Hydrophobicity and Super Hydrophobicity

Your first question would be – what is the difference between hydrophobicity and superhydrophobicity? “Superhydrophobic” has become the common nomenclature to describe ANY surface which easily repels liquids. Sometimes it is used correctly. Most times, however, it is incorrect.

A hydrophobic surface is a water-repelling, low energy surface that resists wetting. On the other hand, a superhydrophobic surface, also known as an ultra-hydrophobic surface, is a non-wettable surface with a high water contact angle, which facilitates the sliding of water droplets. Though hydrophobicity and superhydrophobicity seem similar at first, yet when I delved deep into the details, I was amazed at the minute differences they possessed. While a surface can be termed as hydrophobic if it has a contact angle of 90 degrees or greater, to be categorized as a superhydrophobic surface, it has to have a contact angle of at least 150 degrees as well as a sliding angle of less than 10 degrees. The main conditions determining hydrophobicity of surfaces include surface energy and surface texture. It is interesting to note that low surface energy seems to enhance surface hydrophobicity. In addition, a surface texture consisting of nanopillars (columnar structures with a diameter of one nanometer) is also a prerequisite.

The most basic example of hydrophobicity is soap. It has a short ionic part which is a carboxylic salt. This is the polar end and is hydrophilic. The long hydrocarbon chain is the non-polar end and is hydrophobic. On the other hand, an interesting example of superhydrophobicity can be a water drop sliding off a lotus leaf without wetting it. It is also called the self-cleaning property of lotus leaves. Coming to the conditions necessary for a surface to be superhydrophobic, it needs to have hierarchical micro and nano roughness and low surface tension, both at the same time. This is because the former will trap the air present on the surface, thereby increasing the water contact angle and the latter will decrease the bonding tendency of water droplets. Another factor required for superhydrophobicity is a distance of 10-15 micrometres between the protrusions present on the surface.

You must have seen videos and wondered - How does all that mud just fall right off the clothing without staining it!? It appears to be a miracle of engineering. The problem is this: superhydrophobic coatings are NOT durable. The fact is, superhydrophobic coating technology just isn’t “there” yet. The technology hasn’t advanced enough to provide durable performance for a considerable amount of time. Having said that, yes, it is true that one can get a superhydrophobic coating similar to the ones that we see in online videos. The better question, though, is why do you need it?

A hydrophobic coating (90 degrees and greater contact angle) is sufficient for 99% of the day to day activities and projects. Scientists and manufacturers have been able to create stable, covalently-bonded hydrophobic coatings that last. It may not sound as cool, but hydrophobic coatings will normally be just the thing we need.

When used as a technology, the phenomenon of hydrophobicity can find use in dust removing mechanisms, management of oil spills, chemical separation of polar and non-polar surfaces as well as in the production of stain-resistant clothing. On the contrary, superhydrophobic surfaces are used as anti-freeze surfaces, anti-bacterial surfaces, anti-fog coatings, in oil and water separation and also find many applications in the medical fields. Further research about hydrophobicity and superhydrophobicity will help us unravel many more applications of these two domains which are still left undiscovered.

- Trisha Daftari