Ultrasonic foaming of polyurethane was investigated experimentally and theoretically. Polyol resin was saturated with nitrogen gas at various pressures and ultrasonic excitation was applied to the system. The ultrasonic nucleation was modeled by utilizing the classical nucleation theory by considering the negative pressure generated by the ultrasonic disruption as the environmental pressure at the moment of nucleation. The theoretical prediction indicated that high rate of nucleation can be obtained if the ultrasonic excitation generates large enough negative pressure. Final bubble size could be calculated by considering the pressure equilibrium between inside and outside of the gas-liquid interface by applying surface tension of the resin.
