Broadband study of the scattering of ultrasound by polystyrene-latex-in-water suspensions

Ultrasonic absorption spectra have been measured between 0.1 and 2000 MHz of aqueous suspensions of polystyrene latex globules. Samples of different diameter (60 to 100 nm) and different concentration of the polystyrene latex particles (volume fraction between 0.05 and 0.38) have been studied. The broadband spectra exhibit a Debye-type relaxation term in the frequency range around some MHz and a contribution with special characteristics at higher frequencies. The former term is assumed to be due to counterion effects in the surface layer of the globules. No special attention is payed to this effect. The latter contribution is attributed to scattering and mode conversion mechanisms. An approximate explicit and an exact formulation of the theoretical description of the sound wave attenuation by the scattering process are discussed. The explicit formulation enables valuable insights into the mode conversion mechanisms but does not allow for a complete representation of the measured spectra. Application of the exact version leads to the conclusion that in the frequency range under consideration monopole and quadrupole oscillations of the polystyrene latex globules are the dominant cause of ultrasonic wave attenuation. It is found that even at the elevated concentrations used effects of multiple scattering are of minor importance here.