INVESTIGATION OF VISCOSITY AND HEAT-CONDUCTION EFFECTS ON THE EVOLUTION OF A TRANSIENT PICOSECOND PHOTOACOUSTIC GRATING

The production of acoustic waves by the absorption of radiation, known as the photoacoustic effect, is described by two coupled differential equations for pressure and temperature. Here, solutions to the coupled equations are given for sinusoidal deposition of heat in space, as is typically generated using two picosecond laser beams to form a transient grating. The solutions to the coupled equations, along with the equation of state of the fluid, gives the pressure, temperature, and density of the fluid following rapid deposition of heat. The time dependence of the grating diffraction efficiency is readily calculated from the modulation of the density and temperature. For small values of the viscosity and heat conduction parameters, closed form expressions for the state variables are given which describe thermal and acoustic modes of wave motion in the fluid. The method is applied for computation of the state variables using a frequency-dependent viscosity. Experiments with ethylene glycol are reported where the viscosity and sound speed are measured as functions of temperature.