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Super hydrophobic coatings, What are they and how they work
Have you heard of Neverwet or UltraEverDry?
These are commercial consumer super hydrophobic coatings, and they are cheap. You can by a bottle of Neverwet spray at Amazon for just over 1 $ per ounce.
Ok, what the super hydrophobic coatings are for, you may ask.
They are for making surfaces extremely repellent to water. By surfaces, I mean almost any surfaces, sneakers, boots, dress and coat, window, car, patio, chair, road, wall, … the list goes on.
To use the products, no special tools are need, you just need to spray the products according to the instruction, and wait for it to dry.Then, voila !, you got a super hydrophobic surface. It stays dry even in water, in rain, and most importantly, stays clean !
Now let’s open the bottle to uncover the ‘secret’ ingredients, and reveal how they work.
Commercial super hydrophobic coatings generally comes with two separate parts. The base coating is a cocktail of adhesives and other chemicals, and the top coating is a solution of nanoparticles dispersed in a liquid that easily dries. As the name suggests, you apply the base coating first then the top coating.
The top coating is the most important, and probably expensive one, whereas the base coating is essentially a layer of adhesive that holds the surface and the nanoparticles together so it does not easily come off.
The nanoparticles used are generally silica, similar to sand and quartz, except they’re much smaller in size, only one thousands of the thickness of a human hair, and their surfaces are modified with hydrophobic molecules, molecules that ‘hate’ water.
When the hydrophobic nanoparticles are sprayed and dried, they form a thin layer that looks smooth to the naked eye but is very coarse on microscopic scale. When a droplet of water is placed, only the ‘hill’s of the coarse surface touches the water whereas ‘pockets’ of air is trapped between the ‘valleys’ and the water droplet.
A water droplet sitting on a super hydrophobic coated surface is supported by water-hating hydrophobic nanoparticles and ‘pockets’ of air that water does not like. Furthermore, the air, to disappointment of water, always tries to encircle.
The water molecule can not just sit there ! It stands up, by curling, and tries to rise up ! Unfortunately, gravity kicks in. The droplet beads up and tries to fly away, so close, only if a bit more energy, but eventually caves in.
The bead-like shape of water droplets on a super hydrophobic surface is the result of a struggle and negotiation between various forces. Water droplets on such surfaces are susceptible to even tiny bit of forces, like wind or gravity that can blow it away or carry off from the surface.
Besides being repellent to water, super hydrophobic coatings also repels dust and dirt.
Hydrophobic surfaces does exist in nature, including leafs of most plants, butterfly wings, and goose feathers. You can see bead-up droplets on leafs after rain or dew. Super hydrophobic surface are the extreme case of being hydrophobic, and also present in nature, such as lotus leaf which actually inspired scientist and engineers to fabricate artificial ones that have practical application.
I hope you got something by reading this piece. If you have any comments or questions, please fire away.
