Effects of pulse repetition rate and incident beam energy on the dynamic ultrasound scattering data

The dynamic ultrasound scattering (DSS) technique is a useful tool for investigating the dynamics of micron-sized particles in optically turbid suspensions. Owing to its use of a megahertz ultrasound pulse and its analysis method being based on auto-correlation function approaches, the evaluation of particle size, hydrodynamic interactions, the dispersibility of particles, and the mode of particle motion has become possible for a wide variety of microparticles. However, even if low-intensity ultrasound is employed, the effect of ultrasound energy on the system remains a concern. In this study, we demonstrated how an ultrasound beam affects the DSS results and describe a method of avoiding artifacts caused by ultrasound waves. Although the average velocities were correctly evaluated irrespective of pulse repetition frequency (PRF) using a setup placed along the Z-direction (parallel to the sedimentation), the velocity fluctuations obtained using the Y-setup (perpendicular to the sedimentation) were overestimated when a focused beam or a high PRF was employed. Such artifacts could be eliminated using plane wave transducers operated below 1 kHz.