Walk past a glass facade after a monsoon shower and you will see the problem every glass coating sets out to solve. Rain lands on dusty glass, spreads unevenly, dries in place and leaves streaks and spots behind.
Two very different coating strategies promise to fix this. Hydrophobic coatings make water bead up and roll away. Hydrophilic coatings do the opposite: they make water spread into a thin, even sheet that rinses the glass. Here is the science behind each, and how to choose for windows, facades, shower screens and solar glass.
Key takeaways
- Hydrophobic coatings make water bead and roll off, carrying dirt with it. They work indoors and outdoors, with or without sunlight.
- Hydrophilic, photocatalytic coatings make water sheet off and slowly break down organic grime, but they need UV light and regular rain or rinsing to do their job.
- Neither type makes glass truly self-cleaning. Both reduce how fast dirt builds up and how hard it is to remove.
- For bathrooms, interiors, sheltered glazing and places with long dry seasons, a hydrophobic coating is usually the more practical choice.
Why water decides how clean glass stays
Most marks on glass are what water leaves behind. Rain or shower water carries dust and dissolved minerals, sits on the glass and evaporates, leaving them behind as spots, rings and streaks.
So the real question for any glass coating is what it makes water do. Either get the water off quickly, before it dries in drops, or spread it so evenly that it drains as a sheet. Hydrophobic and hydrophilic coatings take those two routes.
Contact angle: the number behind both coatings
Scientists describe how a surface treats water with the contact angle: the angle the edge of a resting droplet makes with the surface. A low angle means the drop flattens and spreads. A high angle means it stays rounded and touches very little of the surface.
| Contact angle | What water does | Term |
|---|---|---|
| Close to 0° | Spreads into a thin, continuous film | Superhydrophilic |
| Below 90° | Spreads and wets the surface | Hydrophilic |
| 90° to 150° | Forms beads that roll off a tilted surface | Hydrophobic |
| Above 150° | Near-spherical drops that roll off at a slight tilt | Superhydrophobic |
Perfectly clean glass is actually very hydrophilic. In practice it never stays that way. Researchers at the University of Strathclyde found that freshly cleaned glass and titania films both became noticeably more water-repellent within hours on an open laboratory bench, as invisible airborne organic contamination settled on them (Mills and Crow, International Journal of Photoenergy). Water then neither sheets nor beads, and dries in blotches.
How hydrophilic, photocatalytic coatings work
Hydrophilic “self-cleaning” glass usually relies on a very thin layer of titanium dioxide (TiO₂). In 1997, researchers at the University of Tokyo reported in Nature that ultraviolet light turns a titanium dioxide surface strongly water-attracting, and that glass coated this way resists fogging and cleans itself.
- Photocatalysis: UV in sunlight activates the TiO₂, which slowly oxidises organic grime such as traffic film, bird droppings and greasy residues, loosening its hold on the glass.
- Superhydrophilicity: rain spreads across the activated surface as a sheet instead of drops, rinsing away the loosened dirt and drying with fewer spots.
The catch is that both effects depend on conditions you cannot control. The photocatalytic action needs UV light, so it is weak indoors, behind tinted or laminated glass and on shaded elevations. The rinsing needs water, so glass under a canopy, a balcony slab or a deep reveal does not get washed. Inorganic dust and mineral deposits are not broken down by photocatalysis at all; they still have to be washed off.
How hydrophobic coatings work
A hydrophobic coating takes the opposite approach. It lowers the surface energy of the glass so water cannot wet it, and fills the microscopic roughness that dirt would otherwise grip. Water gathers into beads that roll off a vertical or tilted pane, picking up loose dust on the way. Nothing here depends on sunlight, so it works on a shower screen as well as a sunlit facade.
Modern versions are often built on silica chemistry. A sol-gel coating based on silicon dioxide (SiO₂) bonds chemically to the silica in the glass itself, so it becomes part of the surface rather than a film sitting on top. Our guide to nano coatings explains this bonding in more detail.
