  With exposure to air, many liquid metals spontaneously generate an oxide
layer on their surface. In oscillatory rheological tests, this skin is found to
introduce a yield stress that typically dominates the elastic response but can
be tuned by exposing the metal to hydrochloric acid solutions of different
concentration. We systematically studied the normal impact of eutectic
gallium-indium (eGaIn) drops under different oxidation conditions and show how
this leads to two different dynamical regimes. At low impact velocity (or low
Weber number), eGaIn droplets display strong recoil and rebound from the
impacted surface when the oxide layer is removed. In addition, the degree of
drop deformation or spreading during the impact is controlled by the oxide
skin. We show that the scaling law known from ordinary liquids for the maximum
spreading radius as a function of impact velocity can still be applied to the
case of oxidized eGaIn if an effective Weber number $We^{\star}$ is employed
that uses an effective surface tension factoring in the yield stress. In
contrast, no influence on spreading from different oxidations conditions is
observed for high impact velocity. This suggests that the initial kinetic
energy is mostly damped by bulk viscous dissipation. Results from both regimes
can be collapsed in an impact phase diagram controlled by two variables, the
maximum spreading factor $P_m = R_0/R_m$, given by the ratio of initial to
maximum drop radius, and the impact number $K = We^{\star}/Re^{4/5}$, which
scales with the effective Weber number $We^{\star}$ as well as the Reynolds
number $Re$. The data exhibit a transition from capillary to viscous behavior
at a critical impact number $K_c \sim 0.1$.
