Acoustic behavior at the fluid/solid transition of kaolinite suspensions

Anomalously high ultrasonic attenuation is observed in kaolinite/water susp ensions near 40% solid-volume concentrations. Within the range of frequenci es used (3-7 MI-It), the concentration of this loss maximum is nearly indep endent of wave frequency. Velocity extrema are observed near this same conc entration. This behavior is attributed to viscous losses in the oscillating fluid between adjacent particles. At low concentrations (<40%), this mecha nism becomes stronger with decreasing interparticle separation and therefor e with increasing concentration as well. As particles begin to touch at hig h concentrations, their oscillatory motion is inhibited and the wave dampin g decreases. Thus, the loss maximum can be used as a signature of the onset of rigidity in suspensions and colloids. The 40% concentration is consiste nt with water contents of published Atterberg liquid limits for kaolinite/w ater mixtures. Full elastic rigidity is not likely to be realized until the two-phase material achieves a dense particle packing, such as close packin g (62% concentration) for well-sorted particles.