Stable Liquid Water Droplet on a Water Monolayer Formed at Room Temperature on Ionic Model Substrates

Using molecular dynamics simulation, we show direct evidence of the unexpected phenomenon of “water that does not wet a water monolayer” at room temperature. This phenomenon is attributed to the structure of the water beneath the water droplet, which exhibits an ordered water monolayer. Remarkably, there remains a considerable number of dangling OH bonds in this room temperature water monolayer, in contrast with the absence of dangling OH bonds at cryogenic temperature.

Figure 1

(a) Geometry of the solid surface model. Red and blue spheres represent the atoms of the solid with positive and negative charges, respectively, while the green spheres represent neutral solid atoms. (b) Side view snapshot of a water droplet on a water monolayer on a modeled surface.

Figure 2

Cosine values of the contact angle $\theta$ vs the charge value $q$. From the exponential function $\cos \theta \approx 3.5\exp (-q/0.38)$ fitted to the data (black line).

Figure 3

Average number of H bonds formed by a water molecule of the monolayer under the droplet with other water molecules in this same monolayer (■), and by a water molecule of the monolayer under the droplet with the water molecules above the monolayer (●, corresponding to right red axis) with respect to $q$, together with their sum (□). Black triangles (▲) show the average number of H bonds formed by a water molecule of the monolayer outside the droplet.

Figure 4

Structure of water molecules in the monolayer on the solid surface. (a) A snapshot of the monolayer water molecules outside the droplet showing regular 2D ordered hexagons together with the H bonds that form between neighboring water molecules. Magenta arrow shows the direction we used as the crystallographic direction to compute $\phi$. (b) Probability distribution of the angle $\phi$ between the $x\mathrm{\text{−}}y$ plane projection of one water molecule dipole orientation and a crystallographic direction. Black solid squares (■) and magenta circles (●) correspond to $q=0.5e$ and $q=0.6e$, respectively, in the context of a bond length $l=0.142\mathrm{nm}$. Red triangles (▲) and blue triangles (▲) represent the water monolayer outside the droplet and the monolayer molecules under the droplet, respectively, for $q=1e$ and $l=0.142\mathrm{nm}$. Green squares (■) show the case for a larger neighboring bond length $l=0.17\mathrm{nm}$ with $q=1e$.