Determining liquid substrate cleanliness using infrared imaging

Experiments conducted in a modified Langmuir trough are described. Measurem ents of surface tension were obtained using a Wilhelmy plate, while infrare d (IR) images of the water surface adjacent to the Wilhelmy plate were simu ltaneously recorded. A contaminating surfactant film was allowed to form on the surface while the experiments were being conducted. The data reveal a substantial change in the IR imagery due to the film in all cases. The diff erence between the appearance of the clean and surfactant-covered regions o f the surface is clear and distinct. For some of these experiments the chan ge in the IR imagery is accompanied by no observable change in surface tens ion. This, along with several other aspects of this work, suggest that IR i magery may be a superior tool, compared to traditional surface tension meas urements, for ascertaining the cleanliness of a liquid substrate. (C) 2001 American Institute of Physics.