Definition
A superhydrophobic coating is a surface treatment that makes water bead up into round drops and roll off on their own, instead of spreading out or soaking in. It repels water so strongly that the angle between a drop and the surface goes above 150 degrees. The idea came from the lotus leaf in nature.
The secret of the dewdrop on a lotus leaf
Look at a lotus leaf in a pond on a rainy day. The rain does not soak in. It rolls off in little beads. You might guess the leaf is as smooth as glass, so the water just slides. In fact, it is the exact opposite.
Zoom in on the leaf tens of thousands of times with an electron microscope, and you will be surprised. The surface is packed with rough bumps. On top of hills measured in micrometers, much thinner than a human hair, sit even smaller wax bumps, tens to hundreds of times smaller, measured in nanometers. The leaf has two layers of bumps.
Air gets trapped between these tiny bumps. When a water drop lands on the leaf, it does not touch the leaf surface directly. It rests lightly on the tips of the bumps and the layer of air between them. It is as if the drop is floating on an invisible air cushion.
Water drops roll and clean as they go
On a superhydrophobic surface, a water drop does not spread out. It stays close to a perfect ball. Tilt the surface just a little, and the drop does not slide. It rolls down like a marble.
Something remarkable happens next. When rain falls on an ordinary window, dust mixes with the water and leaves dirty streaks and spots. On a superhydrophobic surface, the rolling drop picks up dust particles like a magnet and sweeps them away.
That is because dust sticks to the water drop much more strongly than it sticks to the surface. So a single rain shower is enough to wash the dust off. This is the self-cleaning effect. It is how outer walls and solar panels can stay clean without any soap.
To be a bit more precise
To be a bit more precise, you cannot make a surface superhydrophobic just by smearing on something that hates water, like oil. You need both the right chemistry and a physical structure at the nanometer scale.
The angle a water drop makes with the surface is called the contact angle. Below 90 degrees, water spreads out, and the surface is hydrophilic. Above 90 degrees, water is pushed away, and the surface is hydrophobic. But even the smoothest, most water-hating oily surface tops out at about 120 degrees.
Only when you carve in tiny nanoscale bumps that trap pockets of air under the drop does the contact angle shoot past 150 degrees. Physicists call this state, where a drop rests on top of a layer of air, the Cassie-Baxter state. In the end, superhydrophobic technology is a triumph of precise structural design that copies nature's wisdom.
Common misconceptions
A superhydrophobic surface is perfectly smooth, like a sheet of glass, and that is why water bounces off.
It is actually a very rough surface, packed with nanometer-sized bumps. Air trapped between the bumps holds the drop up, and that is why water bounces off.
Where you meet it
It is used on solar panels and building windows, which wash off dust by themselves when it rains.
It is used in waterproof clothing and shoes that keep water and rain from soaking in (it may not stop oily liquids).
It is being studied for ship hulls, to cut friction with water and save fuel.
In one sentence
A superhydrophobic coating traps a cushion of air between nanoscale bumps to repel water strongly, and its rolling drops wash away dust too.