Acoustic and electroacoustic spectroscopy characterizing concentrated dispersions emulsions

We describe two different techniques (acoustics and electroacoustics), both of which employ ultrasound instead of light for extracting information abo ut the properties of liquid-based dispersions. Ultrasound can propagate thr ough samples that are not transparent for light, which open up many new app lications not possible with classical light scattering methods. Acoustic an d electroacoustic techniques offer a unique opportunity to characterize con centrated dispersion, emulsions and microemulsions in their natural states. Elimination of a dilution step required for most other techniques (light s cattering, sedimentation, electrophoresis) is crucial for an adequate chara cterization of liquid dispersions, especially when the high concentration l eads to structured systems. As with any macroscopic method, ultrasonic tech niques characterize the sample in two steps. The first step is to measure s ome macroscopic property. The second step involves some theoretical treatme nt of the measured raw data which yields the desired information. Acoustic spectroscopy deals with measuring the attenuation of ultrasound within a ce rtain frequency range. Electroacoustic spectroscopy has two implementations depending on the driving force. We emphasize here on the so-called Colloid Vibration Current (CVI) which is generated by the sound wave as it passes through the dispersion. A review of the theoretical basis of acoustics and electroacoustics is given, with emphasis on models that have been applied t o concentrated systems. Recently, new theories have been developed for both acoustics and electroacoustics using a 'coupled phase model' and 'cell mod el concept'. The coupled phase model is widely used for describing a relati ve motion of the particles and liquid in the sound wave. The cell model app roach opens the way to include both particle-particle interactions and poly dispersity into the theoretical model. Experimental evidence is presented t hat shows that this new approach is successful in concentrated systems up t o 45% vol. A short review of the possible applications of acoustics and ele ctroacoustics measurements to a range of systems is presented including: ce ramics, mixed dispersed systems, chemical-mechanical polishing abrasives, e mulsions, microemulsions and latex materials. (C) 2001 Elsevier Science B.V . All rights reserved.