New insights into the mechanisms of ultrasonic emulsification in the oil–water system and the role of gas bubbles

W.H Wu,D.G Eskin,A Priyadarshi,T Subroto,I Tzanakis,W Zhai

doi:10.1016/j.ultsonch.2021.105501

Abstract

Ultrasonic emulsification (USE) assisted by cavitation is an effective method to produce emulsion droplets. However, the role of gas bubbles in the USE process still remains unclear. Hence, in the present paper, high-speed camera observations of bubble evolution and emulsion droplets formation in oil and water were used to capture in real-time the emulsification process, while experiments with different gas concentrations were carried out to investigate the effect of gas bubbles on droplet size. The results show that at the interface of oil and water, gas bubbles with a radius larger than the resonance radius collapse and sink into the water phase, inducing (oil–water) blended liquid jets across bubbles to generate oil-in-water-in-oil (O/W/O) and water-in-oil (W/O) droplets in the oil phase and oil-in-water (O/W) droplets in the water phase, respectively. Gas bubbles with a radius smaller than the resonance radius at the interface always move towards the oil phase, accompanied with the generation of water droplets in the oil phase. In the oil phase, gas bubbles, which can attract bubbles nearby the interface, migrate to the interface of oil and water due to acoustic streaming, and generate numerous droplets. As for the gas bubbles in the water phase, those can break neighboring droplets into numerous finer ones during bubble oscillation. With the increase in gas content, more bubbles undergo chaotic oscillation, leading to smaller and more stable emulsion droplets, which explains the beneficial role of gas bubbles in USE. Violently oscillating microbubbles are, therefore, found to be the governing cavitation regime for emulsification process. These results provide new insights to the mechanisms of gas bubbles in oil–water emulsions, which may be useful towards the optimization of USE process in industry.

Highlights

Emulsion is composed of at least two immiscible liquids, in which the dispersed phase is distributed in the continuous phase in the form of small droplets [1,2,3]
A high-speed camera was utilized to capture in real-time the process of interaction of gas bubbles with the oil and water phases during ultrasonic emulsification (USE)
The effect of gas concentration on the ef ficiency of USE was quantified through the droplet size distribution and the stability of the droplet sizes

Summary

Introduction

Emulsion is composed of at least two immiscible liquids, in which the dispersed phase is distributed in the continuous phase in the form of small droplets [1,2,3]. Stable oil-in-water (O/W) type emulsions can efficiently deliver oil-soluble components into foods [6]. The keys to generate stable droplets in the continuous phase are high energy shear, mixing and turbulence [7]. There are many methods to pre pare emulsion, such as rotor–stator type devices, high-pressure ho mogenization (HPH), microfluidization (MF) and ultrasonic emulsification (USE) [8,9]. USE possesses many advantages, such as high energy efficiency, small droplet size and easy operation [10]

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Conclusion