Contents

Spreading

Pronunciation: /ˈsprɛdɪŋ/

Spreading is the movement of a liquid across a solid (or another liquid) as it wets the surface, driven by the balance of interfacial tensions. It stops at a finite equilibrium contact angle in partial wetting, or continues into a thin film in complete wetting.

Also called

Wetting (dynamic) Drop spreading Spontaneous spreading

Related terms

Surfactant Dynamic surface tension

Topics

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

At a glance

What spreading is, what drives it and where it stops.

What it is

When a drop touches a surface, its edge moves outward as the liquid wets the solid. That motion is spreading. Its end point is set by thermodynamics; its speed is set by viscosity, drop size and how fast surfactants reach the new surface.

  • Partial wetting: stops at a finite contact angle
  • Complete wetting: continues until a thin film forms

Typical values

Rough guide to the final state at room temperature:

  • Water on clean glass: spreads to a very low angle, often below 20°
  • Water on PTFE: stops at about 108 to 115°
  • Hexadecane on glass: spreads to a film

Our contact angle of water on common surfaces dataset lists more reference values.

How it is measured

Record the drop from the side over time and follow the contact angle and base diameter until they stop changing. The contact angle measurement guide covers time resolved sessile drop measurements, and our free contact angle tool reads angles from individual frames.

Expert view

“A contact angle is a thermodynamic property: fix the liquid and the solid and the same angle appears whether the drop sits on a plate or climbs a capillary.”

Dr. Alidad Amirfazli, Co-founder and Scientific Advisor, Droplet Lab

What the terms mean

The two end states of spreading and the motion in between.

Partial wetting (S < 0)

The edge moves outward until the contact angle reaches its equilibrium value, then stops. Water on most polymers behaves this way.

Complete wetting (S ≥ 0)

No finite angle can balance the tensions. The liquid keeps spreading into a thin film, slowed only by viscosity. Oils on clean glass or metal behave this way.

Dynamic spreading

While the edge moves, the dynamic contact angle is larger than the equilibrium angle. It relaxes toward that value as the drop slows.

The spreading coefficient decides where spreading ends. Viscosity and drop size decide how quickly it gets there.

S Spreading coefficient, γSV − (γSL + γLV), mN/m
θ Equilibrium contact angle, degrees
θd Dynamic contact angle while the edge moves, degrees
R Drop base radius, mm
t Time since contact, s

Key equations

The condition for spreading and the classic law for how fast a wetting drop spreads.

Equation: S = γSV − (γSL + γLV)

Variable legend:

  • S: spreading coefficient (mN/m)
  • γSV, γSL, γLV: solid vapor, solid liquid and liquid vapor tensions (mN/m)

Interpretation: If covering the solid with liquid lowers the energy, S is zero or positive and the liquid spreads fully. If not, it stops at a finite angle.

When to use: Use it to predict the final state before running an experiment.

Assumptions:

  • Clean, uniform solid
  • No evaporation or reaction during spreading

Common mistakes:

  • Expecting a positive S from a drop that stops at a finite angle

Equation: R(t) ∝ t^(1/10)

Variable legend:

  • R: drop base radius (mm)
  • t: time since contact (s)

Interpretation: A small, completely wetting drop spreads quickly at first, then slows sharply. Doubling the radius takes about a thousand times longer.

When to use: Use it for small, viscous, completely wetting drops where gravity and inertia are negligible.

Assumptions:

  • Complete wetting
  • Drop smaller than the capillary length
  • Viscous forces dominate inertia

Common mistakes:

  • Applying it to partially wetting drops that stop at a finite angle
  • Applying it to the first milliseconds, when inertia still matters

How to read it

What a spreading curve usually tells you.

Plot contact angle and base diameter against time. The shape of the curve separates surface chemistry from formulation effects. Surfactant solutions often spread slowly because molecules need time to reach the new surface; our article on why you need dynamic surface tension explains this.

From spreading behavior to meaning

If you see It usually means Check next
Angle falls quickly then holds steady Partial wetting reached equilibrium Repeat at several spots to check uniformity
Angle keeps falling toward zero Complete wetting, or absorption into the substrate Drop volume over time to separate spreading from soaking in
Angle falls slowly over many seconds Surfactant diffusion or evaporation is controlling the rate Dynamic surface tension of the liquid
Edge jumps in steps Pinning on defects or roughness Advancing and receding angles on the same sample

Common misconceptions

Where spreading is most often misread.

A drop that keeps flattening is always spreading

On porous or absorbent materials, the drop may be soaking in or evaporating. Track drop volume as well as angle. Fabrics often need a different test altogether, such as the AATCC TM22 water repellency spray test.

Faster spreading means better wetting

Speed depends on viscosity and drop size, not only on surface chemistry. A viscous liquid can wet completely yet spread slowly. Compare final angles, not just how fast the drop moves.

Related terms

Where spreading sits among other wetting terms.

Broader term

  • Wettability
  • Wetting

Related terms

  • Spreading coefficient
  • Surfactant
  • Dynamic surface tension
  • Dynamic contact angle
  • Work of adhesion

Often confused with

  • Absorption into porous substrates
  • Evaporation of the drop

FAQ

Short answers to the questions people ask most about spreading.

What makes a liquid spread on a surface?

A liquid spreads when wetting the solid lowers the total surface energy. That happens when the solid has a high surface energy compared with the liquid surface tension. Clean metals and glass are spread on by most oils, while low energy plastics resist spreading by water unless surfactants are added.

How do surfactants help a liquid spread?

Surfactants lower the liquid surface tension and can lower the solid liquid tension as well. Both raise the spreading coefficient and lower the equilibrium contact angle. They act only once they reach the new surface, so fast processes such as spraying need surfactants that move quickly to the interface.

What is the difference between spreading and wetting?

Wetting describes how much a liquid likes a surface, usually expressed as a contact angle. Spreading is the motion of the liquid as it moves toward that state. A liquid can wet well but spread slowly, for example when it is very viscous.

References

Primary sources and further reading.

1. de Gennes, P. G. (1985). Wetting: statics and dynamics. Reviews of Modern Physics, 57(3), 827 to 863. https://doi.org/10.1103/RevModPhys.57.827
2. Bonn, D., Eggers, J., Indekeu, J., Meunier, J., and Rolley, E. (2009). Wetting and spreading. Reviews of Modern Physics, 81, 739 to 805. https://doi.org/10.1103/RevModPhys.81.739
3. 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
4. Contact angle measurement guide
5. Surfactant
6. Dynamic surface tension