Molecular Dynamics Simulation Study Of The Surface Wetting Characteristics Of Alkane Droplets

Natural gas is a clean energy source and reduces the consumption of coal and oil,while its combustion products are also more environmentally friendly.The development and utilization of natural gas in stratigraphic deposits often involves multiple stages of condensation and heat transfer on the heat exchanger wall surface,and wettability,which characterizes the degree of liquid adhesion on the wall surface,significantly affects the condensation and heat transfer of alkanes on the heat exchanger wall surface,so the development of efficient heat exchangers requires an understanding of the wettability of alkanes.Most of the current studies involving alkane wettability are focusing on the effect of mixing alkanes and other components in some microstructure,such as gas sequestration in various geological formations and the adsorption of alkanes in pore structure in shale gas extraction,and the effect of adding alkane molecules as an guest object to study the wettability change of the original system,but none of them can directly show the wettability characteristics of pure alkane droplets.Experimental conditions to maintain the stable presence of methane/ethane droplets while measuring their contact angles are not easily achieved,experimental measurements of pure methane and pure ethane wettability have not been reported yet.The extremely small time and space scales of molecular dynamics provide new perspectives for many problems that cannot be experimentally addressed at present condition.This paper uses molecular dynamics to study the wetting characteristics of pure methane,pure ethane,and mixed alkane droplets on different wetting types of surfaces.The simulation study found that the wetting contact angle of both pure methane and pure ethane droplets decreases with the increase of the surface interaction potential well parameter,and the change trend and magnitude of both contact angles show a decreasing trendency,although their decreasing degree is not the same.The wetting contact angle of pure ethane droplet is larger than that of pure methane droplet in the studied potential well parameter range,and the difference between the two contact angles gradually increases with the increase of the surface potential well parameter.Therefore,the pure methane droplets have a higher spreading and flow degree on the same surface,while ethane has a certain wetting hysteresis compared to methane.It is found that the increase in temperature leads to an increase in the motion tendency of alkane molecules for the wetted surface,and the cumulative effect leads to an overall motion tendency of alkane droplets on the surface,thus enhancing the tendency of pure methane and pure ethane to wet on the surface,i.e.,the contact angle of pure alkane droplet increases on hydrophobic surface,remains the same on neutral surface,and decreases on hydrophilic surface.When studying the wetting characteristics of methane-ethane mixed droplets,it is found that the ethane molecules in mixed droplets are more stable and have a lower movement tendency than the methane molecules,so the ethane molecules replaced into methane droplets act like a "glue" that prevents the droplet molecules from further moving and spreading,resulting in a higher contact angle,while the methane molecules replaced into ethane droplets act as a "lubricant" that enhances the spreading motion of the mixed droplets,resulting in a lower contact angle.The change magnitude in contact angle of mixed droplets is also correlated with the wettability difference of methane and ethane on the corresponding surfaces.And the nearly linear variation trend of the contact angle curve on the same surface corresponds to the linear increasing trend in action intensity of the guest alkane.This paper investigates the wetting characteristics of alkane droplets and their causes from a microscopic perspective,which fills the gap in the wetting characteristics study of pure alkane,especially pure methane/ethane droplet,and provides a certain degree of guidance for the development and design of high-efficiency heat exchangers.