Hydrophobic is the term used when a surface “pushes away water”. The word means as much as “fear/anxiety for water”. Although there is no actual fear or anxiety, it refers to the effect that looks as if it is pushing away the water. (there is no actual repulsive force involved)
Why the need for Hydrophobic
A side effect of a protective product is often that it has a very low surface energy, lower then water molecules. This creates beading and sheeting. This effect is called “wetting”. Wetting refers to the angle of contact between water and the surface underneath. The greater this angle, the higher the wetting and the higher the bead.
A Hydrophobic surface has the benefit that water has great trouble “sticking” to the surface, which allows for the water to run off the surface when energy (like motion) is applied.
For example: a Hydrophobic windshield will cause raindrops to slide of the glass when the wind blows against them. In this scenario there is a case of high contact angle, combined with a low roll-off angle. These do not always have to co-exist. In simple terms this means that beading and sheeting don’t always have to appear together.
The benefit of a Hydrophobic surface
When water moves from a surface, it is much more likely to carry dirt and other contaminants away from the surface. Since the beads runs off, it can’t dry up and leave its trace elements and mineral on the surface. Simply put, it will help in keeping the surface more clean. It is also less likely that dirt can stick to the surface, which means it is more easy to clean and there is much less need for claying in the future. Drying after washing the vehicle, will also be more easy.
A detailed explanation
Wikipedia says the following:
The Hydrophobic interaction is mostly an entropic effect originating from the disruption of the highly dynamic hydrogen bonds between molecules of liquid water by the nonpolar solute forming a clathrate-like structure around the non-polar molecules. This structure formed is more highly ordered than free water molecules due to the water molecules arranging themselves to interact as much as possible with themselves, and thus results in a higher entropic state which causes non-polar molecules to clump together to reduce the surface area exposed to water and decrease the entropy of the system. Thus, the two immiscible phases (Hydrophilic vs. Hydrophobic) will change so that their corresponding interfacial area will be minimal. This effect can be visualized in the phenomenon called phase separation.
The contact angle gives the amount of wetting. The higher the contact angle, the less wetting is happening. This basically means that the bead will be more round. In many cases this helps the beads to have better sheeting, but does not always have to be the case.
The roll-off angle is the angle at which a surface needs to be lift to make the droplet of liquid shift. For example, if the roll-off angle is 10 degrees, it means the surface needs to be held at an angle of 10 degrees to make the droplet of water move. This is often called “sheeting” in the world of detailing. Sheeting and beading don’t have to go hand in hand. It is possible to have a very high roll-off angle (which is not good) with a very high contact angle (which is good).
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