Surfactant-free emulsification in microfluidics using strongly oscillating bubbles

Abstract

In this study, two immiscible liquids in a microfluidics channel has been successfully emulsified by acoustic cavitation bubbles. These bubbles are generated by the attached piezo transducers which are driven to oscillate at resonant frequency of the system (about 100 kHz) [1, 2]. The bubbles oscillate and induce strong mixing in the microchamber. They induce the rupture of the liquid thin layer along the bubble surface due to the high shear stress and fast liquid jetting at the interface. Also, they cause the big droplets to fragment into small droplets. Both water-in-oil and oil-in-water emulsions with viscosity ratio up to 1000 have been produced using this method without the application of surfactant. The system is highly efficient as submicron monodisperse emulsions (especially for water-in-oil emulsion) could be created within milliseconds. It is found that with a longer ultrasound exposure, the size of the droplets in the emulsions decreases, and the uniformity of the emulsion increases. Reference: [1] Tandiono, SW Ohl et al., “Creation of cavitation activity in a microfluidics device through acoustically driven capillary waves,” Lab Chip 10, 1848–1855 (2010). [2] Tandiono, SW Ohl et al., “Sonochemistry and sonoluminescence in microfluidics,” Proc. Natl. Acad. Sci. U.S.A. 108(15), 5996–5998 (2011).