Contact angle
- Also known as
- wetting angle, Young's equation
Contact angle is the angle, measured through the liquid, where the edge of a liquid droplet meets a solid surface. It is the standard measure of wetting: low angles mean the liquid spreads, and angles above 90 degrees mean the surface repels it.
What is contact angle?
Place a drop of liquid on a solid and it settles into a shape somewhere between a flat film and a bead. The contact angle describes that shape: it is the angle between the solid surface and the tangent to the droplet’s surface at the point where liquid, solid and surrounding gas or liquid meet, measured through the droplet. It is the most widely used number for describing wetting.
What contact angles mean
| Contact angle | Behavior | Description |
|---|---|---|
| Near 0° | The liquid spreads into a film | Complete wetting |
| Below 90° | The droplet spreads partly | The surface is wetted; for water, hydrophilic |
| Above 90° | The droplet beads up | Poor wetting; for water, hydrophobic |
| Above about 150° | The droplet is nearly spherical and rolls off | Superhydrophobic |
Young’s equation
Contact angle reflects a balance of three interfacial tensions at the edge of the droplet: between solid and gas, solid and liquid, and liquid and gas. Young’s equation relates them:
solid-gas tension = solid-liquid tension + liquid-gas tension × cos(contact angle)
It shows why surfactants improve wetting: by lowering the liquid’s surface tension and the solid-liquid tension, they reduce the contact angle.
How contact angle is measured
The usual method is the sessile drop: a droplet is placed on the surface and photographed from the side with an instrument called a goniometer, and software fits the droplet profile. Real surfaces are rough and chemically uneven, so the angle measured as a droplet advances over a dry surface differs from the angle as it recedes. The difference, contact angle hysteresis, indicates how strongly droplets stick.
Where contact angle matters
- Sprays and coatings, where a lower contact angle means better coverage.
- Reservoir engineering, where the contact angle of oil and water on rock is one measure of wettability and enters the capillary pressure equation.
- Mineral flotation, where collectors raise the contact angle of water on the target mineral so it attaches to air bubbles.
- Soils, where high contact angles signal soil water repellency.
Frequently asked questions
A water contact angle above 90 degrees is generally considered hydrophobic, and above about 150 degrees superhydrophobic. Angles below 90 degrees indicate a hydrophilic surface.
Because real surfaces are rough and chemically uneven, a droplet’s edge sticks at features as it moves. The advancing angle is usually larger than the receding angle, and the gap between them is called contact angle hysteresis.
Sources
- contact angle, IUPAC Compendium of Chemical Terminology (Gold Book)
- Capillarity and Wetting Phenomena, P.-G. de Gennes, F. Brochard-Wyart and D. Quéré, Springer, 2004