Everyone loves beading and hydrophobic products. We certainly do! But why does it happen and how do they work? What do people mean when they say does it bead or sheet?
I saw a post on Facebook the other day asking for a recommendation for a product that would sheet instead of bead. I was astounded at the amount of answers from self proclaimed “experts” who were just wrong. They just added more and more misinformation on what the difference between beading and sheeting actually was so… here’s the actual difference:
We’ve all seen pictures of vehicles with crazy beading, but why does it bead? Well it’s actually down to a few factors:
- Adhesion and Cohesion – Adhesion is the tendency for dissimilar surfaces to cling to each other, for example glue has a massive adhesion which is why it sticks to most things. Cohesion is the opposite it is the tendency for similar surfaces to cling to each other, for example have you ever seen a water droplet merge with another? That’s because it has high cohesion with itself.
- Wettability – Wettability is the ability for a liquid to maintain contact with a solid surface. This is due to balance between the adhesion and cohesion of the liquid and the surface.
- Surface Tension – Due to cohesive forces a molecule of water in a droplet is pulled equally in every direction by neighbouring water molecules. The molecules at any surface being either the water-air or water-solid do not have the same forces acting on all sides and therefore are pulled inwards this is why droplets of water dripping from a tap for instance are spherical.
On a hydrophobic surface cohesion dominates over adhesion, that means the water prefers to be with itself than with the surface and forms droplets. Surface tension causes them to become almost spherical and the wettability is low as there is hardly any surface area of the droplet touching the surface.
That’s why you get beading!
On the other hand on a hydrophilic surface adhesion dominates over cohesion, that means the water prefers to be with the dissimilar surface than with itself and forms a film. Surface tension causes them to become almost flat and the wettability is high as there is a large amount of surface area of the droplet touching the surface.
That’s why you get sheeting!
The common misconception is that hydrophobic products cause sheeting. This is technically incorrect. What you see is sheeting but it is not caused by the hydrophobic product. This is where the misinformation comes from. The sheeting comes from the water on top of the hydrophobic surface.
When spraying a fine mist over a freshly waxed or sealed paintwork you can see incredible beads. That’s because the protection is hydrophobic and you can see as mentioned above the water hates the surface and tries to avoid touching it at as much as possible forming a bead.
However… if you then hose water onto the same paintwork you will see what appears to be sheeting. This is because the surface of the paintwork is hydrophobic but it is over saturated with water and although there is beading happening on a microscopic level at the surface the majority is suffocated. The first layer of water has suffocated the protection and instead any additional water is therefore only touching more water which stick to each other and form a now hydrophilic layer and appears to sheet.
So in the same adage of “guns don’t kill people, people kill people” – no, technically hydrophobic products don’t cause sheeting, the water they repel do.