(color online) Snapshots of coalescence dynamics and the center-of-mass displacement of metal drops. (a) Coalescence processes of the Al and Pb drops on the Al and Pb sheets, respectively. The green dashed line represents the initial boundary between the two metal films. Center-of-mass displacement of (b) Al and (c) Pb along the x direction (CMD ), (d) Al and (e) Pb along the z direction (CMD ) as a function of time. The changes of curves along the positive direction of x axis (the rise of curves) mean that the drop moves to the Pb side, and the change of curves along the negative direction (the fall of curves) indicates the Al side.