(color online) (a) Definition of contact angle θ. (b) Comparison between contact angle of liquid Al on the Al substrate and that on Pb substrate. The contact angles of liquid Al on the Mg and Fe substrates are not presented in the graph because of the abnormal wetting behavior of liquid Al on the Fe substrate and the dewetting behavior on the Mg sheet. (c) Plots of interaction energy between the liquid Al and the substrates (Al–Al and Al–Pb) versus time, the interaction energy is actually a kind of potential energy.