Acoustic Spectroscopy for Concentrated Polydisperse Colloids with Low Density Contrast
The mechanisms of sound attenuation are different in dispersions with low and high density contrast. “Viscous losses” are dominant in high density contrast dispersions whereas “thermal losses” predominate in dispersions with low density contrast. The rutile dispersion is chosen as an instance of the high density contrast system. The dispersion of the neoprene latex is an instance of the low density contrast system. The dilution experiment performed with both systems shows that the role of the particle−particle interaction is quite different in these two dispersions. The measured spectra show that attenuation remains a linear function of the volume fraction in the latex dispersion even at 30 vol %. At the same time, attenuation exhibits a nonlinear dependence on the volume fraction for the rutile dispersion even at 10 vol %. This difference means that particle−particle interaction contributes more to the “viscous losses” than to the “thermal losses”. We associate this effect with the difference between “viscous depth” and “thermal depth”. These parameters characterize the penetration of the shear wave or thermal wave into the liquid. The observed insensitivity of the thermal losses to the particle−particle interaction supports the application of dilute case theory to calculate the particle size distribution in the concentrated (up to 30 vol %) emulsions and latices.