Investigation of Thermal Contact by the Photoacoustic Effect

We are studying the application of the photoacoustic effect to the measurement of small concentrations of aerosols deposited on a surface. Our investigation has been essentially focused on the detection of asbestos (chrysotile) fibers collected on a membrane filter. Detection is made possible by a sharp absorption band of the mineral at 2.7 μm and is improved by fiber alignment. The smallness of the fiber diameters with respect to the wavelength insures that the heat source generated by infrared absorption, is proportional to the mass of collected dust. However, the proportionality between the observed photoacoustic signal and the mass may be affected by the dependence of heat transfer to the subtrate upon deposition conditions. This effect is rather difficult to investigate with a low concentration of collected dust and a cell of minimum volume required for convenient detection: when the frequency of modulation is varied, interfering effects, such as another weak absorption or heat diffusion to the cell walls, are likely to complicate the interpretation. Therefore, we have used a very different set-up which is described below. We have also oriented our investigation towards the general study of the effect on the photoacoustic signal of an imperfect thermal contact between a thermally thin sample and its substrate. Results obviously can be applied to the understanding of the detection of collected aerosols, but also to other problems such as non-destructive adherence testing.