Ultrasonic dispersion and attenuation in bubbly liquids

Ultrasonic examination of bubbly liquid like fresh concrete supplies important information from very early age. While the traditional pulse velocity is very indicative of the setting and stiffening processes, it utilizes essentially only one point of the waveform (first threshold crossing) without supplying specific information on the microstructure. More recently, ultrasonic dispersion has been proposed as a strong upgrade to monitoring as the level of velocity dependence on the frequency is related to the degree of heterogeneity. Cavities or air bubbles suspended in the viscous matrix are strong wave scatterers. The air bubbles pose strong dispersion and attenuation in the liquid state while their influence is minimized in a stiff matrix. Although, dispersion curves are indicative of the current condition, their measurement is a delicate process that involves calculations of the phase in the FFT domain of the received signals. To establish some references, experiments are conducted in water considered ideal medium and shampoo with and without bubbles . Results indicate that initially, shampoo exhibits stronger dispersion, seen by the strong influence of frequency on the propagation velocity, while gradually as bubbles are released to the surface due to settlement (in shampoo) the dispersive trend weakens reaching towards the nearly flat dispersion curve of water.