Collective particle motion and velocity fluctuations in micron-sized suspension by dynamic ultrasound scattering

Micron-sized particles are useful for avoiding the extreme agglomerative nature of nanoparticles while circumventing the problems of processing and controlling the physical properties of cosmetics and paints. Although determination of the condition of micron-sized particles from their dynamic behavior is more informative than from microscopic images alone, the motion of micron-sized particles is much more complicated and troublesome than that of nanoparticles. In this study, ultrasonic waves were irradiated onto a particle dispersion containing micron-sized particles, and the ultrasonic scattering field was analyzed to visualize the transient structures formed by the particles using a single element transducer. This technique is one of the methods to investigate the velocity of particles from time relaxation by continuously recording the ultrasonic scattering waves from the microparticles against time. The micron-sized particles appear to be randomly dispersed, but the collective structure that actually forms is at the millimeter level, far exceeding the wavelength of ultrasonic waves.