Ultrasonic Analysis of Liquid Samples

Ultrasound propagation is adiabatic in homogeneous media at the frequencies typically used in US-based detection techniques. Therefore, although temperature fluctuations inevitably accompany pressure fluctuations in US, thermal dissipation is small and it is adiabatic compressibility which matters. As a second derivative of thermodynamic potentials, compressibility is extremely sensitive to structure and intermolecular interactions in liquids (e.g. the compressibility of water near charged ions or atomic groups of macromolecules differs from that of bulk water by 50-100%). 

Measuring US velocity is the only direct way of determining adiabatic compressibility. This type of measurement has opened the door to a new world of materials characterization, a world that hitherto has remained the province of specialists. Thus, US allows biochemists to determine protein hydration, food scientists to monitor changes in solid fat content, physical chemists to measure solute-solute and solute-solvent interactions, and physicians to measure cell aggregation, just to name a single use in some key areas. 

Most applications of US-detection in liquids use compressive, longitudinal waves shear waves propagate over macroscopic distances in solids, but not in liquids, and other modes of US propagation are of little use with liquid samples. 

Variables influencing ultrasound measurements in liquid systems 

The most influential variables on US measurements in liquids are temperature, dissolved air oonoentration and pressure. Also, moisture is a key variable with non-aqueous media. 

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