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dimensional coils which can be more compressible, so that the film elastic-
ity decreases.
5 Conclusions
Laboratory measurements of the damping coefficient and wavelengths
of cm-mm-scale surface waves due to ordinary surfactants (oleic acid and
oleyl alcohol) and polymer (polyoxyalkylene glycol - “Emkarox”) organic
films in a wide concentration range and at different wave frequencies are
carried out using a method of paramertically excited waves.
It is shown that the dependencies of the damping coefficient on surfac-
tant concentration are quite different for the ordinary surfactants and for
the polymer films. For the ordinary surfactants the damping coefficient
achieves a maximum at concentrations of the order of the concentration of
saturated monolayers; at higher concentrations the damping coefficient is
high and practically constant. For the polymer films the damping coeffi-
cient has a low-concentration peak similar to the case of ordinary surfac-
tants and a "plateau"-like maximum at high concentrations.
The dynamic elasticities of the ordinary and polymer films are estimated
from the measured damping coefficient using a theory of wave damping
for purely elastic films. It is obtained that the elasticity of the ordinary sur-
factants at the concentrations larger that the saturated monolayer concen-
tration is quite large and does not depend on concentration, while the static
elasticity is practically zero in this concentration range. This can be ex-
plained by the fact that the ordinary surfactant films at large concentrations
represent a saturated monolayer with microscopic drops. The drops weakly
affect the wave damping and corresponding dynamic film elasticity in this
case is as large as for saturated monolayers.
The estimated dynamic elasticity of the polymer film also differs from
the static elasticity. The dynamic elasticity increases with concentration,
achieving a maximum at intermediate concentrations (between the two
damping coefficient maxima), and then decreases monotoniously to small
values at large concentrations.
Acknowledgements.
The authors thank Dr. J. Scott for his valuable re-
marks and discussions of this work. The work was supported by the UK
Ministry of Defense through the DERA, Winfrith, UK, via the Interna-
tional Science and Technology Center (Project 1774p), by INTAS (Project
03-51-4987 “SIMP”), and RFBR (projects 05-05-64137, 04-05-64763).
