Dependence of ultrasonic attenuation on the material properties

Ultrasonic spectroscopy is a promising measurement technique for the characterisation of emulsions and suspensions over a wide range of particle size and concentration. It appears highly suitable for on-line applications, in particular for dense nano-sized particle systems, where the system stability may be very sensitive to changes in the concentration. In the case of colloidal dispersions the particle sizes are usually smaller than the sound wavelength. Then dissipative processes rather than scattering govern the acoustic behaviour of such systems. The dissipative processes, however, are affected by several material properties, whose significance for the overall acoustic behaviour depends on the type of the material system, e.g. thermal properties are important in the case of emulsions and non-watery suspensions but not for watery suspensions. Often the information on these parameters is incomplete and not sufficiently accurate. In this paper the stability of ultrasonic particle size measurement against incorrect values of the relevant material properties is investigated. This was done firstly by analytical consideration. From this, the degree of influence of the respective material properties on the analysis of spectrometric measurements was derived for oil-water-emulsions, watery and non-watery suspensions. It could be shown that the single properties affect the analysis very differently. In a second step, the conclusions obtained analytically could be confirmed by analysing experimental attenuation spectra with slightly changed material property data. The paper is intended to give users of ultrasonic spectroscopy a practical guide for deciding which material properties have to be obtained with high accuracy and which can be estimated.