Rheology in Longitudinal (Ultrasound) Mode. Review

This review is dedicated to the topic of longitudinal rheology, a branch of rheology that is complementary to traditional shear rheology, and yet not as widely explored. Longitudinal rheology differs from typical shear rheology by the type of stress applied. Longitudinal stress is a wave, which causes liquid expansion and compression that occurs when ultrasound propagates through such liquid. The penetration depth of a longitudinal stress is much longer than for a shear stress, which allows this method to be used for studying liquid bulk properties at the MHz range. The concept of longitudinal rheology has been known for centuries, but only became available in commercial instruments as recently as the 1990s. We describe here the main principles of this technique, as well as present an overview of existing published applications, which include: —Bulk viscosity —Microviscosity —Hookean coefficient of inter-particle bonds —Newtonian liquid test at MHz range —Compressibility —Sol-gel transition —Micellar systems —Dissolution of polymers —Liquid mixtures