Molecular Dynamics Study of Friction Reduction of Two-Phase Flows on Surfaces Using 3D Hierarchical Nanostructures

The use of superhydrophobic surfaces is one the most promising methods for reducing the friction and increasing the flow rate in fluid transfer systems. Because in such systems the surface structure plays a key role, in this study, we explore the performance of the hierarchical nanostructures. These nanostructures are inspired by the superhydrophobic surface of the lotus leaf. We consider a flow between two walls with hierarchical nanostructures and simulate the system via the molecular dynamics method. The size of the nanostructures and the distance between them have been studied to find whether a design with a maximum flow rate exists. The nanostructures have two parts, a bigger part on the wall which is a half-sphere and a smaller part which is a cylinder on top of the bigger part. Twenty-four different nanostructures are placed on the two walls and three different distances are selected. The effect of wall materials was also examined by considering four different materials, namely, carbon, silicon, and two other hypothetical materials. In the second part of this study, a two-phase flow consisting of water and air have been simulated to study the effect of the trapped airs in the performance. The results show that in the carbon-made walls for the design with minimum pressure drop, the slip length increases by 97% and the flow rate increases by 200%. The increases for silicon-made walls and similar sizes are 99 and 183%, for the slip length and the flow rate, respectively. The slip length for carbon-made walls is almost 3 times larger than the silicon-made walls. Finally, by increasing the air fraction up to 30% in the carbon-made walls, the slip length increases by 430% and the flow rate increases by 310%, and also, for the silicon-made walls, the increases are 380 and 360% for the slip length and the flow rate, respectively.