The first video shows water droplets rolling off a tilted metal sheet. As you can clearly observe no trace of water is observed on the treated surface. The treated Zn sheet has become superhydrophobic and so it is fully protected against the corrosion caused by water.The Zn sheet is placed outside. The treated surface is left outside to test whether the coating will remain superhydrophobic after being exposed to sun, wind, rain, snow and atmospheric pollutants.

The second video shows the same coating applied this time on a wall. The coating was applied on the wall again by brush, about a year before the video was captured.

The upper part of the picture shows the untreated wall. The untreated wall is fully wetted. The rest of the wall which was protected with the coating is superhydrophobic. Consequently the wall is protected against water.

The same coating has been applied on various substrates like stones, concrete, glass, wood, fabric and the results were excellent. Every substrate became superhydrophobic!

Contact information here: http://superhydrophobiccoating.com/aboutus/

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Name: Panagiotis Manoudis
Address: Santas Str. 7, 54453 Thessaloniki, Greece
Phone: (+30) 6972913805
Email: panagiotis.manoudis@superhydrophobiccoating.com

Education:
Chemical Engineer, Ph.D
Aristotle University of Thessaloniki, Greece

Fields of experience:
Water repellant surfaces, coatings, surface protection

How we might work together:

• Seminars
• Consulting and advisory
• On site interim researcher
• Guest lectures etc.

Feel free to contact me with any ideas and proposals!

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The video features Neil Shirtcliffe of test-tube.org.uk

Please feel free to comment and discuss the video below!

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Watch it at: http://www.grcblog.com/?p=364

Of course the bouncing of a droplet on a surface is not as simple as it looks. There is a great number of parameters that determines how a droplet can impact on a superhydrophobic surface; drop size and impact velocity are examined in a new published study. The theoretical study of the impact of a water droplet on superhydrophobic surface is presented at: http://pubs.acs.org/doi/full/10.1021/la802897g

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For further information: newscientist.com

Straight to the Journal :http://www3.interscience.wiley.com/journal/121497094/abstract

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Super-hydrophobicity has also been observed on insects.

The super-hydrophobic biological surfaces have been under thorough study. On such surfaces there is a special morphology. There are surface structures in the micron-scale and also in the nano-scale. The surface structure prevents the water form touching the whole surface. So a water droplet would sit on mostly on the air like the fakir state. In some plants (e.g. Indian Cress) there is a fur on the surface. In particular there are elastic hairs on which the water is resting and so easily removing.

SEM images of the micro- and nano- structure on the surface of the Lotus leaf.

Water droplet on Lotus leaf

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Most of the real surfaces are rough. There two options for the water droplet.

i) To follow the roughness of the underlying substrate. When the substrate is hydrophilic the surface roughness reduces the water contact angle and when it is hydrophobic it enhances it.

So the parameter r (the ratio between the true surface area over the apparent one) controls θ* contact angle (Wenzel equation):

cosθ*= rcosθ

θ: Young’s contact angle

ii) The droplet does not follow the roughness of the surface

In this case the droplet is like a fakir. It is sitting upon a patchwork of solid and air. The more it is sitting on air, the higher the measured water contact angle.

The contact angle θ* is an average between the angle on the solid (cos θ) and the one on the air (cos180^o = -1):

cosθ^*= -1+f_s(cosθ+1)

f_s: fraction of the liquid that contacts the solid

The fakir state is also called Cassie – Baxter state.

A surface is called super-hydrophobic when water contact angle exceeds 150⁰. In this case usually a water droplet can bounce on the surface and also can roll-off on it with a tilt of less than 5⁰. In other words the surface is water-repellant. This kind of surfaces is also called self-cleaning. On a hydrophilic or on a hydrophobic surface the pollutants adhered to the water droplet, but the water droplet sticks on the substrate. So the water remains and evaporates on the surface and so the pollutants remain on it. Consequently the pollutants do not remove from the substrate. On a super-hydrophobic substrate the water rolls-off and leaves the surface taking the dirt with it. So the water cleans the surface. This is a very useful application especially for glasses of buildings. The self-cleaning mechanism is presented in the next figure:

Click for larger image

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A hydrophilic material is a friend of water. Friends like to come closer, so a water droplet would spread on a hydrophilic surface. Furthermore water can enter the pores of a hydrophilic material and totally wet it. Most natural materials are hydrophilic.

The evaluation of hydrophilicity is made through water contact angle measurements. A water droplet would occupy as long surface of the hydrophilic material as possible. So the water contact angle will be significantly low.

Hydrophobicity comes also from the Greek ύδωρ, (water) and φόβος (fear). It refers to the physical property of a material (known as a hydrophobic) that repels a mass of water.

Water would be afraid of a hydrophobic material. So a water droplet would try not to touch large area of the surface and the shape of the droplet would be spherical. Hydrophobic natural materials are waxes, oils and fats.

The evaluation of hydrophobicity is made through water contact angle measurements. A water droplet would be spherical so the water contact angle will be significantly high.

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Feel free to contact us with comments, questions, suggestions, offers etc: info@superhydrophobiccoating.com – yes, we do actually read and answer! :-)

We are pointing the following additional domains to this website: hydrophobiccoating.com, hydrophobicpaint.com, superhydrophobiccoating.com, superhydrophobicpaint.com, superwaterrepellent.com, waterrepellentcoating.com, waterrepellentpaint.com

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Inspired by the “lotus-leaf effect” a lot of techniques have been developed to create super-hydrophobic surfaces. The aim of the scientists is to create a coating with a surface morphology that mimics the morphology of the lotus-leaf. Some of the developed methodologies are: Coating of substrates with various nanoparticles mixed with polymers, electrochemical deposition of gold and silver aggregates followed by chemisorption of a monolayer of n-dodecanothiol, electrodeposition of copper combined with lithography or copper wet etching, close-packed polystyrene microsphere topography, casting of polymer solutions under humid conditions, replication of the lotus-leaf structure in PDMS by nanocasting, mechanical assembly of monolayers on elastomeric surfaces and a gelation process for polypropylene and tetraethyl orthosilicate mixed with an acrylic polymer.

Plasma techniques have also been used either to modify the chemistry and the surface morphology of polymeric surfaces, such as polybutadiene and polypropylene to deposit ribbon like randomly distributed microstructures, or to etch an inhomogeneous material. Some methods for the production of super-hydrophobic surfaces have been also based on the formation of densely packed high-aspect-ratio structures such as polymer nanofibres, aligned carbon nanotubes, Si pillars fabricated by photolithography and plasma etching, and Si nano-rod arrays fabricated by Si vapour deposition.

Most of the developed methodologies have been tested only in lab conditions and not outdoor.

Water droplet on calcium carbonate treated with a perfluorinated polymer and silica nanoparticles

Pros and Cons of Today’s Super-Hydrophobic Coatings Technologies.

Superhydrophobic coatings technologies are in fact under development. Most of the methodologies have been tested only in the lab. This is the major disadvantage. The behaviour of these coatings at outdoor condition has not been tested. It is also known that it is very difficult to produce robust super-hydrophobic coatings. The surface morphology is in general sensitive to mechanical wear. Finally most of the methodologies demand expensive materials, special knowledge, or some special equipment and can not be applied on all the materials. A simple methodology demanding materials of low-cost, with good results is the one with the polymers and the nanoparticles.

The advantages of the Super-Hydrophobic Coatings Technologies come from their properties and their potential applications. Self-cleaning glasses, clothes etc, protection against corrosion of all the concrete constructions (e.g bridges, marines, under water constructions etc), anti-snow sticking, even monument protection are only some of them.

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