Superhydrophobe - Recent Research

Recent Research

The self-cleaning property of superhydrophobic micro-nanostructured surfaces was reported in 1977, and perfluoroalkyl and perfluoropolyether superhydrophobic materials were developed in 1986 for handling chemical and biological fluids. Other biotechnical applications have emerged since the 1990s.

Many very hydrophobic materials found in nature rely on Cassie's law and are biphasic on the submicrometer level with one component air. The Lotus effect is based on this principle. Inspired by it, a lot of functional superhydrophobic surfaces were prepared.

Research in superhydrophobicity recently accelerated with a letter that reported man-made superhydrophobic samples produced by allowing alkylketene dimer (AKD) to solidify into a nanostructured fractal surface. Many papers have since presented fabrication methods for producing superhydrophobic surfaces including particle deposition, sol-gel techniques, plasma treatments, vapor deposition, and casting techniques. Current opportunity for research impact lies mainly in fundamental research and practical manufacturing. Debates have recently emerged concerning the applicability of the Wenzel and Cassie-Baxter models. In an experiment designed to challenge the surface energy perspective of the Wenzel and Cassie-Baxter model and promote a contact line perspective, water drops were placed on a smooth hydrophobic spot in a rough hydrophobic field, a rough hydrophobic spot in a smooth hydrophobic field, and a hydrophilic spot in a hydrophobic field. Experiments showed that the surface chemistry and geometry at the contact line affected the contact angle and contact angle hysteresis, but the surface area inside the contact line had no effect. An argument that increased jaggedness in the contact line enhances droplet mobility has also been proposed.

There have been a few efforts in fabricating a surface with tunable wettability. For the purpose of spontaneous droplet mobility, a surface can be fabricated with varying tower widths and spacings to gradually increase the free energy of the surface The trend shows that as tower width increases, the free energy barrier becomes larger and the contact angle drops, lowering the hydrophobicity of the material. In addition, increasing tower spacing will increase the contact angle, but also increase the free energy barrier. Droplets naturally move towards areas of weak hydrophobicity, so to make a droplet spontaneously move from one spot to the next, the ideal surface would consist of small width towers with large spacing to large width towers with small spacing. One caveat to this spontaneous motion is the resistance of stationary droplets to move. Initial droplet motion requires an external stimulus, from something as large as a vibration of the surface or as small as a simple syringe “push” as it is released from the needle.

An example of readily tunable wettability is found with special developed fabrics. By stretching a dip-coated commercial fabric, contact angles were typically allowed to increase. This is largely caused by an increase in tower spacing. However, this trend does not continue towards greater hydrophobicity with higher strain. Eventually, the cassie-baxter state reaches an instability and transitions to the wenzel state, soaking the fabric.

An example of a biomimetic superhydrophobic material in nanotechnology is nanopin film. In one study a vanadium pentoxide surface is presented that can switch reversibly between superhydrophobicity and superhydrophilicity under the influence of UV radiation. According to the study any surface can be modified to this effect by application of a suspension of rose-like V₂O₅ particles for instance with an inkjet printer. Once again hydrophobicity is induced by interlaminar air pockets (separated by 2.1 nm distances). The UV effect is also explained. UV light creates electron-hole pairs, with the holes reacting with lattice oxygen creating surface oxygen vacancies while the electrons reduce V5+ to V3+. The oxygen vacancies are met by water and this water absorbency by the vanadium surface makes it hydrophilic. By extended storage in the dark, water is replaced by oxygen and hydrophilicity is once again lost.

Another example of a biomimetic surface includes micro-flowers on common polymer polycarbonates. The micro/nano binary structures (MNBS) imitate the typical micro/nanostructure of a lotus leaf. These micro-flowers offer nanoscale features which enhance the surface's hydrophobicity, without the use of low surface energy coatings. Creation of the superhydrophobic surface through vapor-induced phase separation at varying surrounding relative humidities caused a likewise change to the contact angle of the surface. Surfaces prepared offer contact angles higher than 160° with typical sliding angles around 10°.

Low surface energy coatings can also provide a superhydrophobic surface. A self-assembled monolayer (SAM) coating can provide such surfaces. To maintain a hydrophobic surface, the head groups bind closely to the surface, while the hydrophobic miscelles stretch far away from the surface. By varying the amount of SAM you coat on a substrate, one could vary the degree of hydrophobicity. Particular superhydrophobic SAMs have a hydrophobic head group binding to the substrate. In one such work, 1-dodecanethiol (DT; CH₃(CH₂)₁₁SH) is assembled on a Pt/ZnO/SiO₂ composite substrate, producing contact angles of 170.3°. The monolayers could also be removed with a UV source, decreasing the hydrophobicity.

Superhydrophobic surface are able to stabilize the Leidenfrost effect by making the vapour layer stable. Once the vapour layer is established, cooling never collapses the layer, and no nucleate boiling occurs; the layer instead slowly relaxes until the surface is cooled.