Hydrophobic coatings have limits too. On a horizontal surface, beads sit still until they are wiped or blown away. And if hard water is left to dry as beads, each bead can still leave a small mineral spot, though one that is far easier to remove than scale on untreated glass.
Side by side: which keeps glass cleaner?
A review of anti-soiling coatings for solar glass, published in the journal Materials, looked at studies of both types and found that each can slow the loss of light transmission caused by dust. In one year-long outdoor study it reviewed, a hydrophilic coating gave solar modules about 1 to 1.5% more power at the start, with the gap growing to around 5% in dry summer months. The authors also warn that some commercial coatings reduce transparency. The right choice depends on climate and setting, not on one type being better everywhere.
| Factor | Hydrophobic coating | Hydrophilic, photocatalytic coating |
|---|---|---|
| What water does | Beads and rolls off | Spreads into a sheet and drains |
| Needs sunlight (UV) | No | Yes, for photocatalysis |
| Needs rain or rinsing | Helps, but beads also roll off on their own | Yes, to carry dirt away |
| Indoors and shower screens | Works well | Little effect without UV |
| Sheltered or shaded glazing | Works well | Limited |
| Organic grime | Easier to wipe off | Slowly broken down in sunlight |
| Hard water and limescale | Much easier to remove | Minerals still deposit and need washing |
| How it is applied | Often applied on site to existing glass | Usually applied in the glass factory |
What this means in Indian conditions
Much of India gets most of its rain in a few monsoon months, then goes through a long, dusty dry season. A coating that depends on rain to rinse it has little to work with for much of the year. A hydrophobic coating does not wait for rain: dust has less to cling to, and a quick hose-down or a squeegee pass clears it.
Hard water is the other big factor. In bathrooms, kitchens and on glass washed with borewell or tanker water, mineral spots build up fast. Indoors there is no UV for a photocatalytic layer, so a hydrophobic coating is the practical option for shower screens, mirrors and glass partitions.
Hydrophilic glass suits sunlit, rain-exposed facades specified at the glazing stage. For existing glass, a coating applied on site is the realistic route. Our earlier article on hydrophobic glass coating for facades in India covers cost, coverage and application on large buildings.
Where Nanoman Glass + Ceramic fits
Nanoman Glass + Ceramic is a hydrophobic, PFAS-free nano coating. It is alcohol-based, supplied as a concentrate that is diluted on site, and uses about 10 ml per m². It forms an invisible layer on glass, glazed tiles and sanitaryware and lasts up to 5 years, depending on use and exposure.
- Clean the glass thoroughly and remove any limescale first. The coating bonds to the glass, not to the dirt on it.
- Apply in the shade to dry glass, and buff off any haze.
- Re-coat any area where water stops beading. There is no need to strip the rest.
Maintain coated glass with plain water and a soft squeegee or microfibre cloth. For more on surfaces and application, see our FAQ.
Frequently asked questions
Is hydrophobic or hydrophilic glass coating better?
It depends on the setting. Hydrophilic, photocatalytic coatings work best on sunlit glass that gets regular rain. Hydrophobic coatings work indoors and outdoors, in sun or shade, so they suit bathrooms, interiors, sheltered glazing and regions with long dry seasons.
Does self-cleaning glass work indoors?
Photocatalytic self-cleaning glass relies on UV light and rain, so it has little effect indoors or on sheltered glass. A hydrophobic coating does not need sunlight, which is why it is the usual choice for shower screens, mirrors and glass partitions.
Will a glass coating stop hard water stains?
No coating stops minerals in the water. A hydrophobic coating makes water roll off faster and makes it harder for scale to bond firmly, so spots wipe away easily instead of building up into limescale.
How long does a hydrophobic glass coating last?
Nanoman Glass + Ceramic lasts up to 5 years, depending on use and exposure. Check whether water still beads on the glass; where it no longer does, clean that area and re-coat it.
Want glass that stays clearer between cleans? Tell us the type of glass, the area and the setting, and we will recommend the right approach. Email info@nilimanano.com and we reply within one business day.