Ultrasound propagation in concentrated suspensions: shear‐mediated contributions to multiple scattering

Ultrasonic techniques offer many advantages for process monitoring for suspensions of particles; this type of material occurs in a wide variety of process industries. However, the application of ultrasonic techniques has been limited by the inaccuracy of the models used to interpret the measured ultrasonic propagation parameters (typically speed and attenuation spectra) in terms of particle size, concentration and physical properties. Multiple scattering models have been used with great success in relatively dilute suspensions (up to 10%w/w) for colloidal particles, but are inadequate at higher concentrations, smaller particles, and low frequencies. The principal reason is believed to be the shear‐mediated contributions to multiple scattering which have previously been neglected. We report analytical and numerical results for a modified scattering model that includes these shear‐mediated effects. The model is a development of the multiple scattering formulation published by Luppé, Conoir and Norris (J Acoust Soc Am, 2012 (131) 1113) which incorporates thermal and shear wave mode conversions to and from the compressional wave mode. We consider a silica‐in‐water suspension, identify the dominant scattering contributions and develop analytical forms for them. Numerical calculations demonstrate the contribution of the additional shearmediated effects to the compressional wave speed and attenuation through the suspension. The calculations are compared with previously published experimental data. The work follows a similar formulation to a recently published model for concentrated emulsions in which thermally‐mediated effects are considered (Pinfield, J Acoust Soc Am, 2014 (136) 3008).