Super-hydrophobic surfaces (SH) are well-known in the surrounding nature which has optimized these properties over thousands of years. The extreme water repellent properties of super-hydrophobic surfaces are found in plants (e.g. lotus leaf) and animals (e.g. water striders, or pygmy gecko.
Fig. 1 Water droplet on the surface of superhydrophobic leaf
In the last years, many attempts have been made to mimic these properties found in nature. The lotus leaf effect has been studied carefully, and attention has been paid to reproduce the roughness structure of this plant. However, the development of a super-hydrophobic surface is a complex process involving physical and chemical aspects. A super-hydrophobic surface can be defined as a surface presenting a water contact angle exceeding 150°. In this case usually, a water droplet can bounce on the surface and roll-off at a tilt angle of only 10°.
Super-hydrophobic surfaces can only be achieved by a combination of hydrophobicity (low surface energy materials) in combination with appropriate surface textures. Super-hydrophobicity is usually explained by the Cassie-Baxter model according to which air is trapped in the microgrooves of the rough surface and water droplets rest on the “composite” surface comprising air and the tops of micro-protrusions. SH surfaces may act on the prevention of moisture build-up (reduction of corrosion rate), the prevention of ice formation, avoidance of bio-fouling, the development of breathable super-hydrophobic fabrics, self-cleaning properties, anti-microbial activities, reduction of the energy required to pump fluids in porous networks (ability to slide without resistance), etc. Hence, these surface properties will find numerous applications in the field of microelectronic devices, micro-electromechanical systems (MEMS) and devices, optics, medical devices, textiles, antiquities, photovoltaic cells, self-cleaning surfaces, just to mention a few highly relevant industrial-scale applications.
Fig. 2 Water droplets on superhydrophobic textile
Fig.3 Coffee and red wine droplets on superhydrophobic textile
Fig.4 Water droplets on superhydrophobic glass
Fig. 5 Superhydrophobic coating in action