Self-pinning of a nanosuspension droplet: Molecular dynamics simulations
BO Shi and EB Webb, PHYSICAL REVIEW E, 94, 012614 (2016).
Results are presented from molecular dynamics simulations of Pb(l) nanodroplets containing dispersed Cu nanoparticles (NPs) and spreading on solid surfaces. Three-dimensional simulations are employed throughout, but droplet spreading and pinning are reduced to two- dimensional processes by modeling cylindrical NPs in cylindrical droplets; NPs have radius R-NP congruent to 3 nm while droplets have initial R-0 congruent to 42 nm. At low particle loading explored here, NPs in sufficient proximity to the initial solid-droplet interface are drawn into advancing contact lines; entrainedNPs eventually bind with the underlying substrate. For relatively lowadvancing contact angle theta(adv), self-pinning on entrained NPs occurs; for higher theta(adv), depinning is observed. Self-pinning and depinning cases are compared and forces on NPs at the contact line are computed during a depinning event. Though significant flow in the droplet occurs in close proximity to the particle during depinning, resultant forces are relatively low. Instead, forces due to liquid atoms confined between the particles and substrate dominate the forces on NPs; that is, for the NP size studied here, forces are interface dominated. For pinning cases, a precursor wetting film advances ahead of the pinned contact line but at a significantly slower rate than for a pure droplet. This is because the precursor film is a bilayer of liquid atoms on the substrate surface but it is instead a monolayer film as it crosses over pinning particles; thus, mass delivery to the bilayer structure is impeded.
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