Molecular Dynamics Studies On The Interaction Between Nanodroplets And Alkane Modified Graphite Surfaces

With the help of scanning electron microscope, both micrometer scale bumps as well as nanometer scale hair-like structures are observed on the surface of the lotus leaf. It is the complicated structure that is ascribed to the superhydrophobic behavior of lotus leaves. Although several attempts have been made to synthesize surface structures that mimic the superhydrophobicity, the mechanism of how the nanometer scale hair-like structures influences the behaviors of small water droplets remains unknown.When a hydrophobic surface comes in contact with water, an interface between them will be formed,and the surface may significantly influence the structure and dynamics of nearby water molecules. A molecular level understanding of how specific hydrophobic solid influences the interfacial structure of water will benefit a number of industrial applications involving water-solid interfaces. Although the experimental studies on the interfacial properties of chemical modified surfaces have been widely performed, computer simulation in this field is seldom reported up to now. In the present article, we investigate for the first time the interaction between water droplets and alkane modified graphite surfaces. The main conclusions of this study are as follows:(1) When the number of alkane molecules decreases, the mass center of the nanodroplet fluctuates on a stronger manner. This also means that the existence of alkane molecules immersed in the droplet hinders the motion of the water droplet.(2) For alkane molecules, their angle with z axis is found to decrease with the increase of the number of alkane molecules, and a some trend is found on the head-tail distance for alkane molecules as well as the height of alkane molecules. For the alkane molecules inside the droplet, the fluctuation of the angle between the alkane molecule and substrate normal as well as the height of the alkane molecules also increases with the increase of the number of alkane molecules, but the averaged head-tail distance for alkane molecules decreases.(3) We find that the length of alkane molecules does not affect their angle with respect to the substrate normal, but the longer alkane molecules show a greater fluctuation over their average angle because they have a shorter persistence length.(4) The alkane-substrate interaction potential has a significant effect on the average head-tail distance for alkane molecules outside the water droplet, but the potential seems to have nothing to do with the head-tail distance for alkane molecules inside the water droplet. The high alkane-substrate interaction potential will induce the alkane molecules to keep in a straight state.As the second part of the work, we also study the effects of confinement on the interaction between water droplet and the alkane molecules modified substrate. We chose the size of slit pore, one wall of which is modified graphite surface while the other is not modified, to4nm,6nm, and8nm. Our simulation results indicate that contact angle increases as the pore size decreases, showing a weakened hydrophobicity induced by the confinement.