In-situ observation of phase separation dynamics for immiscible aqueous solution within ultrasonic field

A high-speed imaging technique was used to observe the phase separation process of water (H₂O)-20 %succinonitrile (SCN) immiscible solution within ultrasound field. Combining with numerical simulation, the effects of ultrasonic cavitation and acoustic streaming on the fragmentation and migration of secondary droplets were revealed. It was found that the previously spherical or near-spherical secondary H₂O-rich droplets formed under static condition were dynamically transformed into several novel forms, such as tadpole-like, string-beads, gourd-like, and threadlike patterns. The calculated results showed that the cavitation could fragment micron-scale H₂O-rich droplets because of the produced higher shock wave pressure than the droplets' Laplace pressure, and the subsequent droplet morphology evolution mainly depended on the liquid ejection volume determined by the distance between the droplets and the collapsing bubbles. Meanwhile, acoustic streaming, which generated shear force exceeding the surface tension of H₂O-rich phase, stretched, split and finally fractured millimeter-sized or even larger secondary droplets into several smaller spherical sub-droplets. In comparison, the observed secondary droplet distribution characteristics in H₂O-20 %SCN solution were similar to the Bi-rich particles in the ultrasonic solidification microstructures of Al-30 %Bi immiscible alloy, confirming that this work provided a deep understanding of the liquid phase separation mechanism within ultrasonic field.