Chemistry Reference
In-Depth Information
tank walls by the vertical shaking motion creating an inward travelling cir-
cular wave that is reflected at the centre (now an outward travelling wave)
with the net result a quasi-circular standing wave field. These edge devia-
tion effects are not discernable in the outer portions of the field near the
tank edge but are magnified near the centre of the tank where the wave
field is focused. This effect appears as a distorted central crest (or trough).
Nevertheless, image after image shows a consistent concentration increase
for a centre crest and a concentration decrease for a central trough albeit
with a large confidence limit. Consequently, we believe the measurement
is capturing the correct change in concentration for this region. The slight
distortion in the wave field may also account for slight deviations from the
expected absolute surfactant concentrations measured in moving from the
centre of the field outward. The overall trends recorded are, however, con-
sistent with the insoluble monolayer under compression at the wave crest
and dilated at the wave trough. In future experiments we will expand the
incident probe laser beam to encompass more of the wave field in the im-
aging.
The existing measurements of the crest and trough surfactant concentra-
tions are precise and accurate enough to demonstrate that the reflected
SHG imaging technique can measure capillary wave generated surfactant
concentration gradients. The measurements obtained in these experiments
will be subjected to a comparison with theoretical predictions in a future
publication. In addition, experimental work on the technique and its appli-
cation to capillary wave induced surfactant concentration gradients con-
tinues as well as work adapting the method for
in situ
ocean work.
4 Conclusions
We have demonstrated that capillary waves do induce concentration gra-
dients for an insoluble surfactant monolayer on a water surface. In the ex-
periments described above, reflected SHG imaging was shown to provide a
direct measurement of these surfactant concentration gradients with a tem-
poral resolution of 3 nanoseconds. As far as we know, this is the first such
direct, nonintrusive measurement of these monolayer concentration gradi-
ents on a capillary wave field which does not require mechanical contact
with the system under study. Elegant surface potential measurements were
used in similar experiments but this latter technique requires part of the
measuring device to be located under the water and in close proximity to
the water surface that may affect the subsurface motion associated with the
wave (Hühnerfuss et al. 1985, Hühnerfuss et al. 1985, Lange et al. 1986).
In these surface potential measurements, a travelling wave was used which
