In the first part of this work, a core−shell extension of the isolated-particle framework of Epstein, Carhart, Allegra, and Hawley (ECAH theory) has been developed for the simulation of sound attenuation in concentrated emulsions and suspensions. In the present paper, the model predictions are validated by comparison with a systematic set of experimental data. The systems under consideration include silica/water, poly(tetrafluoroethylene) (PTFE)/water, and corn oil/water at a range of particle sizes, particle volume fractions, and frequencies. These data cover a wide variety of situations where energy dissipation is dominated by viscoinertial losses, thermal losses, or combinations thereof, thus providing a significant test for the reliability and generality of the model. Moreover, the potential of the model as a basis for the estimation of particle sizes from acoustic data is investigated.

Acoustic spectroscopy is a relatively recent method for the characterization of suspensions and emulsions. Measurement of the attenuation of ultrasonic waves is often the first step of such analyses. Coupled with appropriate theoretical models, these data allow one to infer information such as particle size distribution and particle concentration. Several of such models are available, describing the behaviour of isolated particles. However, the validity of the isolated-particle assumption is limited to low particle concentrations, where nonlinear multiple-particle effects are negligible. To investigate this aspect, this paper is concerned with the multiple-particle effects as they arise in concentrated colloidal dispersions of high density contrast between the phases. The mechanism of multiple lossless scattering is demonstrated to be of minor influence in these systems. This suggests the presence of another multiple-particle effect, namely particle-particle interactions in the dissipative energy absorption.

This paper presents theoretical approaches for the acoustic analysis of concentrated suspensions and emulsions. The aim is an extension of the well-established single-particle theory of Epstein, Carhart, Allegra, and Hawley (ECAH), by introducing effective ways of accounting for the effect of interacting neighboring particles. After discussion of three modeling approaches and their implementation into the general ECAH framework, and demonstration of their effect on the acoustic field, the behavior of each thus defined model is analyzed as a function of particle concentration. In all cases, convergence to the well-established ECAH result is observed in the dilute limit. At high concentrations, particularly good behavior is found for a core−shell (pseudofluid) model, both for suspensions and emulsions. This model correctly predicts an increase of attenuation with particle concentration which is less than linear, in agreement with experimental observations.

In this paper, the use of ultrasonics for the determination of particle size distributions (PSDs) in suspensions and emulsions is discussed. Focusing on systems with a large density difference between the dispersed and continuous phases, a mathematical model is used to correlate the primary measurementthe attenuation of acoustic waves as a function of frequencyto the PSD, and it becomes clear that the quality of the PSD thus obtained is directly related to the accuracy of the model parameters. To overcome possible problems in the case of incomplete physical-property information, we have developed a generalized procedure for particle sizing, based on a parametric sensitivity analysis and parameter calibration. The latter allows for computation of physical properties from acoustic data provided that the PSD is known. Moreover, particle sizing results are presented for dispersions of different sizes and materials, and compared to the findings of electron microscopy and dynamic light scattering.

The possibility of using acoustic spectroscopy for the on-line and in-situ monitoring of particle size distributions and volume concentrations in batch crystallization processes is investigated. First, the acoustic technique is discussed, along with the evaluation of the involved model parameters. Second, for a seeded cooling crystallization of potassium sulfate (K 2 SO 4 ) in a supersaturated aqueous solution, it is shown that both size and concentration of the growing crystals can be estimated simultaneously from measured acoustic attenuation spectra. The results are discussed in view of the potential application of this technique to real-time and in-situ monitoring and control of industrial processes.