Bulk viscosity is a somewhat obscure parameter that appears in the hydrodynamic equations for Newtonian liquids when compressibility is important and, together with the dynamic viscosity, controls sound attenuation. Whereas dynamic viscosity reflects only “translational” molecular motion, in contrast the bulk viscosity reflects the relaxation of both “rotational” and “vibrational” degrees of molecular freedom. Several molecular theories yield predictive expressions for both bulk and dynamic viscosities, but experimentally the situation is quite out of balance, in that there is extensive data for the dynamic viscosity of all sorts of liquids, but a paucity of data for bulk viscosity, just a few values for water and a handful of exotic liquids. We compare three possible experimental techniques for measuring bulk viscosity, namely, Brillouin spectroscopy, Laser transient grating spectroscopy, and acoustic spectroscopy. We then formulate some arguments suggesting that acoustic spectroscopy is not only the most suitable for measuring bulk viscosity, but that it also offers a verification procedure that can confirm that the measured parameter agrees with theoretical definition of bulk viscosity for a Newtonian liquid. In addition, acoustic spectroscopy provides a measurement of sound speed, which cannot only improve the attenuation measurement but as a side benefit can also be used to calculate liquid compressibility. We apply this technique for measuring the bulk viscosity and compressibility of twelve commonly assumed Newtonian liquids, two of which surprisingly fails to pass a verification test described here to test the Newtonian hypothesis. Then, we test correlation between measured bulk viscosity and several other intensive properties of these liquids, such as density, dynamic viscosity, dielectric permittivity, and compressibility. We have not discovered any meaningful correlation. This suggests that bulk viscosity is an independent parameter that reflects peculiar properties of liquids and can be used in the set of independent equations describing molecular interaction in liquids.

It has been known for a long time that acoustic measurements offer some unique features for characterizing liquid food products in their intact state, without any preparation or destruction of the product sample. Acoustic characterization can yield information about fat content, droplet size distribution, and kinetics of product variation with time. Furthermore, acoustic methods are very attractive for on-line process control. This paper addresses several questions: Why does ultrasound attenuate when propagating through a heterogeneous system? What properties of dairy products can be extracted from such ultrasound measurements? Which measurement is better for product characterization: attenuation or sound speed? What measurement precision is required to adequately characterize product properties? What frequency range is of most value for determining these product properties?It is possible to provide answers to many of these questions using experimental data, thereby avoiding, for the moment, any complex mathematical analysis. We present several applications of acoustic spectroscopy for characterizing dairy products, including characterization of the fat content in a wide variety of dairy products; calculation of the fat droplet size distribution in milk without dilution; and calculation of water droplet size in butter, without dilution or melting.