This work includes two separate studies regarding superhydrophobicity: 1) analytical modeling of superhydrophobic surface with inverse-trapezoidal microstructures and 2) reduction of platelet adhesion by superhydrophobic cylindrical nanoshell arrays. The abstracts of each part are as follows.
Part 1. Analytical modeling of superhydrophobic surface with inverse-trapezoidal microstructures
A polydimethylsiloxane (PDMS) elastomer surface with perfectly ordered microstructures having an inverse-trapezoidal cross-sectional profile (simply PDMS trapezoids) shows superhydrophobic and transparent characteristics under visible light. The addition of a fluoropolymer (Teflon) coating enhances both features and provides oleophobicity. This paper focuses on the analytical modeling of the fabricated PDMS trapezoids structure and thermodynamic analysis based on the Gibbs free energy analysis. Additionally, the wetting characteristics of the fabricated PDMS trapezoids surface before and after the application of the Teflon coating are analytically explained. The Gibbs free energy analysis reveals that, due to the Teflon coating, the Cassie-Baxter state becomes energetically more favorable than the Wenzel state and the contact angle difference between the Cassie-Baxter state and the Wenzel state decreases. These two findings support the robustness of the superhydrophobicity of the fabricated Teflon-coated PDMS trapezoids. This is then verified via the impinging test of a water droplet at a high speed. The dependencies of the design parameters in the PDMS trapezoids on the hydrophobicity are also comprehensively studied through a thermodynamic analysis. Geometrical dependency on the hydrophobicity shows that overhang microstructures do not have a significant influence on the hydrophobicity. In contrast, the intrinsic contact angle of the structural material is most important in determining the apparent contact angle. On the other hand, the experimental results showed that the side angles of the overhangs are critical not for the hydrophobic but for the oleophobic property with liquids of a low surface tension. Transparent, oleophobic and robust superhydrophobic properties will make the Teflon-coated PDMS trapezoids surface useful in various applications.
Part 2. Reduction of platelet adhesion by superhydrophobic cylindrical nanoshell arrays
Inhibition of the blood coagulation that arises from the natural defense system of humans is of crucial concern for implanted blood-handling devices, such as a heart valve and stent. Unless the coagulation is effectively suppressed, patients with an artificial heart valve may be burdened with the inconvenience of taking an anticoagulant for the rest of their lives. The anti-blood-coagulation effect is demonstrated on the surface of an array of cylindrical nanoshells. The resultant blood contact area is 99.2% less than that of the referenced flat surface, indicating a high level of water repellency. The significant reduction in the effective contact area leads to the notable anti-blood-coagulation effect because the clotting process relies on the surface reaction.
1) 깁스 자유 에너지량 계산을 이용해서 잘 정렬된 마이크로 구조 표면의 이론적인 접촉각과 계면 상태를 예측할 수 있는 모델을 개발하였다. 또한 개발된 모델을 이미 만들어진 마이크로구조의 접촉각 측정치와 비교하여 잘 맞음을 확인하였다.
2) 개발된 마이크로 구조가 물과의 접촉면이 작음을 이용하여, 초소수성 용도 외에 다른 응용처에 대해 사용될 수 있는지 연구를 수행하였다. 특히, 제작된 나노쉘 마이크로구조 표면이, 혈액이 엉겨 붙지 않는 항혈액응고성 표면으로 사용될 수 있음을 증명하였다.