Determination of the nonlinearity parameter of the liquid using spherical finite-amplitude photoacoustic pulse

A method for measurement of the nonlinearity parameter B / A of liquids by means of a spherical finite-amplitude photoacoustic pulse is described. The photoacoustic pulse is generated by the laser-induced breakdown mechanism. The relation between B / A and the decrement of rise time of wavefront of the acoustic pulse during its propagation is given. The nonlinearity parameter of water is obtained by the present method. 1 .INTRODUCTION The existing methods for determination of the nonlinearity parameter B / A of liquids can be separated into two categories: thermodynamic method [I] and finite-amplitude acoustic wave method. The latter can be separated further into harmonic wave method [2] and pulse method. Karabutov et a1 [3] and Bozkhov et a1 [4] used a plane pulsed acoustic wave with bounded beam and determined the nonlinearity parameter of liqu~ds from the distortion of its waveform during propagation. However, in this case, the diffraction of plane wave with bounded beam influences the waveform also. Sigrist et al[5] used two laser pulses with different intensities to excite successively two plane acoustic pulses with different peak pressures in the liquid and recorded their waveforms at the same distance. From the d~fferences of the peak pressures and the rise times of wavefronts of two acoustic pulses, they determined B / A ratio. Although in this case the diffraction effect may be avoided, two acoustic pulses with different peak pressures must have the same rise time initially in order to ensure that the difference of rise times of two pulses received at the same distance is caused by the nonlinearity of the liquid. But it is difficult in practice usually. A method for determination of the nonlinearity parameter of the liquid using a spherical photoacoustic pulse is reported in this paper. The finite-amplitude acoustic pulse is generated in the liquid by a focused laser pulse via the breakdown mechanism. From the variation of the rise time of wavefront of the propagating acoustic pulse, the B / A ratlo of the liquid is Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19947179 JOURNAL DE PHYSIQUE IV determined. 2.METHOD When the propagation distance r IS in the cm range and the sound absorption coefficient of the liquid is not very large, the effect of sound absorption can be neglected in general and the liquid may be considered as a lossless fluid. When a spherical finite-amplitude acoustic pulse propagates in such fluid, the velocity of its wave peak may be approximated by the following expression: where C , is the phase velocity of the infinitesimal acoustic wave; p, is the density of the liquid in the equilibrium state; A,=r,p, and p, is the peak pressure at a certain reference distance r,. The propagation time of the wave peak from r, to r is then given by B A where a = (G + l)*and t o = ( r r o ) / C o . Obviously, the decrement of the rise time of its wavefront is At = t o t . Under condition a / C o < < r o , the B / A ratio can be expressed as follows: