Contents

Wettability

Pronunciation: /ˌwɛtəˈbɪlɪti/

Wettability is the tendency of a liquid to spread on and stay in contact with a solid surface. It is set by the balance between adhesion to the solid and cohesion within the liquid, and is usually expressed as a contact angle.

Also called

Wetting behavior Surface wettability Degree of wetting

Related terms

Work of adhesion Work of cohesion

Topics

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

At a glance

What wettability means, how it is expressed, and why so many processes depend on it.

What it is

Wettability describes how a given liquid behaves on a given solid. A liquid that wets well spreads into a thin, flat drop. A liquid that wets poorly pulls itself into a bead.

  • It belongs to a solid, liquid and vapor system, not to the solid alone
  • It is most often reported as a contact angle measured through the liquid
  • Water angles below 90° are called hydrophilic, above 90° hydrophobic

Worked example

Water at 20 °C (γLV = 72.8 mN/m) on a surface with θ = 40°.

  • cos 40° = 0.766
  • Work of adhesion: Wa = 72.8 × (1 + 0.766) ≈ 128.6 mJ/m²
  • Work of cohesion: Wc = 2 × 72.8 = 145.6 mJ/m²
  • Spreading coefficient: S = Wa − Wc ≈ −17.0 mN/m

S is negative, so water wets partially and forms a drop. Our surface free energy calculator takes the next step, from angles to surface energy.

Expert view

“Wettability is set by the top five to ten ångström of a surface: what the liquid actually sees. It doesn't matter what lies beneath; that outermost layer is what you are really measuring.”

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

What the terms mean

The wetting regimes, from a liquid film to a bead that barely touches the surface.

Complete wetting (θ = 0°, S ≥ 0)

The liquid spreads into a film and no finite contact angle forms. Typical of low surface tension oils on clean metals and of water on freshly cleaned glass.

Partial wetting (0° < θ < 180°, S < 0)

The liquid forms a drop with a finite angle. With water, angles below 90° are called hydrophilic and angles above 90° hydrophobic.

Superhydrophobic (θ > 150°, low hysteresis)

Water sits on a rough, low energy surface as a near spherical bead and rolls off at small tilt. This needs both chemistry and texture.

Wettability compares two pulls on the liquid: adhesion to the solid and cohesion within the liquid. The contact angle is where they settle.

θ Contact angle measured through the liquid, degrees
γLV Surface tension of the liquid, mN/m
S Spreading coefficient, mN/m
Wa Work of adhesion, mJ/m² (numerically equal to mN/m)
Wc Work of cohesion of the liquid, mJ/m²

Key equations

The energy balances that turn wettability into numbers.

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: S compares the energy of a dry solid with the energy of the same solid covered by liquid. S ≥ 0 means the liquid spreads; S < 0 means it forms a drop.

When to use: Use it to decide whether a liquid will spread completely or settle at a finite contact angle.

Assumptions:

  • Smooth, chemically uniform solid
  • Equilibrium with the vapor; no reaction or swelling

Common mistakes:

  • Treating S as directly measurable: γSV and γSL are not measured, they are inferred from contact angles

Equation: Wa = γLV (1 + cos θY); Wc = 2 γLV

Variable legend:

  • Wa: energy to separate a unit area of liquid from the solid (mJ/m²)
  • Wc: energy to split the liquid itself across a unit area (mJ/m²)
  • θY: Young contact angle (°)

Interpretation: Wetting improves as adhesion to the solid approaches the cohesion of the liquid. At θ = 0°, Wa equals Wc.

When to use: Use them to compare how strongly the same liquid holds to different surfaces.

Assumptions:

  • Same as the Young equation: smooth, uniform, rigid solid at equilibrium

Common mistakes:

  • Using an apparent angle on a rough sample as θY
  • Comparing Wa values measured with different liquids

Equation: S = Wa − Wc = γLV (cos θY − 1)

Variable legend:

  • Follows from the Young equation, γSV − γSL = γLV cos θY

Interpretation: For any finite angle, cos θY is below 1, so S is negative. S only reaches zero at θ = 0°, the threshold of complete wetting.

