predicted and experimental values. To do this, it is necessary For aqueous charge-stabilized polystyrene dispersions the ultrato carry out ultrasound experiments over a relatively large sonic sound velocity was observed to change with time. The drift range of concentrations, since the theories in question may could be as high as 17% per day. For certain systems, particularly require estimates of several parameters.
concentrated dispersions at very low salt concentrations, sudden
Based on the above arguments, our original aim was to jumps in the sound velocity occurred. Dilute dispersions with measure the speed of sound in carefully purified polystyrene added electrolyte were more stable, the variation being within the dispersions with sufficient precision to determine the acousppm range. By investigating different mechanisms (sedimentation, tical parameters of the colloidal particles accurately. The aggregation, ultrasonic streaming or coagulation) which might be concentration range had to be wide. However, the experiresponsible for the phenomenon, it was concluded that spontaneous ordering of the particles in the suspension is the cause of this ments resulted in a somewhat unexpected time dependence drift. Ultrasound measurements appear to be more sensitive for of the sound velocity. It was impossible to get a stable velocthe detection of the ordering process of the dispersions than optical ity value within ppm precision.
investigation, since they sense the ordering of the particles well
Time dependence of the attenuation of ultrasound in colbefore the colloidal crystals become observable visually. ᭧ 1997 loidal polystyrene dispersions has been reported earlier (2).
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A possible explanation is that changes in the surface proper-Key Words: ultrasound velocity; time dependence; polystyrene ties of the particles occurred during the time of the measuredispersion; colloidal crystal; particle ordering. ments. Another paper on the velocity and attenuation of aqueous dispersions of polystyrene particles has also been published (3). Here the velocity was found to be indepen-