Study on the fabrication of superhydrophobic surfaces by bottom-up approach and their applications

Abstract

Generally, a superhydrophobic surface has properties of extreme water repellency, showing a very high water contact angle greater than $150^\circ$ and a very low water drop roll-off angle less than $10^\circ$. Water drops on a superhydrophobic surface can move freely, while keeping the surface clean by detaching and removing dust from the surface. These surfaces can be easily found in nature, such as a lotus leaf, a clover leaf, a dragonfly and a water strider. There have been various studies on the applications of these unique features. Various applications, such as self-cleaning effect, drag-reduction, a stain-free fabric, water-oil collecting system, and droplet guiding system, have been studied by applying a superhydrophobic surface. For a wide-range of applications of superhydrophobic surfaces, it is necessary to develop low-cost and quick process for the fabrication of a superhydrophobic surface. In this study, simple fabrication of superhydrophobic surfaces by bottom-up approach and their applications are studied. A stable superhydrophobic surface is quickly created by fixing fragile candle soot particles with paraffin wax. It is interesting that a superhydrophobic surface can be made with only a candle. The fabrication does not require further materials and processes, such as additional solvent, surface treatment, drying process or post-treatment. Coating layer fabricated in this way has an improved durability against drop impact by up to a factor of 50, compared to a bare soot coating without paraffin wax treatment. The mechanism behind the durability enhancement has been explained. The simple coating method can be applied to various surfaces, such as metal, ceramic, wood, plastic and even paper. Proof-of-concept demonstrations in the application of this coating method are provided. For a transparent superhydrophobic surface, coating materials should be transparent and hydrophobic. Also, the surface roughness of the final coating should be as small as about 100 nm due to the Mie scattering effect, while maintaining superhydrophobicity. In order to satisfy those particular conditions, usually cumbersome and time-consuming processes were used. In this work, it was found that the controlled combustion of PDMS oil generates hydrophobic silica nanoparticles. A transparent superhydrophobic coating can be successfully created with commercial silicone oil and a spray system. The silicone oil-based process does not require any additional solvent and further processes. It is expected that this process will provide inexpensive and quick fabrication of a transparent superhydrophobic surface and has a strong potential in a wide range of applications. A superhydrophobic magnetic elastomer actuator has been developed for the fast droplet transportation. The large area surface of the magnetic elastomer actuator is successfully coated with an airbrush by employing a solvent/non-solvent method with isotactic polypropylene. The movement of the droplet can be controlled through a local deformation of the surface of the elastomer by the magnetic actuation. The applicability of the devices as a new type of open-surface digital microfluidics is also demonstrated with a simple chemical reaction.