  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.
