Publications

The spreading of liquid nanodroplets of different initial radii R0 is studied using molecular dynamics simulation. Results for two distinct systems, Pb on Cu(111), which is nonwetting, and a coarse-grained polymer model, which wets the surface, are presented for Pb droplets ranging in size from ∼55000 to 220000 atoms and polymer droplets ranging in size from ∼200000 to 780000 monomers. In both cases, a precursor foot precedes the spreading of the main droplet. This precursor foot spreads as rf2(t)=2Defft with an effective diffusion constant that exhibits a droplet-size dependence Deff∼R01/2. The radius of the main droplet rb(t)∼R04/5 is in agreement with kinetic models for the cylindrical geometry studied. © 2005 The American Physical Society.

Reactive wetting in the eutectic AgCu system is studied with molecular dynamics simulations. As Ag(l) spreads on the Cu surface, Cu dissolves into the liquid. The results for reactive wetting are compared to simulations in which no mixing is permitted, demonstrating that wetting kinetics are enhanced by dissolution reactions. The time dependent radius of the droplet R(t) is used to quantify kinetics for the wetting geometry of an infinitely long cylinder spreading on a substrate. Data show that, when dissolution is dominant, spreading is well described by R(t) ∼ (R0t)1/2, where R0 is the starting cylinder radius. Contact angle θ(t) data were calculated via a method that accounts for structure near the contact region and compared to data obtained using circular fits to the droplet profile. Significant differences were observed due to molecular scale structure that rapidly evolves near the contact line. This structure exhibits markedly lower θ than what is predicted from droplet profile data and it is proposed to exist throughout most stages of dissolutive wetting. Simulations of AgCu binary liquids spreading on Cu demonstrate that wetting kinetics decrease with increasing Cu in the liquid, further emphasizing that wetting kinetics are intrinsically linked to dissolution kinetics. After dissolution is complete, a Ag-rich monolayer of atoms advances diffusively across the Cu surface. © 2005 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

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Atomistic simulations were performed to investigate high temperature wetting phenomena for metals. A sessile drop configuration was modeled for two systems: Ag(l) on Cu and Pb(l) on Cu. The former case is an eutectic binary and the wetting kinetics were greatly enhanced by the presence of aggressive interdiffusion between Ag and Cu. Wetting kinetics were directly dependent upon dissolution kinetics. The dissolution rate was nearly identical for Ag(l) on Cu(100) compared to Cu(111); as such, the spreading rate was very similar on both surfaces. Pb and Cu are bulk immiscible so spreading of Pb(l) on Cu occurred in the absence of significant substrate dissolution. For Pb(l) on Cu(111) a precursor wetting film of atomic thickness emerged from the partially wetting liquid drop and rapidly covered the surface. For Pb(l) on Cu(100), a foot was also observed to emerge from a partially wetting drop; however, spreading kinetics were dramatically slower for Pb(l) on Cu(100) than on Cu(111). For the former, a surface alloying reaction was observed to occur as the liquid wet the surface. The alloying reaction was associated with dramatically decreased wetting kinetics on Cu(100) versus Cu(111), where no alloying was observed. These two cases demonstrate markedly different atomistic mechanisms of wetting where, for Ag(l) on Cu, the dissolution reaction is associated with increased wetting kinetics while, for Pb(l) on Cu, the surface alloying reaction is associated with decreased wetting kinetics.

The spreading of polymer droplets is studied using molecular dynamics simulations. To study the dynamics of both the precursor foot and the bulk droplet, large hemispherical drops of 200 000 monomers are simulated using a bead-spring model for polymers of chain length 10, 20, and 40 monomers per chain. We compare spreading on flat and atomistic surfaces, chain length effects, and different applications of the Langevin and dissipative particle dynamics thermostats. We find diffusive behavior for the precursor foot and good agreement with the molecular kinetic model of droplet spreading using both flat and atomistic surfaces. Despite the large system size and long simulation time relative to previous simulations, we find that even larger systems are required to observe hydrodynamic behavior in the hemispherical spreading droplet.

Wetting in a system where the kinetics of drop spreading are controlled by the rate of formation of a precursor film is modeled for the first time at the atomistic scale. Molecular dynamics simulations of Pb(l) wetting Cu(111) and Cu(100) show that a precursor film of atomic thickness evolves and spreads diffusively. This precursor film spreads significantly faster on Cu(111) than on Cu(100). For Cu(100), the kinetics of drop spreading are dramatically decreased by slow advancement of the precursor film. Slow precursor film kinetics on Cu(100) are partly due to the formation of a surface alloy at the solid-liquid interface which does not occur on Cu(111). © 2003 The American Physical Society.