New approach to achieving smaller bubbles with various microwave irradiation modes
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Previous studies have measured the sizes of bubbles in suspension during microwave irradiation to clarify the mechanism of bubble formation. Those results indicate that the particle number density and the dielectric constants of the particles and solvent are the most important factors determining bubble size during irradiation. The aim of this study is to determine whether bubble nucleation can be controlled by changing the microwave mode between continuous irradiation and the two-stage irradiation proposed in this study. The first irradiation of higher power rapidly accelerates bubble nucleation, whereupon the bubbles grow more slowly during the second irradiation of lower power. Because the absorbed microwave energy is distributed to the liquid–air interface of each highly suspended small bubble produced by the first irradiation, the absorbed energy per bubble decreases during the second irradiation. Finally, smaller bubbles are achieved at the target temperature. Because the bubble number density and size can be controlled by the rapid thermal response that is characteristic of microwaves, this method could be useful for preventing superheating and bumping during nano-particle synthesis.
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