Multiscale Design and Fabrication of Bioinspired Multifunctional Superomniphobic Surfaces

Abstract

In this thesis, we studied the multiscale design and fabrication method of superomniphobic surfaces using bio-inspired approaches for multifunctional device applications. The water and oil repellent surfaces are based on polymeric structural design with chemical/plasma treatment. Theoretical analysis for water and oil wetting behaviors is presented based on the microfluidics. Through the various properties such as omniphobicity, directional liquid sliding, self-cleaning and anti-reflection effect, the multiscale bio-inspired surfaces can demonstrate useful applications to industrial energy device fields. First, we presented bio-inspired polymeric superomniphobic surfaces by using low-expertise yet robust over-etching fabrication process. The structure of gecko foot in nature has been used for various applications particularly in dry adhesive researches. However, we had an inspiration from the end tips of a gecko foot which have similar shape with the re-entrant structures. Using the conventional deep reactive ion etching process with silicon on insulator (SOI) wafer, we could make a wide-tip microstructure through the over-etching step at the etch-stop layer (SiO2), and then replicate the structure on flexible and transparent polymer surfaces. To evaluate superomniphobic properties of fabricated polymer surfaces, we measured static contact angle and hysteresis from water (γ = 72.1 mN/m) to ethanol (γ = 22.3 mN/m). Additionally, the transmittance and aging effect were analyzed for enhanced applicability. Next, we developed overhang micro line arrays inspired from microgrooves of rice leaf to merge two features, omniphobicty and anisotropic wetting property, on the desired surfaces. As previously reported, the UV-assisted micromolding process with a mixture of photoinitiator and acrylate functionalized prepolymer containing aluminum oxide nanoparticles (Al2O3) is a controllable, cheap and low-expertise route to mass production for fabricating a superhydrophobic hierarchical structure. We proposed three different cross-sectional shapes of the micro line arrays, which are prism, rectangle, and overhang structures, to evaluate the surfaces directional sliding effect for water and oil. It was found that the surfaces with overhang structure line arrays have the allowable anisotropic oil sliding effect resulting from contact angles and roll-off angles (sliding angles) of mineral oil. Also, it was worthwhile noting that we successfully mimic the rice leaf, using dual roughness prism arrays pattern. Furthermore, we demonstrated that the directional non-wetting movement of photoresist (AZ 1512) on the fabricated surfaces and the line tracing of water and mineral oil, verifying wide application in several fields. Finally, we showed a facile fabrication method to make robust and flexible PDMS films with lotus leaf-inspired dual-scale hierarchical pyramidal arrays for high efficiency of perovskite solar cells, by securing bifunctional roles of anti-reflection and self-cleaning. Fabricated H.P. PDMS film by micromolding and Ar ion surface treatment was easily attached to a bare FTO glass, which is the substrate of perovskite solar cell devices, without any additional adhesives. It turned out that the H.P. PDMS film has the anti-reflection property with structural advantages of the pyramid shapes, leading to an improvement in the Jsc and PCE of perovskite solar cells. Also, the film demonstrated exellent superhydrophobicity and self-cleaning behavior with long-term stability like the well-known lotus leaf. The experimental results revealed that this simple, yet durable anti-reflection and self-cleaning PDMS film would be useful in various energy applications in which transparent substrates require both light-harvesting and self-cleaning.