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

Abstract

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.

Keywords: Inorganic materials; Materials chemistry; Materials science.

Graphical abstract

Figure 1

Macroscopic photographs of liquid metal droplets as a function of time The images show that the oxide skin enhances the supercooling of Ga drops (∼1.1 g) resting on smooth surfaces (SiO₂/Si wafers, surface roughness RMS ∼0.3 nm). During the experiments, the temperature was ∼15°C. Acid (HCl) and base (NaOH) remove the oxide and the metals freeze, as evident by the transformation of the metal drops into non-spherical shapes. Scale bar is 0.5 cm.

Figure 2

Supercooling observation for smaller Ga drops (∼0.055 g), with and without oxide skin The Ga drops were placed on smooth surfaces (SiO₂/Si wafers, surface roughness RMS ∼0.3 nm). Copper wires were used to trigger the nucleation at the end of each experiment to prove the metal can freeze by heterogeneous nucleation at these conditions. During these experiments, the temperature was ∼15°C. Scale bar is 0.5 cm.

Figure 3

Differential scanning calorimeter (DSC) measurements for cooling Ga (A) Ga in air with the native oxide present. (B) Ga in 1 M HCl. (C) Ga in 1 M NaOH. Each sample was measured five times sequentially under the same conditions. The freezing points of Ga in air, in HCl, and in NaOH were found to be −15.6 ± 3.5°C, 6.9 ± 2.0°C, and 6.3 ± 3.7°C, respectively.