Gas Enrichment at Liquid-Wall Interfaces

Molecular dynamics simulations of Lennard-Jones systems are performed to study the effects of dissolved gas on liquid-wall and liquid-gas interfaces. Gas enrichment at walls, which for hydrophobic walls can exceed more than 2 orders of magnitude when compared to the gas density in the bulk liquid, is observed. As a consequence, the liquid structure close to the wall is considerably modified, leading to an enhanced wall slip. At liquid-gas interfaces gas enrichment which reduces the surface tension is found.

Figure 1

Starting configurations to study (a) liquid-gas interfaces, and (b), (c) liquid films at walls. Initially the particles are located on a lattice, which “melts” during equilibration, forming a liquid while the vacuum is filled by a vapor-gas phase. Because of periodic boundary conditions in (b), (c) the wall terminates the vapor-gas phase in the $z$ direction. The scale is given by $l\approx 16\sigma$.

Figure 2

Density profiles for liquid-gas mixtures at phase coexistence. Gas enrichment at the interface can be observed (see inset). The liquid profiles were similar for the different gases, wherefore only one is displayed. Varying parameters for the gases $(A)–(D)$ are $({ϵ}_{\mathrm{gl}}/{ϵ}_{\mathrm{gg}},{\sigma }_g/\sigma )=(1,1)$, (1.73, 1.47), (1.73, 1.62), and (1.78, 1.62).

Figure 3

Liquid density profiles of droplets at walls, to characterize the walls. Tuning the microscopic attraction ratio ${ϵ}_{\mathrm{lw}}/{ϵ}_{\mathrm{ll}}$ (with fixed ${ϵ}_{\mathrm{ll}}$) results in a change of the macroscopic observable $\theta$. Left, (a) hydrophilic wall, ${ϵ}_{\mathrm{lw}}/{ϵ}_{\mathrm{ll}}=0.617$ with a measured contact angle $\theta \approx 80°$. Right, (b) hydrophobic wall, ${ϵ}_{\mathrm{lw}}/{ϵ}_{\mathrm{ll}}=0.333$ with $\theta \approx 115°$. The $x$ extension of the initial droplets (fluid cube) equals the $x$ extension of the simulation box, leading to (hemicylindrical) droplets, translationally invariant in the $x$ direction.

Figure 4

Liquid and gas density profiles (same symbols for liquid and gas) for liquid films in contact with walls. (a) Hydrophilic wall: the liquid exhibits the usual layering which is not altered by the presence of the gases. (b) Hydrophobic wall: note the tremendous increase of the gas density and the greatly diminished liquid density at the wall. Varying parameters for the gases $(A)–(D)$ are $({ϵ}_{\mathrm{gl}}/{ϵ}_{\mathrm{gg}},{\sigma }_g/\sigma )=(1,1)$, (1.73, 1.47), (1.73, 1.62), and (1.78, 1.62).

Figure 5

Velocity profiles of the liquid close to the wall. The gas enrichment at the hydrophobic wall clearly enhances the average velocity, contrary to the hydrophilic case, where the gas has no influence. The dashed lines are quadratic fits which are used to estimate the slip length (see text).