An ultrasound velocity and attenuation scanner for viewing the temporal evolution of a dispersed phase in fluids

Ultrasound scanning using the group velocity of sound to determine the conc entration of a liquid or solid phase dispersed in a fluid has been used for many years to characterize dispersions with regard to their long-term stab ility. The technique has the twin advantages of speed and operation in conc entrated, optically opaque dispersions. In this work, the group velocity te chnique is combined in a single instrument with phase velocity and attenuat ion spectroscopy measurements to give valuable additional information about particle size and the microscopic particle distribution, related to import ant destabilization phenomena such as particle flocculation. This provides earlier evidence of the processes that finally lead to gravitational destab ilization and reduced shelf life of fluid dispersions such as emulsions. A further advantage is the ability to compare the measured spatial and tempor al variation with computer models. The technique works in optically opaque materials and in concentrated colloids, giving a quantitative picture of th e macroscopic spatial distribution of the dispersed phase and a semiquantit ative picture of microscopic particle aggregation processes. Since these mi croscopic particle rearrangements are often responsible for the ultimate gr avitational destabilization of colloidal systems, the Acoustiscan, as we ha ve called the ultrasonic scanner described herein, may indicate product ins tability long in advance of visual evidence. New data are presented for pro tein containing sunflower oil-in-water emulsions, destabilized with Tween 2 0, in order to exemplify the use of the Acoustiscan for the characterizatio n of food emulsions. The Acoustiscan instrument provides quantitative infor mation about the destabilization of emulsions, dispersions, and colloidal s ystems in a rapid and informative manner. It simultaneously measures change s in the dispersed phase and follows microscopic changes in the arrangement of particles. The instrument has many other uses, for example, for charact erizing crude oil, pharmaceuticals, cosmetics, and agrochemicals. It can al so be used to follow crystallization processes. It does all this in materia ls over an extremely wide concentration range, from a few percent up to the highest concentrations obtainable. Moreover, when data are compared with c omputer models, it is possible to infer the presence of gels whose yield st ress is far lower than any measurable by contemporary rheological equipment . This makes the Acoustiscan ideal for the study of the new soft solid mate rials currently in development. (C) 2001 American Institute of Physics.