Contents

Contact angle

Pronunciation: /ˈkɒntækt ˈæŋɡəl/

Contact angle is the angle a liquid makes with a solid surface, measured through the liquid at the line where liquid, solid and vapor meet. A low angle means the liquid wets the surface; a high angle means it beads up.

Also called

wetting angle water contact angle (when the liquid is water)

Related terms

Sessile drop method

Topics

Wetting Contact angle
Book a 15 minute demo Read the contact angle measurement guide

At a glance

The definition in three parts.

What it tells you

Contact angle runs from 0° to 180°. Below 90° the liquid wets the surface; above 90° it beads up. With water, the same split is called hydrophilic and hydrophobic. At the extreme, a drop on a very hot plate floats on its own vapor and touches nothing, as our article on the Leidenfrost effect explains.

How it is measured

A drop is placed on the surface and its side profile is imaged; the angle is read where the profile meets the solid. The contact angle measurement guide covers methods, drop size and reporting. To try it on your own image, use the free contact angle tool.

Key equation

The force balance that defines the ideal contact angle.

Equation: γSV = γSL + γLV cos θY

Variable legend:

  • θY: Young (equilibrium) contact angle (°)
  • γSV, γSL, γLV: solid vapor, solid liquid and liquid vapor tensions (mN/m)

Interpretation: The angle is set by the balance of three interfacial tensions at the contact line.

When to use: Applies to a smooth, uniform, rigid solid at equilibrium. Real surfaces show a range of angles.

Related terms

Terms that build on the contact angle.

Broader term

  • Wettability

Related terms

  • Young equation
  • Contact angle hysteresis
  • Dynamic contact angles
  • Sessile drop method
  • Hydrophobic vs hydrophilic

FAQ

Two quick answers.

What is a good contact angle?

It depends on the job. Coatings meant to repel water aim high, often above 90°, while surfaces prepared for bonding, printing or coating aim low so the liquid wets them. Set the target from your own process data and the liquid you use, not from a general number.

Can a contact angle be above 180° or below 0°?

No. The angle is measured through the liquid, so it runs from 0° for a liquid that spreads into a film to 180° for a drop that touches the surface at a single point. Values outside that range point to a fitting or baseline error in the analysis.

References

Primary sources.

1. Young, T. (1805). An essay on the cohesion of fluids. Philosophical Transactions of the Royal Society of London, 95, 65 to 87. https://doi.org/10.1098/rstl.1805.0005
2. de Gennes, P. G., Brochard-Wyart, F., and Quéré, D. (2004). Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves. Springer, New York. https://doi.org/10.1007/978-0-387-21656-0
3. Contact angle measurement guide
4. Sessile drop method