Photoacoustic effect in a flowing liquid

It is found experimentally that the photoacoustic signal in a gas microphone cell depends on the velocity of a flowing liquid sample. The signal is constant up to a threshold velocity, above which there is a characteristic oscillatory behavior to the velocity dependence. The threshold velocity and characteristic oscillations vary systematically with both modulation frequency and the position of the incident beam. A simple theory which considers only the temperature dependence in the direction of fluid flow successfully accounts for the principal features of this velocity dependence. It is found that the cell boundaries are primarily responsible for the structure in the observed velocity dependence. Simple approximate expressions for the velocity dependence are derived, and the various simplifications made in the theory are discussed. The results have implications for in vivo photoacoustic measurements on a flowing blood stream.