Preparation of a bionic lotus leaf microstructured surface and its drag reduction performance

Abstract

Reducing machinery surface friction resistance can improve the efficiency of energy utilization. The lotus leaf, as everyone knows, has a strong capacity for self-cleaning and hydrophobicity. In this paper, the bionic surface of the lotus leaf was prepared in large-area, and its drag reduction performance was studied by both numerical simulation and experimental analysis. Experimental results showed that the maximum drag reduction rate of the bionic surface was 6.29% which appeared at a velocity of 3 m s^-1. The contact state between liquid and bionic surface changed from Cassie state to Wenzel state with the increase of water flow velocity. The surface free energies of the bionic surface and smooth surface were 1.09 mJ m^-2 and 14.26 mJ m^-2, respectively. In the droplet rolling experiment, the water droplet was a hemisphere when it rolled on the smooth surface, while it was an ellipsoid on the bionic surface. This study provides a theoretical basis for the structural design of bionic drag reduction surfaces, which are expected to be applied in underwater vehicles.

Fig. 1. The structure of lotus leaf. (a) Lotus leaves in the pond. (b) SEM image of the lotus leaf. (c) Magnified papilla microstructures.

Fig. 2. Establishment of simulation model. (a) The flow field model for numerical simulation. (b) The sample structure for numerical simulation. (c) The size of microstructure.

Fig. 3. Fabrication process of bionic surface.

Fig. 4. Droplet rolling experiment. (a) Schematic diagram of the experimental device. (b) Inclining bench.

Fig. 5. Schematic diagram of the water tunnel experiment device.

Fig. 6. The morphologies and elements distribution on the bionic sample surface. (a and b) The micro morphologies of the bionic surface at 1000 and 5000 times magnification. (c) The cross section structure of the bionic surface. (d) The elements distribution of ESD results.

Fig. 7. Component analysis of the bionic surface. (a) X-ray analysis (b) Raman analysis.

Fig. 8. The result of numerical analysis. (a) Drag reduction rate of the bionic surface. (b) The velocity gradient of bionic surface and smooth surface.

Fig. 9. The contact state between liquid and wall. (a) Depinned-out state. (b) Pinned state. (c) Pinned-in state. (d) Fully wetted state.

Fig. 10. The wettability of smooth surface and bionic surface. (a) Static contact angle of smooth surface (b) Static contact angle of bionic surface (c) Rolling state of droplet on a smooth surface (d) Rolling state of droplet on the bionic surface.