Rheology is the study of deformation and flow caused by applied stresses. In solid bodies, as well as in liquids, stresses can act in both tangential and normal directions. In certain cases both tangential and normal stresses are present simultaneously. However, in rheometric devices designed for measuring both stresses and deformation history, stresses are typically applied in either tangential (shear) or normal (extension/compression) direction. This leads to either shear or extensional rheology.
These two types of rheological measurements can provide significantly different information about the material. For instance, many polymeric liquids have much higher viscosities in extension than in shear.
There is a general overview of relationship between shear and extensional rheologies in the book: Dukhin, A.S. and Goetz, J.P. “Characterization of Liquids, Nano- and Microparticulates, and Porous Bodies using Ultrasound”, Edition 3, Elsevier, 571 pages, 765 references, (2017).
Despite the recognized importance of extensional measurements, there are relatively few extensional studies described in the literature primarily because it is difficult to generate homogeneous extensional flows, especially with low viscosity liquids. This problem can be easily resolved with ultrasound. Actually ultrasound propagating through the media is a wave of oscillating stress that deforms media. Therefore Acoustic measurement is Rheological in its nature. This type of “extensional rheology” is called “longitudinal rheology”.
Longitudinal rheology yields information on processes with much shorter relaxation times than shear rheology due to much higher frequencies, usually from MHz range. Attenuation of ultrasound can be converted to “longitudinal viscosity”, whereas sound speed is linked to “elastic bulk modulus” and “compressibility”. In the case of Newtonian liquids ultrasound attenuation can be used for calculating “bulk viscosity”. This obscure viscosity coefficient characterizes rotational and oscillating degrees of freedom for molecular motion.
There is a paper dedicated to longitudinal rheology of a dozen of Newtonian liquids: 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).
There is a model DT-600 that is High frequency longitudinal Rheometer. Similar function can be performed with the Model DT-1202, which must be equipped with the Option OP0010.