Drag Reduction of Biomimetic Composite Surface Texture

Skin-friction drag at a solid-liquid interface is prevalent in a wide range of engineering fields such as pipeline transportation, maritime navigation, and microfluidic devices, and it is a significant factor affecting energy utilization efficiency. In recent years, the excellent tribological and hydrophobic properties of non-smooth structured surfaces have led to their increasing application in engineering. Over billions of years of evolution, living organisms have developed capabilities to adapt to harsh environments, resulting in numerous excellent geometric structures, ingenious material topologies, simple yet effective control methods, and functionally rich surface textures. Many biological surfaces exhibit high energy utilization efficiency and remarkable friction reduction performance. Therefore, biomimetic surface texture design emerges as an efficient and accurate approach, seeking optimal solutions from nature's designs. Addressing flow drag reduction problem at a solid-liquid interface, this work proposed a method of reducing flow resistance using biomimetic composite surface texture. Inspired by the surfaces of rice leaves and shark skin, a biomimetic composite surface texture combining ribs with micro pillars was designed. The deep silicon plasma etching method was employed to fabricate the composite surface texture on silicon wafer surface. Then the textured silicon wafer was subsequently plasma-bonded with PDMS to create rectangular channels with textured surfaces. A related experimental setup was established to measure the pressure drop of water, liquid paraffin oil, and glycerol passing through textured channels with different velocities. Contact angles of water, liquid paraffin oil, and glycerol on the biomimetic textured surface were measured, and corresponding drag reduction mechanisms were proposed. The experimental results showed that the biomimetic composite surface texture exhibited drag reduction effects for water, liquid paraffin oil, and glycerol. For water flow with higher velocity, the maximum drag reduction of the biomimetic composite surface texture was about 25.2%. For lower velocity liquid paraffin oil flow, the maximum drag reduction of the biomimetic composite surface texture was about 2.07%. For even lower velocity glycerol flow, the maximum drag reduction was about 22.5%. The contact angles of water droplets and glycerol droplets on the biomimetic composite surface texture were about 132.1° and 102.9°, respectively, indicating strong hydrophobicity and good drag reduction performance. Water and glycerol exhibit Cassie-Baxter wetting states on the biomimetic composite surface texture, whereas liquid paraffin oil showed very low contact angles, only 20.7°, indicating a Wenzel wetting state. The drag reduction mechanism of water or glycerol flow was as follows: the biomimetic composite surface texture exhibited hydrophobic and glycerophobic properties, forming partial gas layers on the surface, thus transitioning the liquid-solid interface to a liquid-gas interface, achieving drag reduction. That was to say, the entrapped gas caused by the hydrophobic biomimetic composite surface texture was the mechanism for drag reduction for water or glycerol flow. While the drag reduction mechanism of paraffin oil flow was as follows: the biomimetic composite surface texture exhibits exhibited oleophilic properties, forming a thin oil film layer on the surface, which acted as a lubricant, thus achieving drag reduction. Therefore, the entrapped oil caused by the oleophilic biomimetic textured surface was the mechanism for drag reduction for oil flow. The findings could provide help for drag reduction for engineering fields where the fluid flow must overcome high drag.