Glass is not hydrophobic. Clean glass is actually one of the more hydrophilic (water-attracting) everyday materials you’ll encounter. A drop of water on clean glass spreads out into a thin, flat sheet rather than beading up, which is the hallmark of a hydrophilic surface. The water contact angle on clean glass typically falls between 0 and 30 degrees, with most measurements landing around 21 degrees. For comparison, a perfectly hydrophobic surface would push that angle above 90 degrees, and superhydrophobic surfaces exceed 150 degrees.
Why Glass Attracts Water
The surface of glass is covered in chemical groups called silanols, which are essentially oxygen and hydrogen atoms bonded to silicon. These groups form hydrogen bonds with water molecules, pulling them toward the surface and encouraging them to spread out. There are two types of these groups on glass: one type forms strong, tight bonds with water (behaving similarly to ice at the molecular level), while the other forms weaker bonds. Together, they make glass strongly water-attracting.
This is why a freshly cleaned window gets a smooth, even sheet of water when it rains, and why water in a glass test tube climbs slightly up the walls (the curved surface you see at the top of water in a drinking glass is this same effect in action).
Why Glass Sometimes Looks Hydrophobic
If you’ve seen water bead up on a glass surface, you’re not imagining things, but you’re not seeing the glass itself repel water. You’re seeing a thin layer of oils, dust, or other organic contamination sitting on top of the glass. Fingerprints, airborne grease from cooking, and skin oils all deposit a film that masks the glass’s natural hydrophilic character. Once you thoroughly clean glass with soap or alcohol, the beading disappears and water spreads flat again.
This is also why the contact angle for glass beads in lab settings (around 30 degrees) tends to run higher than for flat glass plates (around 21 degrees). The beads are harder to clean perfectly, and even tiny amounts of residue shift the measurement upward.
How Glass Can Be Made Hydrophobic
While glass is naturally hydrophilic, it can be chemically treated to become hydrophobic or even superhydrophobic. The most common approach uses silane compounds, molecules that bond to the glass surface and replace those water-attracting silanol groups with water-repelling chains. Fluoroalkylsilanes are especially effective. A flat glass surface coated with a fluorinated silane reaches a contact angle around 103 degrees, crossing into hydrophobic territory.
To push glass into superhydrophobic range (above 155 degrees), you need two things: a low-energy chemical coating and a rough surface texture. Researchers have achieved this by etching glass with hydrogen fluoride gas to create nanoscale roughness, then applying a fluorinated silane coating. This combination has produced contact angles as high as 162 degrees, where water droplets essentially sit on top of the surface and roll off at the slightest tilt. Laser etching is another method for creating the necessary surface texture before applying the chemical coating.
In lab settings, the transition from hydrophilic to hydrophobic is remarkably controllable. By adjusting the concentration of a silane solution, researchers have pushed glass from its natural 21-degree contact angle through intermediate stages (57 degrees, 74 degrees) all the way to 96 degrees and beyond. This tunability makes treated glass useful across a wide range of applications.
Practical Uses for Both Properties
The natural hydrophilicity of glass is actually an advantage for self-cleaning windows. Coatings made with titanium dioxide make glass even more hydrophilic, dropping the contact angle below 5 degrees. When rain hits these surfaces, water spreads into an ultra-thin sheet that washes dirt away instead of leaving streaky droplets behind. These coatings also break down organic grime when exposed to sunlight, generating reactive molecules that decompose oils and pollution on the surface.
Hydrophobic glass coatings serve different purposes. Car windshields treated with water-repellent coatings cause rain to bead up and roll off, improving visibility at speed. Shower doors with hydrophobic treatments resist water spots and soap scum. Solar panels benefit from superhydrophobic glass that sheds water, dust, and debris without manual cleaning. In laboratory and industrial settings, hydrophobic glass prevents liquids from sticking to containers and channels, which matters when working with small volumes where every drop counts.
Some newer coatings skip fluorine-based chemicals entirely, using long hydrocarbon chains instead. These offer a lower environmental footprint while still delivering strong water repellency, though they don’t quite match the performance of fluorinated options for repelling oils and other low-surface-tension liquids.
The Contact Angle Scale
Contact angle is the standard measurement for how hydrophobic or hydrophilic a surface is. It measures the angle where a water droplet meets the surface. The scale works like this:
- Below 10 degrees: superhydrophilic, water spreads into a near-invisible film
- 10 to 90 degrees: hydrophilic, water spreads but still forms a visible droplet
- 90 to 150 degrees: hydrophobic, water beads up noticeably
- Above 150 degrees: superhydrophobic, water forms nearly perfect spheres and rolls off easily
Clean glass sits firmly in the hydrophilic zone at around 21 degrees. It takes deliberate chemical treatment to push it past the 90-degree threshold into hydrophobic territory, and specialized etching plus coating to reach superhydrophobic levels above 150 degrees.