Propagation of a longitudinal stress wave through a visco-elastic media creates dissipation of mechanical energy, similarly to a shear stress wave. The rate of dissipation depends on two parameters: bulk viscosity and longitudinal viscosity. For Newtonian liquid it is bulk viscosity that is analog to the dynamic viscosity for the shear stress. For non-Newtonian liquid it is longitudinal viscosity, analog to the shear viscosity for the shear rheology.
Measurement of ultrasound attenuation is the only known way of characterizing these parameters for both Newtonian and non-Newtonian liquids. Our Acoustic sensor allows very precise measurement of the ultrasound attenuation. It is important to perform this measurement at different frequencies. Frequency dependences allow resolving between bulk viscosity and longitudinal viscosity.
Bulk viscosity is important for verification of molecular theories of Newtonian liquids. We published a paper that reports the value of this parameter for 12 different Newtonian liquids. This paper describes methodology of such study. It can be simply reproduced with any other Newtonian liquid. We show that bulk viscosity is an independent property of liquid with practically no correlation with other parameters, such as dynamic viscosity, dielectric permittivity, or compressibility.
Longitudinal viscosity yields information about relaxation times in non-Newtonian liquids at MHz range. These high frequencies are not accessible with traditional shear rheology. That is why longitudinal rheology is complimentary to the traditional shear rheology. Longitudinal frequency spectra can be used similarly to the shear viscosity - shear rate dependence, but at much higher frequencies.
One of the important advantages of our longitudinal rheology measurement is ability to use it online for continuous non-destructive monitoring of the industrial process. This function is much harder to implement with traditional shear rheology.
For more details read:
Dukhin, A.S. and Goetz, P.J. " Bulk viscosity and compressibility measurement using acoustic spectroscopy," The Journal of Chemical Physics, Vol.130, Issue 12, (2009)