Wetting and evaporation of salt-water nanodroplets: A molecular dynamics investigation

We employ molecular dynamics simulations to study the wetting and evaporation of salt-water nanodroplets on platinum surfaces. Our results show that the contact angle of the droplets increases with the salt concentration. To verify this, a second simulation system of a thin salt-water film on a platinum surface is used to calculate the various surface tensions. We find that both the solid-liquid and liquid-vapor surface tensions increase with salt concentration and as a result these cause an increase in the contact angle. However, the evaporation rate of salt-water droplets decreases as the salt concentration increases, due to the hydration of salt ions. When the water molecules have all evaporated from the droplet, two forms of salt crystals are deposited, clump and ringlike, depending on the solid-liquid interaction strength and the evaporation rate. To form salt crystals in a ring, it is crucial that there is a pinned stage in the evaporation process, during which salt ions can move from the center to the rim of the droplets. With a stronger solid-liquid interaction strength, a slower evaporation rate, and a higher salt concentration, a complete salt crystal ring can be deposited on the surface.

Figure 1

Routine for preparing the MD simulation system of a salt-water nanodroplet on a platinum surface.

Figure 2

Density contours for droplets and contact angle measurements. The black solid line is obtained by applying a circular fit to the points with half of the bulk liquid density. The coordinates and density are normalized by ${\sigma }_{\mathrm{O}}$ (the characteristic diameter of the oxygen site in water molecules) and ${\sigma }_{\mathrm{O}}^{-3}$, respectively.

Figure 3

The MD setup for the calculation of surface tensions. The platinum atoms are fixed at the bottom of the simulation box and the thin film of salt water covers the solid surface. After the system is equilibrated, there are three interfaces formed from bottom to top: solid-liquid, liquid-vapor, and solid-vapor (due to the periodic boundary conditions).

Figure 4

Distribution of water molecules and $\mathrm{N}{\mathrm{a}}^{+}$ and $\mathrm{C}{\mathrm{l}}^{-}$ ions along the $y$ direction in the thin-film case. The overall salt concentration is 10.7% and the solid-liquid interaction strength parameter is 0.25. The densities of $\mathrm{N}{\mathrm{a}}^{+}$ and $\mathrm{C}{\mathrm{l}}^{-}$ ions are increased by a factor of 10 for a clear comparison.

Figure 5

Relation of the increments of the liquid-vapor surface tension to the salt concentration.

Figure 6

Contact angle determined from the density contours of nanodroplets and from Young's equation. In Young's equation, the surface tensions are obtained from MD measurements in our system of a salt-water film covering a platinum surface.

Figure 7

Temporal evolution of the number of vapor molecules for three different concentrations of salt-water droplets. The surface temperature is raised from 300 to 600 K in the period from 4 to 5 ns.

Figure 8

Temporal evolution of the liquid-vapor interface during the evaporation process of (a) pure-water droplets and (b) salt-water droplets with a concentration of 20.5%. Also shown are normalized density contours and velocity vectors (black arrows) at 5.0 ns for (c) pure-water droplets and (d) salt-water droplets with a concentration of 20.5%. The density and velocity are normalized by ${\sigma }_{\mathrm{O}}^{-3}$ and ${\left({\varepsilon }_{\mathrm{O}}/m_{\mathrm{O}}\right)}^{1/2}$, respectively.

Figure 9

(a) Snapshot of the salt-water droplet with a concentration of 20.7% on the surface at 6.8 ns. The red dots represent the vapor water molecules and the salt crystal is deposited on the platinumlike surface. (b) Side view of the salt crystal deposited on the surface: $\mathrm{C}{\mathrm{l}}^{-}$ is cyan and $\mathrm{N}{\mathrm{a}}^{+}$ is blue. The red circle indicates a region with good lattice structure. (c) Lattice structure of an ideal NaCl crystal.

Figure 10

Forms of the final salt crystals (top view) deposited on the solid surface with a solid-liquid interaction strength parameter $\lambda =0.25$.

Figure 11

Forms of the final salt crystals (top view) deposited on the solid surface with a solid-liquid interaction strength parameter $\lambda =0.38$.

Figure 12

Temporal evolution of the liquid-vapor interface during the evaporation of a salt-water droplet with a concentration of 25.1% and (a) a temperature ramp rate of 1500 K/ns and (b) a temperature ramp rate of 75 K/ns. Also shown are the normalized density contours and velocity vectors (black arrows) for (c) a ramp rate of 1500 K/ns (snapshot at 4.4 ns) and (d) a ramp rate of 75 K/ns (snapshot at 4.8 ns). The density and velocity are normalized by ${\sigma }_{\mathrm{O}}^{-3}$ and ${\left({\varepsilon }_{\mathrm{O}}/m_{\mathrm{O}}\right)}^{1/2}$, respectively. The surface is hydrophilic.

Figure 13

Top view of the salt ions in the droplets (salt concentration: 25.1%) during the evaporation process (temperature ramp rate: 75 K/ns). The small red dots represent the water molecules before the start of evaporation. Snapshots are at (a) $t=4.0$ ns, (b) $t=5.4$ ns, and (c) $t=9.0$ ns.