ABSTRACT
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.
ABSTRACT
Pickering emulsions comprising liquid droplets stabilized by solid microparticles have gained much attention in the field of cosmetics, inks, and drug delivery systems. To ensure that microparticles in Pickering emulsions are localized at the surface of liquid droplets, ultrasonic spectroscopy analysis combined with scattering function theory was conducted in this study. Two specific cases were investigated: (1) silica particles and liquid droplets independently dispersed in liquid and (2) silica particles effectively localized at the surface of the droplets. It was found that the core-shell model was effective for analyzing nanoparticles anchored at the surface of oil droplets. Conversely, it was found that an effective shell comprised of solid particles was no longer observed as the particle size or the distance between solid particles increased. When a large solid particle was applied, the ultrasonic spectra resembled those of conventional surfactant-stabilized emulsions without solid stabilizers.
ABSTRACT
Ultrasonic scattering method is a promising technique to evaluate the particle size distribution and/or the elastic properties of particle suspended in liquid. Among the wide variety of scattering theories, the ECAH theory proposed by Epstein-Carhart-Allegra-Hawley is one of the most relevant acoustic scattering theories to reproduce the ultrasonic spectroscopy data for the particle suspensions. However, the original theory assumes that the shear contribution is provided for either elastic solid or viscous liquid. Thus, the viscoelastic effect of particle to the frequency spectra of the attenuation coefficient and the phase velocity has gained less attention and is not fully understood yet. In order to minimize the number of unknown parameters for the analysis of viscoelastic suspensions, we employed the multiple echo-reflection ultrasonic spectroscopy (MERUS) technique, which allowed us to determine the material properties, prior to the complicated scattering analysis. In this study, we prepared the particle suspensions of poly(methyl methacrylate) (PMMA), bisphenol-A polycarbonate (PC) and cross-linked poly(dimethyl siloxane) (PDMS) and the acoustic spectra were measured by the ultrasonic spectroscopy. Then, we demonstrated the effect of the particle viscosity on the spectra by using the viscoelastic ECAH model at the wavelength comparable with the particle size.
ABSTRACT
The size distribution and mechanical properties of microparticle dispersed in liquid can be characterized by ultrasonic spectroscopy with the aid of acoustic scattering theories. In order to carry out the accurate analysis of the particles, the basic properties, such as the density, viscosity, longitudinal and shear velocities and intrinsic attenuation coefficient of the particle must be known prior to the analysis. Particularly, for soft elastomers or rubbers which exhibit complex mechanical properties with comparable real and imaginary parts, such fundamental information should be provided prior to the particle analysis to minimize the uncertainty of estimation associated with the number of adjustable parameters. In this study, we examined the acoustical properties of poly(methyl methacrylate)(PMMA) and cross-linked poly(dimethyl siloxane) sheets having different cross-linker concentrations by Multiple-Echo Reflection Ultrasonic Spectroscopy which simultaneously enabled us to acquire 4 fundamental properties, the ultrasound attenuation coefficient, phase velocity, density, and thickness (MERUS4 for solid plate). In addition, it was confirmed that the acoustic spectra of PMMA particles dispersed in water were reproduced well with the physical properties determined by MERUS4 using the PMMA plates.
