Shear elasticity analysis of supraball by resonant scattering using longitudinal ultrasonic pulses

When a longitudinal ultrasound pulse impinges on a particle suspension with the particle diameter on the order of the wavelength, resonant scattering of the ultrasound occurs, resulting in an inherent frequency dependence of ultrasound attenuation. Since ultrasound is an elastic wave that transmits deformation of material, the attenuation coefficient and phase velocity are strongly correlated with the mechanical properties of the particles. It is known that a peak is observed in the frequency spectrum of the attenuation coefficient, from which the elastic modulus and viscous loss of the particles can be quantified at a single particle level. This method may be applicable not only for uniform spherical particles with uniform density, but also for particle assemblies called supraballs or supraparticles. Among the various factors, the packing fraction of particle and the connectivity between particles may contribute to the stiffness of supraball. The packing factor was then determined from the sedimentation velocity and density obtained by dynamic ultrasound scattering measurements, while particle elasticity was evaluated from the peak of the frequency spectrum of the attenuation coefficient obtained by ultrasonic spectroscopy measurements. To validate the ultrasonic elasticity analysis, an indentation analysis of a single dried particle was performed using a commercially available dynamic hardness tester.