Ultrasonic characterisation of solid-liquid suspensions

Results on ultrasonic attenuation in suspensions of alumina powder in water at high concentration (40% vol.) are presented. When high concentrations of solid material in the host liquid are desired, it is very important to keep the solid particles well separated (deflocculated), This property is achieved by using a dispersant, whose concentration is critical for obtaining a good particle separation. The purpose of our study was to identify weak structural differences which occur between suspensions with different degrees of deflocculation, obtained by slightly varying the quantity of dispersant around the critical value; such differences are difficult to be seen with other methods or devices (optical, Acoustosizer) at high concentration. In this aim the attenuation coefficient of ultrasonic waves travelling through the samples has been determined by using an ultrasonic method. This method, based on techniques used for electric network analysis, allows us to achieve the absorption curve of the suspension under test. To this purpose, a plane ultrasonic wave, whose frequency is linearly swept: is generated inside the liquid by a piezoelectric transducer, acting as a transmitter, and received by a second transducer, frontally placed and aligned on the same axis. The transfer function of the system is proportional to the ratio of the acoustic pressure at the receiver and at the transmitter and depends on the spectral response of the transducers and on the acoustic path inside the liquid. The measurement of the system response is provided by a swept frequency network analyser. From the ratio of the transfer functions H and H-0 measured at two positions separated by a distance Delta d the absorption coefficient cc is obtained as a function of the frequency through the simple relation alpha = -(l/Delta d)log(H/H-0). In the frequency range in which the measurements have been carried out (1-20 MHz) we observed small but significant differences between attenuation dependence on frequency in suspensions with the same basic composition but with slightly different particle size distributions: indeed weakly flocculated suspensions presented lower attenuation than well deflocculated suspensions. In order to decide which mechanism is responsible for attenuation dependence in our experiments, we calculated the attenuation values for different mechanisms: thermal conduction, multiple scattering and viscous drag. Good agreement was obtained with a hydrodynamic model (Harker et al., Ultrasonics 29 (1991) 427) mainly based on viscous losses. Moreover this model is also able to reproduce the dependence of attenuation on slight variations in particle size distributions, as observed in our experiments. (C) 1998 Elsevier Science B.V.