Molecular Origin of Wetting Characteristics on Mineral Surfaces.

Accurate determination of the wetting characteristics on mineral surfaces is critical for many natural processes and industrial applications where multiphase flow in porous media is involved. The wetting behaviors on mineral surfaces are controlled by water-mineral interactions, giving rise to various wetting characteristics, including contact line advancement, formation of precursor films, etc. However, a fundamental understanding of wetting characteristics on different mineral surfaces is still lacking at the molecular level. Here, utilizing a comprehensive set of molecular dynamics simulations, we investigate the wetting characteristics of water on various mineral surfaces and obtain the corresponding water-mineral interaction properties (including the areal density of water-mineral interaction energy and the work of adhesion of the water-mineral interface), mineral wettability, and structural and diffusion properties of water molecules near the surface. We show that the diffusion properties of water molecules on mineral surfaces play an important role in wetting characteristics. We find that the contact line tends to advance forward in the jumping mode or the rolling mode during the wetting process, which depends on the diffusion capacity of the water molecules on mineral surfaces. The corresponding evolution of the solid-liquid friction coefficient during dynamic spreading is also analyzed. We further demonstrate the strong impact of isomorphic substitution and charge-balancing counterions on wetting characteristics on the surfaces of clay minerals. It is shown that the introduction of charge-balancing counterions can shift the mineral surface from strongly hydrophilic to strongly hydrophobic and lead to completely different wetting characteristics. Our results provide a clearer picture of the molecular underpinnings in mineral wetting phenomena and deepen the understanding of the control of water-mineral interactions on the wetting properties.

Roles of energy dissipation and asymmetric wettability in spontaneous imbibition dynamics in a nanochannel.

HYPOTHESIS: The imbibition dynamics is controlled by energy dissipation mechanisms and influenced by asymmetric wettability in a nanochannel. We hypothesize that the imbibition dynamics can be described by a combined model of the Lucas-Washburn equation and the Cox-Voinov law considering velocity-dependent contact angles. METHODS: Molecular dynamics simulations are utilized to investigate the imbibition dynamics. A wide range of wetting conditions is achieved via adjusting the liquid-solid interaction parameters, and the spontaneous imbibition processes are quantified and compared. FINDINGS: The critical condition for the occurrence of spontaneous imbibition is analyzed from a surface energy perspective. The analyses of energy conversion and dissipation indicate that the viscous dissipation is dominant during spontaneous imbibition. The classical Lucas-Washburn equation is modified with the Cox-Voinov law considering the effect of the dynamic contact angle and an effective equilibrium contact angle. We show that the proposed theory well captures the imbibition dynamics embodied in the growth of imbibition length as well as the transient interface shape and velocity for both the symmetric and asymmetric wetting conditions. In nanochannels with asymmetric wettability, the imbibition length difference between the sidewalls and interface oscillations increases with wetting disparity. Our findings deepen the understanding of imbibition dynamics on the nanoscale, and provide a theoretical reference for relevant applications.