생체모방 기반의 기능성 비젖음성 표면 개발 및 응용

Abstract

In this dissertation, research on bio-inspired functional surfaces with liquid repellency and their applications are presented. Many living creatures in nature have evolved features that prevent the detrimental effects of liquids, which come from chemistry and topology of their biological surfaces. By understanding and mimicking biological surfaces that have property of liquid repellency, human being has achieved extensive developments in a broad range of technology. In the area of non-wetting technology, innovative advances have been achieved including developments of a superhydrophobic, superomniphobic and slippery liquid-infused surface. However, liquid repellent functionality of them can be easily lost by many factors such as environment temperature and humidity, dusts, and contaminants. And, many of them suffer from low durability. Moreover, to maximize the benefits of liquid-repellency, liquid repellent surfaces are in need of multifunctionality, and efficient fabrication process in terms of material selection, process complexity and cost. The research included in this dissertation is author’s active endeavors to develop multifunctional liquid-repellent surfaces with improvement in performance and stability in order address above challenges. We developed effective strategies for functional liquid-repellent surfaces for diverse applications. This presentation comprises four individual parts where each promising strategy and its demonstration of application potential is introduced.

In chapter 2, an effective coating method to form superhydrophobic film inside three-dimensional microchannel is introduced for high efficient water transport. Highly transparent, flexible and thermally stable superhydrophobic aerogel film can be achieved inside a closed channel. Due to advantages of the convenience of coating approach, a film can be formed over a large area with complex configurations regardless of materials. Its application potential as a promising tool for reducing friction of droplet transport was demonstrated.

In chapter 3, superhydrophobic surfaces which have nanoscale surface morphology are reported to show good anti-icing performances. However, condensation is detrimental for anti-icing applications because condensate water is formed within structures on the surfaces, leading to loss of superhydrophobicity and eventually the formation of Wenzel ice. Therefore, despite even under high condensation, retaining superhydrophobicity is important for anti-frosting/icing applications. Here, simple approach to superhydrophobic nanostructured Al which maintains low adhesion under high condensation is reported. Interestingly, on this surface, coalescence-induced self-propelled jumping by condensate droplets is shown under high supersaturation (S) condition (S=3.41, 6.39). As a result, suppression of interbridge formation between condensate droplets and high energy barrier to form ice nucleation suppress frost propagation.

Despite many benefits of superhydrophobic surfaces, they readily wet by low surface tension (LST) liquids (e.g., oils, organic liquids), resulting in loss of superhydrophobicity. Liquid repellency against LST liquids as well as water was developed as omniphobic surfaces where a re-entrant curvature of a structure is necessary, which is more difficult and expensive to fabricate. Therefore, little research has been achieved to precisely control surface repellency to LST liquids. Chapter 4 reports the effective approach to durable, scalable, and tunable omniphobicity on stainless steel mesh and this strategy allows for a diverse liquids separation. Variation in re-entrant surface curvatures determines the mesh's sensitivity to liquid-vapor surface tension and changed the apparent contact angles and sliding angles for LST liquids. This feature successfully separates liquids in binary mixtures of general organic solvents.

In chapter 5, a self-lubricating elastic conductor (SLEC) with regenerative liquid repellency is introduced. This material is an organogel that consists of multi-walled nanotubes, oil, and PDMS; the SLEC spontaneously releases lubricant (oil) from inner gel matrices to the outer surfaces, so it has high liquid repellency (roll-off-angle < 5°) that is stable for > 5 months even after several loss of liquid repellency. The SLEC shows high electrical conductivity, stretchability (> 40 % strain), and high flexibility. Furthermore, the SLEC heats up by electrical and photonic stimulations, which can boost syneresis. The combination of self-slipperiness and heat generation by electrical and photonic energies has not been reported, which increases application potential. The electromechanical characteristics of the SLEC are stable under multiple stretching and bending. Lastly, we confirmed the application potential of the SLEC as a droplet movement platform. We believe the multifunctional SLECs provide many practical advantages with soft electronics, anti-icing devices, gas sensing devices, etc.