Chemistry Reference
In-Depth Information
Monomolecular surface films change the structure of the uppermost water
layer within a thickness of around some micrometers to possibly some
hundred micrometers (Hühnerfuss and Alpers 1983, Hühnerfuss 1986).
For example, “ice-like” clathrate structures are induced by OLA films in a
water layer of d d 190 Pm. Furthermore, the surface potential of pure water
of about - 180 mV becomes positive and may approach values of t 400
mV (Gericke and Hühnerfuss 1989), and the dilational surface viscosity is
drastically increased (Hühnerfuss 1985). The relaxation time for disturb-
ances of the surface film order W
comp
attains values of around 10 to 20 min,
which are about 10
13
times larger than the relaxation time W
s
of the water
molecules (Hühnerfuss and Alpers 1983).
Wave-induced compression and dilation of a monomolecular surface
film lead to concentration gradients, which in turn give rise to surface ten-
sion and surface potential gradients (Hühnerfuss 1986, Lange and Hühner-
fuss 1984). A viscoelastic surface film changes the free surface boundary
condition in the tangential direction, i.e., longitudinal waves are formed,
and it thus strongly modifies the flow pattern in the boundary layer. As a
consequence, wave energy is dissipated by enhanced viscous damping in
the short-gravity-wave region due to large velocity gradients induced in
the viscous boundary layer. This phenomenon is called the
Marangoni ef-
fect
. Marangoni damping denotes resonance-type wave damping of water
waves in the short-gravity-wave region, which is connected with the fact
that viscoelastic surfaces can carry two kinds of waves, the well-known
gravity-capillary waves and the Marangoni waves. The latter waves are
predominantly longitudinal waves in the boundary layer which are heavily
damped by viscous dissipation. When these two waves are in resonance,
the surface waves experience maximum damping. The fact that the Maran-
goni waves are strongly damped is the reason why these waves have long
escaped detection. Their existence was verified experimentally only in
1968. A detailed presentation of the theory is given in Hühnerfuss (1986)
and Alpers and Hühnerfuss (1989). In summary, water wave damping by
monomolecular sea slicks can be attributed to the Marangoni effect and to
slick-induced modification of nonlinear wave-wave interaction, by means
of which wave energy is transferred from the longer waves to the energy
sink in the Marangoni resonance region.
By the way of contrast, a water surface covered with a freshly spilled
crude oil exhibits quite different physicochemical characteristics. No sur-
face tension gradients will be generated by the undulating water surface
and thus the Marangoni effect plays no role. On the other hand, the crude
oil layer may also dampen short gravity waves due to its relatively high
viscosity as compared with a pure water surface.
