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 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.
MEASUREMENT and APPLICATION
There are relatively few studies of extensional rheology. Generation of homogeneous extensional flows presents a problem, especially with low viscosity liquids. Ultrasound offers easy resolution of this problem. 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 “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 serves as a raw data for calculating “longitudinal viscosity”. Whereas sound speed provides information for “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 general overview of relationship between shear and extensional rheology 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).
Several published papers present longitudinal rheology measurements for variety 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).
- Dukhin A.S., “Rheology in longitudinal (ultrasound) mode. Review”, Colloid Journal, vol 83, 1, pp. 1-19, (2021)
- Dukhin, A., Parlia, S., Somasundaran, P., “Rheology of non-Newtonian liquid mixtures and the role of molecular chain length.” J. of Colloid and Interface Science. (2019)
Models DT-1202, DT-100 and DT-110 can perform such rheological measurement when equipped with the Option Rheology OP0010.