9daa2db5-02ee-43a8-861e-75ace751d148.doc Ultrasonic Particle Sizing of badly defined material systems F. Babick, M. Stintz, A. Richter Institut für Verfahrenstechnik und Umwelttechnik, TU Dresden, 01062 Dresden, Germany, Frank.Babick@mailbox.tu-dresden.de In the last decade the ultrasonic spectroscopy has received much attention as a measurement technique for the characterisation of concentrated suspensions and emulsions. In the same period the physical understanding of sound propagation through disperse systems has considerably advanced providing us now with mathematical models that can predict sound velocity and sound attenuation up to concentrations of 50 vol.% [1,2,3]. Beside this, the ultrasonic spectroscopy shows further features, like the robust, autoclavable sensor technique, the low requirements in sample preparation and a sensitivity down to nanometre-sized particles, which make it very attractive for on-line applications. In practice however, the interpretation of acoustic spectra encounters some specific problems depending on the type of material to be characterised. The major drawback of this method is the dependency of the acoustic behaviour on a large variety of material properties of the disperse and continuous phase. That means, that the interpretation of velocity or attenuation spectra with respect to particle size distribution requires the exact knowledge of these material properties . For example in the case of oil-in-water-emulsions crucial parameters are the densities of both phases, the specific heat, the expansion coefficient and the sound absorption of the oily phase. All of these parameters are affected by temperature and additives like stabilisers or solutes. That is why for many industrial particle systems a comprehensive knowledge of the relevant material properties will not be available. The authors intend to discuss possible strategies of coping with the problem of unknown material properties. They will particularly focus on fitting these unknown properties from ultrasonic spectra additional to the particle size distribution. Unlike to previous publications  both acoustic signals – sound speed and attenuation – are used for data interpretation. This considerably improves the parameter fitting and thus particle sizing.  DUKHIN, A. S., GOETZ, P. J., (1996), Acoustic Spectroscopy for Concentrated Polydisperse Colloids with High Density Contrast, Langmuir, 12, pp. 4987-4997.  HEMAR, Y., HERRMANN, N., LEMARÉCHAL, P., HOCQUART, R., LEQUEUX, F., (1997), Effective Medium Model for Ultrasonic Attenuation Due to the Thermo-Elastic Effect in Concentrated Emulsions. Phys. II France, 7, pp. 637-647.  HIPP, A. K., STORTI, G., MORBIDELLI, M., (2002), Acoustic Characterization of Concentrated Suspensions and Emulsions. I. Model Analysis. Langmuir, 18, pp. 391-404.  BABICK, F., HINZE, F., RIPPERGER, S., (2000), Dependence of ultrasonic attenuation on the material properties. Colloids & Surfaces A, 172, pp. 33-46  HIPP, A. K., STORTI, G., MORBIDELLI, M., (2002), Acoustic Characterization of Concentrated Suspensions and Emulsions II. Experimental Validation. Langmuir, 18, pp. 405-412.