The Properties of Water on Solid Surface at Micro/Nano-scale

Author:Zhang Kai

Supervisor:wang feng hui


Degree Year:2018





Wettability is one of the important features of the material surface.The study of surface controllable wettability is of great significance in both basic research and industrial applications.The main content of this dissertation is to discuss the properties of water on solid surface at micro/nano-scale using molecular dynamics simulation method.The main work is as follows:(1)A molecular dynamics model of water droplets on the solid surface with different morphological microstructures was established.The water droplets were driven by the surface energy gradient to eliminate the influence of external conditions on the simulation results.Combined with the advantage of excellent motility and high hydrophobic stability,a kind of groove-nail hybrid microstructures is proposed.The groove-nail hybrid microstructure can make the water droplets have less viscous resistance when moving along the direction of the groove on the solid surface without easily losing the hydrophobicity.(2)A controllable wettability graphene surface model was established.From the perspective of the intuitive contact angle,the motion behavior of water droplets on the graphene surface including various wettability combinations is discussed.It has been found that the control of the motion status of water droplets driven by the wetting gradient can be achieved by adjusting the wettability of the solid surface.In addition,the movement of the water droplets driven by the wetting gradient is similar to that of the tank’s tracks,which the water molecules move counterclockwise around its centroid as the water droplets move with its centroid.This mode of movement of water droplets will provide a reference for the manufacture of self-cleaning functional surfaces.(3)A model of graphene nanochannel with uneven wettability was established.The water molecules passed through the uneven wetting graphene nanochannels under different external pressures.In this chapter,by analyzing the flow rate,shear viscosity,capillary force,density distribution along the channel height direction and apparent friction coefficient of water in the channel,we revealed that the change of transport properties of the nanoflow in the uneven wetting nanochannels due to the change of the position of the uneven wetting.(4)A molecular dynamics model of different sizes of nanodroplets spontaneously infiltrating into carbon nanotubes was established.The simulation results show that the capillary filling behavior of nanodroplets at the nano-scale is different from the capillary filling behavior of bulk water,that is,the capillary filling law is no longer completely consistent with the traditional Lucas-Washburn relationship,and the capillary filling behavior is closely related to the size of the nanodroplets.In addition,we also discussed the infiltration behavior of nanodroplets in hydrophilic carbon nanotubes and the effect of the wettability of open-hole graphene plates in the simulated system on capillary capillary filling behavior.The results are expected to help in practical applications such as water harvesting and desalination.