Characteristics of emulsion preparation in an ultrasonic microreactor: cavitation, droplet size and energy efficiency

•An efficient ultrasonic assisted microreactor for emulsification was developed.•A distinct cavitation pattern with an array of bubble clusters was observed.•The bubble cluster was composed of many fine bubbles in emulsion systems.•A semi-empirical correlation was proposed to predict the emulsion size.•A high energy efficiency of 0.03–2.77% was achieved in the novel device. An ultrasonic assisted microreactor (USMR) for emulsification was designed. Uniform sound distribution in the microchannel and effective temperature control were realized. The oscillation behavior of cavitation bubbles in coarse oil-in-water emulsion was investigated in two other ultrasonic microreactors with similar structure. It was found that bubble clusters composed of many fine bubbles vibrated as a whole, similar to the vibration of single bubbles. It was easier for the bubble clusters to oscillate under the transient oscillation mode, compared to a cavitation bubble. Due to the secondary sound field, it formed a uniform array of bubble clusters in straight channel and stagnant bursting cavitation zones with strong shocking wave in meandering channel. Parametric study on the emulsification performance was also conducted. A semi-empirical correlation was proposed and showed excellent predicting performance over the emulsion size. Due to the controlled sound field and cavitation bubbles of the USMR, the energy efficiency ranged from 0.03 to 2.77%, which was at least one order of magnitude higher than conventional ultrasonic devices that do not directly couple the microchannel and vibrator.