BULK VISCOSITY COEFFICIENT OF A LIQUID AND ITS RELATION TO ABSORPTION AND DISPERSION OF ULTRASONIC-WAVES

A brief review of the bulk viscosity coefficient of a liquid is presented. This property which has its origin in hydrodynamics equations vanishes for monatomic fluids and has a large value for many polyatomic fluids. Being associated with normal forces between molecules, it accounts largely for the observed absorption and dispersion of a longitudinal wave passing through a liquid. Various theories of bulk viscosity have been discussed. Since the bulk viscosity coefficient becomes complex and frequency-dependent under the action of a high frequency ultrasonic wave it can be used to evaluate the structural relaxation time of the liquid, using experimental values of ultrasonic absorption and dispersion in the liquid. The distribution of acoustic relaxation times has been evaluated, keeping in view the actual behaviour of this coefficient. Comparisons between the Navier-Stokes equations for this complex and frequency-dependent property and other equations, confirm that the real part of this coefficient, which accounts for absorption, decreases steadily with frequency, while the imaginary part, which accounts for dispersion, exhibits a maxima at the relaxation frequency. Temperature and pressure variation of this property is discussed by considering variation of ultrasonic absorption and dispersion.with temperature and pressure. Ultrasonic absorption at very low frequencies is proportional to ultrasonic dispersion at very high frequencies. This concept of bulk viscosity of a liquid is also used to discuss the visco(bulk) elastic properties of a liquid. Various other theories of bulk viscosity have been briefly discussed. Acoustic streaming in a liquid is closely connected with the bulk viscosity coefficient of the liquid. Experimental determination of acoustic streaming which is closely connected with bulk viscosity coefficient and the possible role of bulk viscosity in understanding ultrasonic attenuation in metals and gases are briefly discussed.