IR Imaging Studies Solid-Phase Monolayers on Water Surfaces
Technique visualizes polluting surfactants.
Daniel S. Burgess
To promote a better understanding of the environmental effects of pollutants, researchers at Clemson University in South Carolina have developed an infrared imaging method that visualizes solid-phase surfactant monolayers on water. They envision its use in the airborne detection and monitoring of effluents such as are produced in agriculture.
A series of 25 × 25-cm infrared images of the 25-μm-thick layer beneath the surface of a tank of water exposed to an airflow of 2 m/s reveals the presence and breakup of a solid-phase monolayer of stearic acid. The imaging technique may be suitable for the airborne detection and monitoring of pollutants.
Monolayers on the surface of water, the scientists note, affect natural processes by altering how heat, water and soluble gases are transferred across the boundary between the atmosphere and lakes, rivers and the oceans. Despite the success of various approaches, including optical ones, for the characterization of monolayers on small scales, they cite no satisfactory macroscopic techniques and none involving IR imaging for the study of solid-phase monolayers.
The new method images the characteristic temperature fields that are produced just below the water surface depending on the conditions at the air/water boundary. In the absence of a surfactant monolayer, heat transfer and/or evaporative cooling drive convection, causing warmer, less dense water to rise to the surface in plumes between volumes of sinking cooler, denser water. The presence of a surfactant monolayer damps those processes and the convection, resulting in the elimination of small-scale variations in the temperature field and in a different thermal signature.
The investigators demonstrated the IR technique in the laboratory by imaging the effects of a 24.4-Å-thick layer of solid-phase stearic acid on a 1003 × 250 × 380-mm tank of warm tap water. They employed a 252 × 238-pixel CCD camera from Inframetrics Inc. (now part of Flir Systems Inc. of Wilsonville, Ore.) to collect images of the 25-μm-thick layer of water under the monolayer at 3 to 5 μm. To simulate natural conditions and to monitor the stability of the monolayer, room-temperature air was blown across the water tank at 2 m/s.
The experiments confirmed that the approach distinguishes regions of water that are covered with a solid-phase monolayer and those that are not. The scientists further illustrated that it enables the visualization of the disintegration of a monolayer under wind shear in a process that they compare to the calving of icebergs from the polar ice pack.
They note that further research will be necessary before the technique may be suitable for real-world environmental monitoring. Although their experiments with liquid-phase surfactants suggest that small waves and ripples in the water do not adversely affect the imaging of the subsurface temperature fields, it remains to be established that this is the case when solid-phase monolayers are present.
Langmuir, Aug. 1, 2006, pp. 6881-6886.
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