The effect of wetting anisotropy (formation of different equilibrium contact angle (CA) of liquid B on monocrystalline substrate A with different surface plane orientations) in binary systems is discussed in the literature since the end of nineteen century [1]. Significant amount of existing experimental data was reviewed recently [1,2]. The present study is focused on the analysis of wetting anisotropy at elevated temperature in binary liquid metal / solid metal (or metalloid) systems. Literature data is discussed together with original experimental results for Pb/Cu and Cu/Si systems and molecular dynamic (MD) simulation results. Two main aspects of the problem is considered: (i) what are the physicochemical factors which are able to distort equilibrium CA measurements in metallic systems preventing correct analysis of wetting anisotropy; (ii) how to interpret correctly CA anisotropy data in terms of interfacial (solid/liquid) and surface (solid/gas) energy anisotropy.
The first question was intensively studied for several decades and thoroughly reviewed in [3]. The main sources of systematic errors are the adsorption of impurities (oxygen etc.) mainly from gas phase and dissolution of substrate in the melt. Moreover both effects are known to be orientation-dependent. Thus CA anisotropy could be artificially enhanced due to these effects. Unfortunately most of experimental data on CA anisotropy was obtained without taking into account dissolution of substrate or pollution by the impurities and could not be used to analyze wetting anisotropy. The lack of reliable data motivates experimental and molecular dynamic study of CA anisotropy on Pb/Cu system performed in the frame of the present work.
The second difficulty under consideration is the analysis of CA anisotropy data in the framework of Young equation. Only difference between solid/gas and solid/liquid interfacial energy could be calculated. Thus this data should be coupled with independent information on the anisotropy of solid/liquid interfacial energy which could be extracted, for example, from the analysis of melt inclusion shape inside solid crystal.
This approach was applied to the data obtained for Pb/Cu system. Both experimental data and MD simulation demonstrates negligible anisotropy of CA. Solid/liquid interface is also characterized by weak orientation anisotropy [4]. Thus we can conclude on negligible anisotropy of solid/gas interfacial energy despite of the different structure of Pb adsorption films on differently oriented copper surfaces obtained from MD simulation.
Financial support of RFBR under contract11-08-01244-а is acknowledged. All simulations have been performed on SKIF-MSU supercomputer.