Acoustic ultrasonic relaxation

Walther, K. (1967). Ultrasonic relaxation at the Neel temperature and nuclear acoustic resonance in MnTe, Solid State Communications, 5 (5), pp. 399. 

In this section, we shall examine first the relaxation behaviour of a polymer material when irradiated with a sound wave, acoustic relaxation. Then we consider how the interactions may be influenced by increasing the intensity of the sound wave. Since most of the work in this area has been carried out in the ultrasonic frequency region, the phenomena are sometimes designated as ultrasonic relaxation. The irradiation of materials with high intensity ultrasonic waves is usually referred to as sonochemistry. 

Rassing et al. [62] studied kinetics of sodium perfluorooctanoate micellar systems using the ultrasonic relaxation method. They observed a fast relaxation process attributed to a micelle formation. The ultrasonic relaxation times revealed that periodic fluctuations in temperature and pressure caused by the acoustic wave are several magnitudes less than the temperature or pressure perturbations of jump techniques. Rassing et al. [62] suggested that the ultrasonic and jump methods measure different modes of micelle formation whose relaxation times differ by several orders of magnitude. Ultrasonic absorption techniques [69-71 ] have also been used to measure relaxation spectra of sodium perfluorooctanoate and cesium perfluorooctanoate [72,73]. 

Winter T. G., Hill G. L. High-temperature ultrasonic measurements of rotational relaxation in hydrogen, deuterium, nitrogen and oxygen, J. Acoust. Soc. Am. 42, 848-58 (1967). 

Schwan was one of the founders of biomedical engineering as a new discipline. Before World War II, in the laboratory of Rajewski at the Frankfurter Institut fiir Biophysik, he had started with some of the most important topics of the field on low-frequency blood and blood serum conductivity, counting of blood cells, selective heating and body tissue properties in the ultra-high-frequency range, electromagnetic hazards and safety standards for microwaves, tissue relaxation, and electrode polarization. He also worked with the acoustic and ultrasonic properties of tissue. In 1950, he revealed for the first time the frequency dependence of muscle... 

The optoacoustic properties of plasmon-resonant gold nanoparticles originate from photoinduced cavitation effects. This process can be summarized as follows (i) thermalization of conduction electrons on the subpicosecond timescale/ (ii) electron-phonon relaxation on the picosecond timescale and thermalization of the phonon lattice, with a subsequent rise in temperature by hundreds to thousands of degrees (iii) transient microbubble expansion upon reaching the kinetic spinodal of the superheated medium, initiated on the nanosecond timescale (iv) microbubble collapse, resulting in shockwaves and other forms of acoustic emission. The expansion and collapse of a cavitation bubble takes place on a microsecond timescale, and are easily detected by ultrasonic transducers. 

The observation that a system in chemical equilibrium can absorb ultrasounds dates back to the 1930s. During this period, Bazulin attributed the absorption of ultrasounds by acetic acid to the existence of hydrogen bonds. When an acoustic wave is made to cross a medium, alternate regions of compression and depletion of the particles that comprise it are produced. Eventually, this acoustic wave is attenuated within the space covered, in the same way that an electromagnetic wave is attenuated, because no medium is completely transparent. If species in chemical equilibrium that absorb ultrasound at the frequency used are added to this medium, and if the relaxation for the equilibrium situation is slower than that in the ultrasonic frequency, the regions of compression and depletion will not occur with the same phase or amplitude as the pure solvent. By comparison of the two situations it is possible to obtain the velocity of relaxation of the system in equilibrium. The... 

Transitions temperatures vary with the method and the rate of measurements. This is a potentially confusing situation. The transitions associated with the relaxation processes are highly frequency-dependent. Glass transition temperature obtained in measurement by dynamic methods [acoustic, dynamic mechanical analysis (DMA), ultrasonic, or dielectric methods] should reasonably be denoted as T to differ it from Tg measured, for instance, by DSC. At low fi equen-cies, that is, at 1 Hz or least, Ta is close to Tg. As the measurement fi equency is increased, increases while Tg remains the same, giving rise to two separate transition temperatures. In the literature, the distinction between the static and dynamic glass transitions is not always made clear. 

---