Chemistry Department, Moscow State University, Moscow
Spreading of molten lead on copper surface has been studied by dispensed drop technique at 450ºC in vacuum or reducing atmosphere of helium-hydrogen gas mixture. Copper (100); (110); and polycrystalline surfaces were investigated. The profile of spreading drop was observed by rapid photography using fast CMOS video camera.
The simulations of spreading have been carried out by classical molecular dynamics (MD) using embedded atom method (EAM) potential. The lateral size of simulation cell was 115x115nm. The Pb droplet was represented as a sphere of 16 nm diameter. The simulations have been performed on (100), (110), or (111) copper crystal surfaces. The calculations have been run on a SKIF-MSU supercomputer using efficient parallel Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS).
The goal of this study was to determine whether the spreading anisotropy could be observed on smooth homogeneous metallic substrate in the absence of dissolution or intermetallic formation reactions.
In experiment two drops were consequently deposited on each substrate. The first drop was placed immediately after the sample was conditioned at experimental temperature. The second drop placed after heating the sample with first drop at 520ºC during 10 min and the cooled back to 450ºC. It was found, that kinetics of spreading is significantly different for the first and the second drops. In the case of the second drops the spreading process was saturated after ~10 ms, but for the first drops the spreading time exceeds 102 s.
Anisotropy of spreading was observed only on Cu (110) surface and only for the first drop. The shape of wetting line for these drops was elliptical with long axis directed at [100] direction, while in other cases the shape was generally circle.
Simulation data has shown that spreading of the drop accompanied by formation of a few monolayer thick precursor film in front of wetting line. On (100) and (111) faces the shape of precursor film is generally circular, but on (110) face the layer is strongly elongated along [100] direction. However the shape of sessile drop was virtually spherical on all studied copper surfaces.
Financial support of RFBF under contract 11-08-01244-а is acknowledged