A super-hydrophobic surface has a contact angle to water that is >150 degrees. The contact angle for water on smooth solids cannot exceed approximately 130°, which is the limit for chemical hydrophobicity.

To alter a hydrophobic surface to create a one that is super-hydrophobic, the surface must be roughened by the addition of micro or nano particles.

Two criteria define super hydrophobicity:

1. The surface has a very high-water contact angle (>150°) and;

2. The surface is micro or nano structured.

Creation of a surface with a greater than 150° contact angle to water is due to the air entrapment in the gaps of the rough structure. It is important to remember a super-hydrophobic surface is therefore a composite structure of air and solid in combination with the presence of low surface energy components on the roughened surface.

The phenomenon of super-hydrophobicity and related wetting mechanisms are attractive scientifically, but also from the industrial point of view, where extreme water repellency is desired. Articles with super-hydrophobic surfaces shed water very efficiently, carrying with it readily dissolved and wetted particulates.

Another advantage of a super-hydrophobic surface is that it dramatically lowers the resistance to water flow at that surface. As the affinity of the surface for air or other gas is much higher than for water, the resistance to water flow can be primarily defined by the viscosity of the water with air or other gas rather than the solid surface. In this way a superhydrophobic coating lowers the coefficient of friction or the drag.

In addition, a very important property of a superhydrophobic coating is increased ice and snow repellency.

In spite of numerous scientific papers on super-hydrophobic coatings most of the commercial super-hydrophobic surfaces have their limitations. Since a super-hydrophobic surface is a composite structure of air and solid in combination with the presence of low surface energy components on the roughened surface, this type of surface can be damaged due to abrasive forces which in turn reduce the level of hydrophobicity, limiting many commercial applications of super-hydrophobic coatings.

Just comparing the contact angles of water to hydrophobic and super-hydrophobic surfaces you would easily think that having a super-hydrophobic surface would be better than a hydrophobic surface. However, there are other types of properties that need to be considered in deciding between a super-hydrophobic and hydrophobic surface such as abrasion resistance, durability, and transparency.

Sometimes an abrasion resistant hydrophobic coating can offer more protection from water and moisture than a super-hydrophobic coating. If transparency is desired to protect against moisture, then a hydrophobic coating is generally more robust and less expensive.

Roughened and particle containing super-hydrophobic surfaces effectively repel water but do not always repel ice. The problem is the mechanism of action of super-hydrophobic coatings. In humid and cold conditions, water droplets can form within the rough super-hydrophobic coating by condensation. After condensation, water droplet nuclei create water-loving and ice-loving patches. In high humidity cold conditions super-hydrophobic surfaces can lose their hydrophobic and icephobic properties.

A surface that has ice adhesion strength of less than 100kPa is considered icephobic or anti-icing. Unfortunately, ice adheres very strongly to many structural materials – aluminum, for example, has ice adhesion strength of 1600kPa.

Since ice is frozen water, a great deal of research has been conducted on making icephobic coatings by increasing water repellency. Newer approaches based on interfacial cavitation using rubbery elastomers have been shown to create more durable icephobic materials. Even a small amount of force can deform the rubbery surface, breaking the solid free.

VIRIDYN SH-2 is a 2-step superhydrophobic coating used on marine and ground-based microwave antenna, 5G antenna, radomes and other telecom gear to prevent rain fade. VIRIDYN SH-2 also significantly reduces snow and ice accumulation. 

To learn more about VIRIDYN SH-2, click here or visit ViriDyn’s Products page.

ViriDyn also offers VIRIDYN SH-1, a 1-step superhydrophobic coating, click here or visit ViriDyn’s Products page.

If you have a technical challenge that could be solved with a superhydrophobic coating, contact us.  We have either already developed it or can use our expertise and technology to engineer a specific solution.