Abstract
Pulsed laser irradiation of a weakly absorbing fluid sphere in a transparent medium results in the production of a large thermal gradient at the surface of the sphere. The rapid transfer of heat from the sphere to the surrounding fluid as a result of the thermal gradient generates high frequency photoacoustic transients which affect the leading edge of a photoacoustic wave. Here, the character of the photoacoustic wave is determined by solving a modified wave equation for the photoacoustic effect. A solution to the heat diffusion equation is determined, which, together with the heating function for the optical source, provides the source term for the wave equation for pressure. The wave equation is then solved with appropriate boundary conditions using Laplace transform techniques to give the photoacoustic waveform. The relative magnitude of the transient to the N-shaped wave is shown to be determined, in part, by the laser pulse length.
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Notes
For water at 20 Celsius, \(\gamma \) differs from unity by less than 1 %. For organic liquids the approximation is not so good. For instance for toluene \(\gamma -1=0.35.\)
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Acknowledgments
The authors are grateful to the US Department of Energy for support of this research under Grant ER16011.
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Frez, C., Diebold, G.J. Thermally Induced Photoacoustic Transients Produced by Laser-Irradiated Fluid Spheres. Int J Thermophys 35, 2171–2177 (2014). https://doi.org/10.1007/s10765-013-1558-0
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DOI: https://doi.org/10.1007/s10765-013-1558-0