Molecular Dynamics Simulation Of Unidirectional Liquid Penetration

Unidirectonal liquid penetration has received great attraction because of the ability to transfer liquid automatically toward a specific location without costing extra energy. Such a "smart" property is promising for many applications such as smart textiles, separation technology, and fuel cell. The liquid penetration is determined by surface wettability, thickness and porous structure. But there is lack of a microscopic explanation for the mechanism. In this work, molecular dynamics simulation has been used to investigate the mechanism and law of unidirectional liquid penetration.For single atom system, molecular dynamics simulation was carried out on the investigation of permeability characteristics of liquid in homogeneous channel. In the hydrophilic channel, the entrance is a potential well for liquid indicating the easiness of infiltration, the exit is a potential barrier indicating the difficulty of exudation. The velocity of front meniscus has a good agreement with the classical capillary equation and results from literatures. In the hydrophobic channel, the entrance is a great potential barrier for liquid indicating the impossibility of infiltration without external force.Heterogeneous channel was built up to investigate the mechanism of unidirectional penetration. When the droplet penetrates from hydrophobic layer to hydrophilic layer (forward direction), the potential energy of liquid is decreasing which indicates a spontaneous process. This process can be divided into two stages. The first stage is a rapid process, liquid penetrate into the channel driven by attraction of hydrophilic layer to overcome the barrier of hydrophobic layer. The second stage where the liquid exude channel is a slow process. However, it is a non-spontaneous process from hydrophilic layer to hydrophobic layer (reverse direction). During this process, liquid contacts the hydrophilic layer which has lower potential energy firstly. Liquid cannot exude channel without external force.The basic law of unidirectional liquid penetration was revealed. The anisotropic ratio of critical breakthrough pressure and penetration speed can be improved by increasing wettability and thickness of hydrophilic layer. The potential barrier of hydrophobic layer can be weakened by decreasing the wettability and thickness, resulting in unidirectional penetration. While enlarging the channel size, it is better for penetration and can improve the speed. It indicates that the gravity can broaden the scope of requirements in wettability, channel size and thickness for unidirectional penetration. Further improving the performance of material by reducing the wettability and thickness of hydrophobic layer, increasing the wettability and thickness of hydrophilic layer, increasing channel radius, unidirectional penetration under anti-gravity field can be achieved.Using a spray-based coating fabrication approach, PS/chloroform solution was sprayed on the superhydrophilic polyester fabric. Wettability gradient from hydrophobicity to superhydrophilicity along the thickness direction was formed. Water could penetrate from the hydrophobic side, but be blocked on the superhydrophilic side. There is a difference of breakthrough pressure between forward direction and reverse direction.