Chemistry Reference
In-Depth Information
less) for solid state Q-switched lasers (high peak intensities needed to gen-
erate the SHG and SFG signals) coupled with the one to two metre per
second speeds (slowest speed while maintaining steerage) of a research
vessel, limit surface resolution to about a centimetre. While this is excel-
lent for general surface profiling, millimetre or finer spatial resolution is
needed to resolve variations in surfactant concentrations on the surface of
capillary-gravity (CG) waves and capillary waves. Such measurements are
important since surface tension gradients, interfacial viscosity, and interfa-
cial elasticity strongly affect the CG and capillary waves, and wind-wave
coupling and wave damping are dramatically altered for these waves when
there are small changes in the interfacial chemical composition at the air-
ocean interface.
The system, which we presented in Part 1, was an imaging detection ap-
paratus for use with the reflected SHG and SFG probes. It was applied to
the measurement of a gradient in an insoluble surfactant monolayer on a
laminar channel flow. A two-dimensional surfactant gradient map was ob-
tained (0.2 millimetre or better spatial resolution) from this flat, surfactant-
covered surface. In this paper, we apply this reflected SHG imaging tech-
nique to the measurement of surfactant concentrations at the crest and
trough of a standing capillary wave field. It is expected that insoluble sur-
factant monolayers will undergo compression and dilation on a capillary
wave surface (e.g. MacIntyre 1971). The expansion of the compressed por-
tion of the monolayer and the contraction of the dilated portion give rise to
additional interfacial forces instrumental in wave damping and the reduc-
tion of wind-wave coupling (Levich 1962). In this paper we present a di-
rect measurement of this capillary wave induced compression and dilation
of an insoluble monolayer.
2 Experimental
Measurements of surfactant concentrations on travelling capillary waves is
complicated by the rapid decay rate of these waves, necessitating measure-
ments close to the source of wave generation. To avoid this complication,
we utilized a field of standing capillary waves. The wave tank was a cir-
cular (6.99 cm, inner diameter) glass vessel. The inner wall was coated
with paraffin to avoid loss of the surfactant to the tank side walls. Triply
distilled water was used as the substrate. The tank was overflowed to clean
the surface prior to spreading the insoluble hemicyanine surfactant mono-
layer at a surface concentration of 0.288 Pg cm
-2
. Hemicyanine, 4-[4-
(dimethylamino)styrl]-1-docosyl-pyridinium bromide, is a stilbazolium
dye molecule to which is attached on one end a saturated twenty two car-
