contact angle
/ KON-takt AN-gul /
Look closely at a drop of water resting on a surface, right at its rim where the water, the solid, and the air all meet. The edge of the drop rises from the surface at some slope. On wax it climbs steeply, nearly straight up; on clean glass it lies low and flat. That slope at the rim — measured through the liquid — is the contact angle, a single number that captures how a drop sits.
More precisely, the contact angle is the angle, measured inside the liquid, between the solid surface and the tangent to the liquid's edge at the line where solid, liquid, and gas meet. A small angle (well under 90 degrees) means the drop spreads out and wets the surface well; a large angle (well over 90 degrees) means the drop balls up and wets poorly. An angle near zero means the liquid spreads into a film; an angle near 180 degrees means it barely touches at all.
Contact angle matters because it is the most direct, measurable readout of wetting, used to judge how clean a surface is, how well a coating will stick, or how waterproof a fabric is. The honest caveat is that real surfaces give messy answers: roughness, contamination, and whether the drop is advancing or receding all shift the angle, so a single tidy number is more of a guide than an exact constant.
On a freshly waxed car, a raindrop sits up as a near-ball with a contact angle well above 90 degrees, so it rolls off. On bare glass the same drop flattens to a low angle and clings, spreading into a film.
Small angle: spreads and wets. Large angle: beads and resists.
A rough convention: below 90 degrees the surface is called wetting (hydrophilic for water); above 90 degrees it is non-wetting (hydrophobic); above about 150 degrees, superhydrophobic, as on a lotus leaf. The angle is always measured through the liquid, not the air.