Introduction
The superhydrophobic surface is easily found in nature, such as the lotus leaf or butterfly. The micro/nanostructure on the lotus leaf makes it extremely hydrophobic. It can help the lotus leaf keep the surface clean and dry. The wings of a butterfly also have this property. If there is dew between the two wings, the capillary force, and the surface tension may cause butterfly wings hard to be re-opened again. However, thanks to the butterfly wings are the superhydrophobic surface, which can prevent dew from staying between the wings; thus, a butterfly can spread wings in a humid place. With the development of micro/nanotechnology in recent decades, the applications of superhydrophobic surfaces, such as waterproof spray, automobile glass, and anti-fouling treatment of solar cell surfaces, have been easily found in the market. In the medical field, many blood cells are damaged during a transfusion. Related studies have shown that superhydrophobic surfaces can slow the rate of blood cell hemolysis. Although there is much-supported evidence that SHS can reduce wall shear stress, the drag reduction mechanism of superhydrophobic surfaces is still an issue. Our research focuses on the mechanisms of shear stress reduction.
Experiment setup
We designed a small water channel according to the needs of the experiment, the cross-sectional area of the test section is 10 mm * 5 mm. The diffusion, contraction, mesh, honeycomb, and porous structure of copper pipes allow water to flow uniformly. Most of the parts are made of acrylic. The test section needs to be replaced with different test pieces, so it is designed as a detachable part. The initial design is a bottom opening type. Due to the difficulty of replacing test pieces, it is changed to the upper opening type. Moreover, the seam surface and the observed surface are staggered to avoid the seam surface blocking the view of the near the wall area. The design of each component also considers the possibility of disassembly and replacement.
In the experiment, we make the flow field reach a steady-state, fully developed flow before reach the test pieces. The pressure difference between the front and back of the pipe flow can be regarded as the total energy loss, which comes from the wall shear stress in our experiment design. We use this principle to measure the pressure difference of the Acrylic surface (AS) and porous superhydrophobic surface (SHS).
Experiment results
Preliminary results show that the pressure difference of the acrylic surface is higher than that of the porous superhydrophobic surface. The wall shear stress of the SHS is less than that of the AS. In present work, we start to use FV (flow visualization) and PIV (Particle Image Velocimetry) to capture the flow structures and analyze the shear stress reduction effect of superhydrophobic surfaces.
