Transition from liquid droplet to solid particle investigated by ultrasonic spectroscopy

The size distribution and elastic modulus of micron-sized particles dispersed in liquid can be quantitatively evaluated by ultrasonic spectroscopy at a megahertz frequency range combined with a scattering theory. Conventional theories dealing with the wavelength comparable with the micron-sized particles consider viscosity for liquid droplets in emulsions and elasticity for solid particles in suspension, but very few studies have simultaneously considered viscosity and elasticity for the dispersed phase. In this study, a toluene (Tol) solution of polystyrene (PS) was dispersed in a continuous phase (water), and the ultrasonic properties of the PS-Tol/water emulsion were investigated. Furthermore, when Tol is dried from the PS-Tol droplet, spherical solid PS particles are obtained in water. Using such a drying-in-liquid method, the processes of solid PS particle formation from PS-Tol liquid droplets were analyzed ultrasonically. Since these solid PS particles are in a glassy state at room temperature, the process from emulsion to solid polymer must go through a rubbery state or transition region, where viscosity becomes important in addition to elasticity for solid particles. The objective of this study is to demonstrate a methodology to quantitatively reproduce the ultrasonic spectra of microparticles associated with the drying of organic solvents using a model that takes account of both elasticity and viscous loss.