Effect of oxidation on the Mechanical Properties of Liquid Gallium and Eutectic Gallium-Indium (1201.4828v1)

Abstract: Liquid metals exhibit remarkable mechanical properties, in particular large surface tension and low viscosity. However, these properties are greatly affected by oxidation when exposed to air. We measure the viscosity, surface tension, and contact angle of gallium (Ga) and a eutectic gallium-indium alloy (eGaIn) while controlling such oxidation by surrounding the metal with an acid bath of variable concentration. Rheometry measurements reveal a yield stress directly attributable to an oxide skin that obscures the intrinsic behavior of the liquid metals. We demonstrate how the intrinsic viscosity can be obtained with precision through a scaling technique that collapses low- and high-Reynolds number data. Measuring surface tension with a pendant drop method, we show that the oxide skin generates a surface stress that mimics surface tension and develop a simple model to relate this to the yield stress obtained from rheometry. We find that yield stress, surface tension, and contact angle all transition from solid-like to liquid behavior at the same critical acid concentration, thereby quantitatively confirming that the wettability of these liquid metals is due to the oxide skin.

Effect of Oxidation on the Mechanical Properties of Liquid Gallium and Eutectic Gallium-Indium

The paper presented in this paper explores the influence of oxidation on the rheological and surface properties of liquid gallium (Ga) and a eutectic gallium-indium (eGaIn) alloy. The authors focus on quantifying changes in viscosity, surface tension, and contact angle under controlled oxidation conditions, using an innovative experimental approach that involves immersing the liquid metals in acid baths of varying hydrochloric acid (HCl) concentrations. This experimental setup allows for the control of oxidation levels, providing a platform to validate hypotheses about the resultant mechanical properties.

Rheological Behavior

The paper provides a comprehensive examination of the rheological behavior of oxidized liquid metals. Rheometry measurements identify a significant yield stress attributed to the oxidized skin on the metal surface, disrupting the intrinsic behavior of the liquids. By varying the acid concentration, the paper achieves a controlled balance between oxidation and reduction, essential in observing the transition from solid-like to liquid states. The use of high-resolution rheometry reveals that at higher Reynolds numbers, inertial effects become pronounced, demonstrating a non-Newtonian behavior characterized by a $\tau \propto \dot{\gamma}^{3/2}$ relationship. The authors develop an empirical scaling technique that collapses data from low- and high-Reynolds number regimes to obtain intrinsic viscosity values with increased precision. This method is validated against known viscosity standards like water, yielding results consistent with existing literature values for these metals.

Surface Tension and Contact Angle

Utilizing the pendant drop method, the paper measures the effective surface tension, finding it significantly increased by the presence of an oxidized skin layer. An empirical model derived within this work successfully relates this increase to the yield stress, suggesting a $\gamma_{\mbox{\scriptsize eff}} \approx \tau_y l/4 + \gamma_0$ relation, where the size of the droplet and oxide layer properties play critical roles. The fitting of surface tension data against drop sizes in different chemical environments allowed the authors to further elucidate the contribution of solid-like behaviors to surface stress, reaffirming the validity of their scaling model.

With regards to wettability, the authors determine the contact angles of Ga and eGaIn on glass surfaces, establishing that the pure, pristine liquid forms exhibit perfect non-wetting properties when free from oxide contamination. The gradual decrease in contact angle with decreasing acid concentration underscores the role of oxidation in enhancing metal wettability. This comprehensive linkage across different property measurements, all displaying a threshold transition at a critical acid concentration, convincingly corroborates the hypothesis that oxidation underpins many unique mechanical behaviors of liquid Ga and eGaIn alloys.

Implications and Future Directions

This research has significant implications in materials science, specifically in applications involving liquid metals where control over surface and bulk mechanical properties is crucial. The methodological approach adopted here provides a robust framework for further studies exploring other liquid metal systems or similar interfacial phenomena in different liquid-solid interfaces. Furthermore, the insights on the stability and control of the oxide layer on these metals open avenues for enhanced formulation of coatings, microelectronic components, and other technologies utilizing the unique properties of liquid metals. Future research can venture deeper into the molecular mechanisms driving oxidation at these interfaces and aim to generalize these findings across other less studied liquid metal alloys.

By systematically detailing the impact of oxidative effects on liquid metals' mechanical characteristics, this paper represents a substantial step forward in understanding and manipulating these unique materials' dynamic and interfacial properties.