Extraordinary Solidification Mechanism of Liquid Alloys Under Acoustic Levitation State

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.

An atomically smooth container: Can the native oxide promote supercooling of liquid gallium?

Metals tend to supercool-that is, they freeze at temperatures below their melting points. In general, supercooling is less favorable when liquids are in contact with nucleation sites such as rough surfaces. Interestingly, bulk gallium (Ga) can significantly supercool, even when it is in contact with heterogeneous surfaces that could provide nucleation sites. We hypothesized that the native oxide on Ga provides an atomically smooth interface that prevents Ga from directly contacting surfaces, and thereby promotes supercooling. Although many metals form surface oxides, Ga is a convenient metal for studying supercooling because its melting point of 29.8°C is near room temperature. Using differential scanning calorimetry (DSC), we show that freezing of Ga with the oxide occurs at a lower temperature (-15.6 ± 3.5°C) than without the oxide (6.9 ± 2.0°C when the oxide is removed by HCl). We also demonstrate that the oxide enhances supercooling via macroscopic observations of freezing. These findings explain why Ga supercools and have implications for emerging applications of Ga that rely on it staying in the liquid state.