When to use: Use it to put a measured angle on an energy scale.

Assumptions:

  • Valid only in partial wetting, where an equilibrium angle exists

Common mistakes:

  • Expecting this form to give a positive S: a positive S has no contact angle to measure

How to read it

What a change in wettability usually points to.

A wettability result is a comparison: before and after a treatment, batch to batch, or sample to reference. A simple version is shown in experiment 4 on treated versus untreated surfaces.

From result to likely cause

If you see It usually means Check next
Water angle drops sharply after plasma or corona treatment Polar groups were added to the outermost layer Whether the angle creeps back up during storage (hydrophobic recovery)
High water angle on glass or metal that should be hydrophilic Organic or silicone contamination on the surface Cleaning, gloves, packaging and handling
Angle keeps changing while the drop sits Evaporation, absorption, swelling or reaction Dynamic contact angles and drop age at capture
Different angles across one sample Uneven treatment, coating or contamination A map of several spots rather than a single drop
Drop spreads to a film within seconds Complete wetting, so the angle no longer separates samples A higher surface tension test liquid or a surface energy analysis

Common misconceptions

Where wettability is most often misread.

90° is a physical boundary between hydrophilic and hydrophobic

The 90° line is a convention tied to the sign of cos θ, not a change in physics. Some authors argue for a lower boundary for water. Report the angle itself, and use reference values such as our surface free energy of common polymers dataset for context.

Wettability is a property of the solid alone

The same solid wets differently with water, oil or a solvent, and at different temperatures. Wettability always names a liquid. Studies such as an analysis of alfalfa and carrageenan packaging films report water angles for exactly this reason.

Good wetting guarantees good adhesion

Wetting is necessary for adhesion but not sufficient. Weak boundary layers, curing and mechanical interlocking also decide whether a bond holds. Our article Beyond 38 dynes explains what surface energy can and cannot predict.

One static angle tells the whole story

A single angle sits somewhere between the advancing and receding angles. Two surfaces with the same static angle can behave very differently when a drop moves. Teaching labs can explore this with equipment discussed in our guide to the best contact angle goniometer for teaching and academic labs.

Related terms

Where wettability sits among other wetting concepts.

Ways to quantify it

  • Contact angle
  • Contact angle hysteresis
  • Roll-off / sliding angle
  • Surface free energy

Energy terms

  • Young equation
  • Work of adhesion
  • Work of cohesion

Wetting on real surfaces

  • Wenzel model
  • Cassie-Baxter model
  • Surface roughness and heterogeneity
  • Lotus effect
  • Dewetting

FAQ

Short answers to the questions people ask most about wettability.

How is wettability measured?

Most often by the contact angle of a liquid drop on the solid. A low angle means good wetting and a high angle means poor wetting. Advancing and receding angles, sliding angles and surface energy calculations from several liquids add detail when a single static angle is not enough.

What makes a surface hydrophilic or hydrophobic?

Mainly the chemistry of the outermost molecular layer. Polar groups such as hydroxyl and carboxyl attract water and lower the angle. Nonpolar groups such as methyl and fluorinated chains raise it. Roughness then amplifies the trend: rough hydrophilic surfaces wet more, rough hydrophobic surfaces less.

Is wettability the same as surface energy?

No. Wettability describes how one liquid behaves on a solid. Surface free energy is a property of the solid alone, calculated from contact angles of two or more liquids with a model such as OWRK. High surface energy solids are usually well wetted, but the angle also depends on the liquid.

References

Primary sources and further reading.

1. 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
2. Bonn, D., Eggers, J., Indekeu, J., Meunier, J., and Rolley, E. (2009). Wetting and spreading. Reviews of Modern Physics, 81(2), 739 to 805. https://doi.org/10.1103/RevModPhys.81.739
3. Kwok, D. Y., and Neumann, A. W. (1999). Contact angle measurement and contact angle interpretation. Advances in Colloid and Interface Science, 81(3), 167 to 249. https://doi.org/10.1016/S0001-8686(98)00087-6
4. Contact angle measurement guide
5. Work of adhesion
6. Work of cohesion
7. Contact angle hysteresis