Previous work showed that solid polar surfaces with a very small dipole length still might be quite hydrophobic even with large values of charge. Using molecular dynamics simulations, we have found that the presence of the point defects on a solid polar surface greatly influences the wetting behavior of water, even at a very low defect ratio of 1%. As the defect ratio increases, the coverage of the water layer over the solid surface also increases. Because of the breakdown of steric exclusion, the water molecules strongly bind to the solid surface mainly through electrostatic interactions between their hydrogen atoms and the negative charges near the positive-vacancy defects on the surface, or between their oxygen atoms and the positive charges near the negative-vacancy defects.

Based on molecular dynamics simulations, we found a nonmonotonic relationship between the contact angle of water droplets and the surface polarity on a solid surface with specific hexagonal charge patterns at room temperature. The contact angle firstly decreases and then increases as polarity (denoted as charge q) increases from 0 e to 1.0 e with a vertex value of q = 0.5 e. We observed a different wetting behavior for a water droplet on a conventional nonwetted solid surface when q ≤ 0.5 e, and a water droplet on an ordered water monolayer adsorbed on a highly polar solid surface when q > 0.5 e. The solid–water interaction, density of water, hydrogen bonds, and water structures were analyzed. Remarkably, there was up to six times difference in the solid–water interactions despite the same value of the apparent contact angle values.

The mechanism of ice nucleation at the molecular level remains largely unknown. Nature endows antifreeze proteins (AFPs) with the unique capability of controlling ice formation. However, the effect of AFPs on ice nucleation has been under debate. Here we report the observation of both depression and promotion effects of AFPs on ice nucleation via selectively binding the ice-binding face (IBF) and the non–ice-binding face (NIBF) of AFPs to solid substrates. Freezing temperature and delay time assays show that ice nucleation is depressed with the NIBF exposed to liquid water, whereas ice nucleation is facilitated with the IBF exposed to liquid water. The generality of this Janus effect is verified by investigating three representative AFPs. Molecular dynamics simulation analysis shows that the Janus effect can be established by the distinct structures of the hydration layer around IBF and NIBF. Our work greatly enhances the understanding of the mechanism of AFPs at the molecular level and brings insights to the fundamentals of heterogeneous ice nucleation.

It is generally accepted that the metal–water interface tensions are quite high; thus, the metal surfaces are usually regarded as hydrophilic. Using the molecular dynamics simulations, we have investigated the microscopic wetting behaviors of a series of metal surfaces at room temperature, including Ni, Cu, Pd, Pt, Al, Au, Ag, and Pb with three crystal faces of (100), (110), and (111). We have found that the wetting of the metals is greatly dependent on both the lattice constants and crystal surfaces. Particularly, stable water droplets are found forming on the first ordered water layer, serving as an evidence of room temperature “ordered water monolayer that does not completely wet water” on Pd(100), Pt(100), and Al(100) surfaces, while water films without ordered water monolayer are found on (110) and (111) faces of all metal surfaces and even (100) face of other metal surfaces (Ni, Cu, Au, Ag, and Pb). The formation of water droplets is attributed to the rhombic ordered water layers on the surfaces, reducing the number of hydrogen bond formation between the monolayers and other water molecules atop the water monolayer. These results demonstrate a tight correlation among the lattice constant, the crystal faces, and the surface wetting behaviors. Our findings of the novel wetting behavior may have potential applications in the surface friction reduction at the metal surfaces, design of the anti-ice materials, and the nonfouling materials.

Using molecular dynamics simulations, we have found that the charge dipole length has a large effect on surface–ethanol adsorption behavior, particularly the orientations of the ethanol molecules of the first ethanol layer and subsequently the wetting behavior. At surfaces with large charge dipole length, an ordered ethanol monolayer forms with upright orientations of the ethanol molecules and an ethanol droplet forms on this ordered monolayer, which can be termed as “ordered ethanol monolayer does not completely wet ethanol”. The upright orientations, with the OH groups buried beneath the monolayer, exclude the possibility of forming hydrogen bonds between the ordered ethanol monolayer and ethanol molecules in contact with the monolayer, leading to a phenomenon wherein the ordered ethanol monolayer does not completely wet ethanol. The strong binding mainly occurs between the negative charges of the surface and the OH group of the ethanol molecules while the hydrophobic ethyl tails point away from the surface. When the surface charge dipole length is small, a flat orientation of the first ethanol layer and an ethanol droplet are observed despite the large charge value. This can be attributed to weak surface–ethanol interactions, where the steric exclusion effect prevents the OH group of the ethanol molecules from attaching to the surface charge dipoles. Our work shows the effect of the surface lattice structure on the orientations of the adsorbed ethanol molecules and the subsequent wetting behavior.

Based on molecular dynamics simulations, we found a nonmonotonic relationship between the contact angle of water droplets and the surface polarity on a solid surface with specific hexagonal charge patterns at room temperature. The contact angle firstly decreases and then increases as polarity (denoted as charge q) increases from 0 e to 1.0 e with a vertex value of q = 0.5 e. We observed a different wetting behavior for a water droplet on a conventional nonwetted solid surface when q ≤ 0.5 e, and a water droplet on an ordered water monolayer adsorbed on a highly polar solid surface when q > 0.5 e. The solid–water interaction, density of water, hydrogen bonds, and water structures were analyzed. Remarkably, there was up to six times difference in the solid–water interactions despite the same value of the apparent contact angle values.