Droplet spreading on a surface exhibiting solid-liquid interfacial premelting

Abstract We study, using molecular-dynamics (MD) simulation, the spreading kinetics and equilibrium shape of liquid Pb droplets on an Al (111) substrate. The Al-Pb solid-liquid interface was found previously [Phys. Rev. Lett, 110, 096102 (2013)] to exhibit solid-liquid interfacial premelting at temperatures below the Al melting point. Because the Al(111) free surface does not exhibit premelting, the spreading of a Pb droplet is accompanied by a simultaneous surface transition from a faceted to a premelted Al substrate. Here, we examine how the coupling of the droplet spreading to the premelting affects the spreading kinetics and compare the results to two standard limiting mechanisms of droplet spreading: hydrodynamic spreading, in which the spreading energy dissipation is dominated by viscous relaxation, and kinetic spreading, where interfacial friction dominates. These two mechanisms predict different power-law dependency for the droplet radius versus time. For temperatures where premelting is present (between 875 K and the melting point of aluminum), kinetic spreading is observed at intermediate times. Because Pb droplets only partially wet the surface, the droplet radius at long times relaxes exponentially to the equilibrium droplet shape. Spreading simulations of a faceted system at 625 K [below the Al(111) roughening temperature] show that this system is consistent with a hydrodynamic spreading mechanism. However, when we examine a system at 922.38 K in which premelting is artificially suppressed by introducing harmonic constraints to give surface vibrations consistent with the Al(111) free surface, we observe a kinetic spreading mechanism - as in the premelted system. When premelting is suppressed, the equilibrium contact angle is observed to be significantly larger than when premelting is present even at the same temperatures. Thus, we conclude that the presence of the premelting layer has a significant effect on the thermodynamics Al(111)/Pb solid-liquid interface, but little effect on the mechanism of spreading. We also observe that the structure of the droplet contact line in the presence of the premelting layer is described by two contact angles (instead of the usual one) due to the presence of the additional premelting layer, and resembles structures seen in reactive wetting systems.