Understanding wetting at nano scale: Atomistic simulation investigations

Abstract

The wetting property of water is crucial in a variety of chemical, biological and technological systems. Finding the relevance between water and the structure of material surface is the key to understand the wettability of water and other implications such as those design nanostructured devices. In those work, we attempt to use the classical and the first-principles molecular dynamics simulations to explore the interactions between water and material surfaces. We have achieved a series of results, mainly including the following aspects: (1) Wettability of graphene and h-BN. (2) Relations between wettability and lattice constant. (3) Wetting of synthesized Bi2Se3. The water contact angle of the ideal Bi2Se3 surface is ~98.4°, and the edge of the terrace on its surface is extremely hydrophilic. (4) Wenzel to Cassie state transition. (5) Characterizing hydrophobicities of amino acid side chains. (6) Hydrophobicity and ion-selectivity. According to these examples, it is found that the molecular dynamics simulation is an extremely important tool to reveal the mechanism of wetting. In wettability of graphene and h-BN, we used Born-Oppenheim quantum molecular dynamics (QMD) simulations and investigated interfacial properties of water droplets on a graphene sheet or a monolayer BN sheet. The QMD simulations reveal that the contact angle of the graphene is 87°, while the contact angle of the monolayer BN sheet is 86°. So they all show weakly hydrophobic. Compared with nanodroplets of water in a supercooled state on the graphene, the results show that under the supercooled condition, water droplets exhibit an appreciably larger contact angle than under the ambient condition. In addition, there are also related introductions about evolution of the interfacial shape in dissolutive wetting and electrowetting. In the relations between wettability and lattice constant, the MD simulation presents a small uniform strain (±3%) applied to the lattice constant of a multilayer hydrophilic surface can generate a marked change in the wetting tendency. Meanwhile, when the lattice constant of a hydrophilic surface matches the projected oxygen-oxygen distance of bulk water to the surface, a contact-angle minimum is resulted. They surmised that the structure of the first water layer next to the surface can strongly affect the contact angle of water droplets. For vdW layered materials surface and nanostructured surfaces by design, the MD simulations show that a modified Wenzel model can proposed for describing the wetting behavior of van der Waals layered materials with topographic surfaces and the water contact angle of the ideal Bi2Se3 surface is ~98.4°. The Cassie state may translate into Wenzel state via precisely designed trapezoidal nanostructures. Two key parameters—The base angle of the trapezoids and the intrinsic contact angle—can be controlled the transition. For example, there will be three regimes for a given base angle. For characterizing hydrophobicity of amino acid side chains in a protein environment, the hydrophobicity of 20 types of amino acids has been studied by measuring the CA ( θ ) of a water nanodroplet. The results show that all nonpolar amino acid side chains are hydrophobic and the difference in θ among the nonpolar amino acid side chains is very small. Finally, we introduced the mechanism of selective ion transport in hydrophobic subnanometer channels, intercalation and diffusion of lithium inos in a carbon nanotube bundle and a series of related issues.