The secret of perpetually stainless stuff hid for centuries in plain sight—at least for those of us living around lotus leaves. Now physics can help you never to clean again, no matter what you spill.
It all comes down to how water sticks to stuff, in other words, how stuff gets wet.
Water molecules are made of oxygen and hydrogen atoms. Oxygen loves electrons, and bullies them away from the tiny hydrogens. So one side of the molecule gets a little negative charge, while the other side has a tiny positive one.
When water approaches a surface oxygen-side-on, its electrons push the ones in the surface a little bit away (and the opposite happens if it comes hydrogen-on). The surface gets temporarily a little bit charge, and water immediately sticks to it. Stuff gets wet because of electricity.
Water molecules also stick to each other. If they like sticking to each other better than to the surface, they curl up in a ball and roll off. Materials that do that are called hydrophobic, meaning that they fear water… Though it’d be more accurate to say that water is scared of them.
Chemical coatings, like those on non-stick pans, can make a surface hydrophobic because their electrons are harder to push around. But there’s a better way—the physics way.
Take the lotus leaves. They are covered in tiny bumps and ridges, a few atoms big. Water droplets rest on the tip of just a couple of bumps, too little to properly stick. Simplifying a bit, this means that cohesion forces within water “win”, keeping the drops rolled in tidy balls that simply roll away, raking up all the dirt.
Studying the leaves, scientists found the trick, and recently managed to create surfaces with the right kind of tiny bumps and ridges. These materials never get wet or dirty, for surgical tools that blood can’t stain, or toilets that don’t need cleaning, saving precious water in places where it’s needed.
If you want more
- This post is part of a running series on wetting and molecular forces. Check it out!
- Turns out, how drops are made is important: lotus leaves absorb a lot small vapor droplets
- Reading around, I’ve come across this hydrophobic knife that cuts water drops. How cool is that?