Chemistry Reference
In-Depth Information
Imaging surfactant concentration distribution at the
air/water interface
Part 1: Surfactant concentration gradient on a laminar
channel flow
Gerald M. Korenowski
1
, Elizabeth A. van Wagenen
1
and Amir Hirsa
2
1
Department of Chemistry
2
Department of Mechanical Engineering, Aeronautical Engineering, and
Mechanics, Rensselaer Polytechnic Institute, Troy, NY, USA
Abstract.
The surface specific spectroscopic probes of reflected second
harmonic generation (SHG) and reflected sum frequency generation (SFG)
have been successfully employed to measure surfactant monolayer concen-
tration gradients on the ocean surface. Reflected SHG was adapted for
areal measurements of monolayer concentrations using an intensified pulse
gated CCD camera for detection and a dichromatic image splitter to allow
for signal normalisation and scaling. As a preliminary test prior to field
use, the imaging probe was used to measure the surfactant concentration
gradients for an insoluble monolayer on a channel flow downstream of the
Reynolds ridge. The resulting concentration measurements had a temporal
resolution of 3 nanoseconds and spatial resolution of 0.21 millimetres or
less per pixel.
1 Introduction
Over the last thirty or so years, the ocean science and ocean engineering
communities have slowly come to recognise the importance of natural
ocean surfactants in modifying the physicochemical properties and pro-
cesses at the ocean/air interface. It is now the belief of many individuals in
these communities that all of the ocean surfaces should be viewed as sur-
factant influenced to some degree or another (Liss et al. 1997). This grow-
ing interest has stimulated a new generation of scientific research directed
at the ocean microlayer. Our own research is a part of this renewed scienti-
fic interest. We have worked at adapting reflected second-harmonic gene-
ration (SHG) and reflected sum-frequency generation (SFG) as nonintru-
sive
in situ
techniques for measuring surfactant concentration distributions
