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 ±5nm, 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 551Hz, 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.

1.
J.
Mertens
, “
Oil on troubled waters: Benjamin Franklin and the honor of Dutch seamen
,”
Phys. Today
59
(
1
),
36
41
(
2006
). This issue of Physics Today focuses on Benjamin Franklin in celebration of the tricentennial of his birth.
2.
For an excellent historical account of Franklin’s scientific experiments and observations see
Charles
Tanford
,
Ben Franklin Stilled the Waves: An Informal History of Pouring Oil on Water with Reflections on the Ups and Downs of Scientific Life in General
(
Duke U. P.
,
Durham
,
1989
). Most of the historical notes in this article come from this text and references therein.
3.
B.
Franklin
, “
Of the stilling of waves by means of oil
,”
Philos. Trans. R. Soc. London
64
,
445
460
(
1774
).
4.
Reference 2, p.
69
.
5.

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.”

6.
Pliny the Elder
,
Natural History
, translated by H. Rackham (
Harvard U. P.
,
Cambridge, MA
,
1938
), Vol.
1
, p.
361
.
7.
Reference 2, p.
71
.
8.

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).

9.
Reference 2, p.
118
.
10.
Reference 2, p.
72
.
11.
R.
Seeger
,
Benjamin Franklin: New World Physicist
(
Pergamon
,
Oxford
,
1973
), pp.
59
60
.
12.

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).

13.
H.
Lamb
,
Hydrodynamics
, 6th ed. (
Dover
,
New York
,
1945
), p.
630
.
14.
K. S.
Birdi
,
Lipid and Biopolymer Monolayers at Liquid Interfaces
(
Plenum
,
New York
,
1989
), p.
142
.
15.

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.

16.
Neil K.
Adam
,
The Physics and Chemistry of Surfaces
(
Oxford U. P.
,
London
,
1930
), p.
91
.
17.
Lord
Rayleigh
, “
Measurements of the amount of oil necessary in order to check the motions of camphor upon water
,”
Proc. R. Soc. London
47
,
364
367
(
1889–1890
).
18.
Agnes
Pockels
, “
Surface tension
,”
Nature (London)
43
,
437
439
(
1891
).
19.
Reference 2, p.
168
.
20.
O.
Reynolds
, “
On the effect of oil in destroying waves on the surface of water
,”
Br. Assoc. Adv. Sci., Rep.
50
, pp.
409
425
(
1880
).
21.

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.

22.
Reference 2, p.
170
.
23.
W. M.
Klipstein
,
J. S.
Radnich
, and
S. K.
Lamoreaux
, “
Thermally excited liquid surface waves and their study through the quasielastic scattering of light
,”
Am. J. Phys.
64
,
758
765
(
1996
).
24.
F.
Behroozi
, “
Fluid viscosity and the attenuation of surface waves: A derivation based on conservation of energy
,”
Eur. J. Phys.
25
,
115
122
(
2004
).
25.
For a more thorough discussion of this effect, see
E. H.
Lucassen-Reynders
and
J.
Lucassen
, “
Properties of capillary waves
,”
Adv. Colloid Interface Sci.
2
,
347
395
(
1969
).
26.
F.
Behroozi
and
N.
Podolefsky
, “
Dispersion of capillary-gravity waves: A derivation based on conservation of energy
,”
Eur. J. Phys.
22
,
225
231
(
2001
).
27.
F.
Behroozi
and
N.
Podolefsky
, “
Capillary-gravity waves and the Navier-Stokes equation
,”
Eur. J. Phys.
22
,
587
593
(
2001
).
28.
F.
Behroozi
and
A.
Perkins
, “
Direct measurement of the dispersion relation of capillary waves by laser interferometry
,”
Am. J. Phys.
74
,
957
961
(
2006
).
29.
F.
Behroozi
,
B.
Lambert
, and
B.
Buhrow
, “
Direct measurement of the attenuation of capillary waves by laser interferometry: Noncontact determination of viscosity
,”
Appl. Phys. Lett.
78
,
2399
2401
(
2001
).
30.

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.

31.
Feredoon
Behroozi
and
Peter S.
Behroozi
, “
Efficient deconvolution of noisy periodic interference signals
,”
J. Opt. Soc. Am. A
23
,
902
905
(
2006
).
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