and composites in the field of hydrothermal synthesis, as well as emulsion polymerization for nano-particles with a monodisperse size distribution [3]. Recently, microwave technology has been applied to industrial processes for material characterization to save energy and time. Although nonthermal reaction effects have been reported [4], the mechanism of monodisperse nano-particle formation under microwave irradiation remains unclear, due to the difficulty of direct observation inside reactors.The movement of particles smaller than a few micrometers is governed by Brownian motion, which causes flow around the particles. The force acting upon Brownian particles is measured using a technique called dynamic light scattering (DLS) [5], which provides the diffusion coefficient and particle size. When observation tools are installed within the reactor, the waveguide tube must be carefully designed to prevent microwave leakage. Commercial nano-particle measuring apparatuses are not necessarily suited to such designs, and it is difficult to adjust the light source power and the angle between the light axis and detector probe. Our laboratory has experience with observation of nano-particle movement [6], and measurement techniques we have developed make in situ observation of nano-particle behavior under microwave irradiation possible. This study presents an in situ observation technique for nano-particles or bubbles in a microwave reactor, and clarifies particle movement and bubble formation in water during and after microwave irradiation. Methods MaterialWe used a suspension of monodisperse polystyrene latex (PSL; 100 nm, Duke Scientific Corp.) particles as the Abstract A microwave reactor was designed for in situ observation of nano-and micro-bubbles, and size profiles during and after irradiation were measured with respect to irradiation power and time. Bubble formation in water during irradiation was observed even at temperatures below the boiling point of water. The maximum size strongly depended on radiation power and time, even at a given temperature. Nano-particles in the dispersion medium were found to play an important role in achieving more stable nucleation of bubbles around particles, and stable size distributions were obtained from clear autocorrelation by a dynamic light scattering system. Moreover, a combination of microwave induction heating and the addition of nanoparticles to the dispersion medium can prevent heterogeneous nucleation of bubbles on the cell wall. Quantitative nano-bubble size profiles obtained by in situ observation provide useful information regarding microwave-based industrial processes for nano-particle production.