Published a paper on ultrasonic analysis of Pickering emulsions and colloidosomes
While water and oil do not mix, oil droplets can be stabilized in water by adding a small amount of oil and surfactant to water followed by vigorous stirring. Although low molecular weight surfactants or polymeric dispersants are generally used to prepare such emulsions, it is known that oil droplets can also be stabilized by nano or micron-sized solid particles. In addition, solid particles covering oil droplets can be thermally fused and linked together to form a capsule. These capsules have a vast specific surface area and can exchange materials through pores, making them promising materials for biological and medical applications in addition to cosmetics and electronic materials. In this paper, Pickering emulsions (PE), in which polystyrene (PS) particles are coated around droplets of an organic solvent, hexadecane, and colloidosomes (CS), in which PS particles are fused together near the glass transition temperature of PS, were fabricated for new ultrasonic studies.
Ultrasonic methods have long been used to study particle size distribution because they propagate even in highly turbid, opaque samples through which light does not propagate. Studies have also been conducted to calculate particle size distributions using information on the physical properties of particles and continuous phase liquids. Conversely, it is also possible to study the properties of particles in liquids without dilution if the particle size has been determined by other methods. Unlike electromagnetic scattering methods such as light or X-ray, ultrasonic methods can study not only the structure but also the propagation of forces inside the structure.
In a Pickering emulsion (PE), oil droplets are coated with solid particles. This condition is important in various material and medical applications, for example, Pickering emulsions coated with magnetic particles for therapy. If the oil droplet is coated with solid particles covering around it, the group of solid particles will behave like a shell covering around the oil droplet. However, if there are large gaps between the solid particles, it is easy to envisage that the solid particles on the surface of the PE are too far away from each other, so that no force can be transmitted between the particles.
Now lets melt a little of the PS particles present on the surface of the PE and let the PS particles interlink with each other. In this state, the shell made of PS will keep its shape even after the oil component, hexadecane, is removed. Such hollow particles are called colloidosomes, which are microcapsules with a core-shell structure, but can exchange substances through the gaps between the linked PS particles.
In this study, we proposed a new ultrasonic analysis: what kind of signal is obtained when PE or CS is irradiated with ultrasonic waves? If the size of the particles and the wavelength of the ultrasound are comparable, resonant scattering of the ultrasound will occur. Unlike spherical particles, which are uniform in density, PE and CS have a core-shell structure. Furthermore, the distance between PS particles, which serve as the shell part in PE, can be chemically controlled by the addition of salt. Also, how strong is the shell of CS, which is made by fusing PS particles? In this study, we analyzed resonant scattering of the core-shell structure to clarify the difference between PE and CS shells. Specifically, we showed that the frequency peak, called quadrupole resonance, is split into two frequencies, one lower and one higher than that of simple spherical particles, which provides a direct means of examining the emergence of elasticity in the shell.