Eutectic gallium-indium (EGaIn) liquid metal, consisting of 75 wt% gallium and 25 wt% indium, recently draws increasing attention in various engineering fields due to its exclusive properties of fluidity and high electrical conductivity. In the atmosphere, EGaIn has a nanometer-thick oxide skin, resulting in the different interfacial phases in tension and compression. We visualize the interfacial phases by controlling the volume of EGaIn droplets to induce expansion or contraction of the surface. The results are reminiscent of the waxing or waning moon.
Waxing Phase We coated silica particles on the oxide skin of an EGaIn droplet to visualize the interfacial creation of the expanding droplet. As the droplet volume increases, the oxide skin was torn locally by the action of tensile force greater than the yield strength of the oxide skin. The newly created interfacial area was covered with the oxide skin in a few milliseconds. Accordingly, the effective surface tension of EGaIn is determined by the yield strength of the oxide skin in the case of expanding interface.
Waning Phase We observed a contracting EGaIn droplet with the bare oxide skin. As the droplet volume decreases, the spherical EGaIn droplet collapsed, losing the initial mirror-like interface. Wrinkles appeared on the oxide skin, and eventually the wrinkles covered the entire interface. In the case of contracting interface, the effective surface tension of EGaIn can be assumed to be zero because the oxide skin can hardly resist compressive forces.