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Dewetting

Pronunciation: /diːˈwɛtɪŋ/

Dewetting is the spontaneous retreat of a liquid film from a solid surface, leaving dry areas that grow and break the film into drops. It happens when a film covers a surface it only partially wets, so the film is unstable or metastable.

Also called

Film rupture Crawling (coatings) Retraction

Related terms

Three-phase contact line

Topics

Wetting Thin films Coatings
Book a 15 minute demo Read the contact angle measurement guide

At a glance

What dewetting is, why films dewet, and where it causes trouble.

What it is

A film can be spread over a surface it does not like, for example by spin coating, dipping or spraying. If the liquid has a finite contact angle there, the film tends to pull back.

  • Dry holes open and grow, pushing liquid into a rim
  • Rims meet and break up into drops
  • The final drops sit at the receding contact angle

Worked example

Below what thickness will a water film on a partially wetting surface dewet? Use the capillary length κ⁻¹ = √(γLV / ρg).

  • κ⁻¹ = √(0.0728 / (998 × 9.81)) ≈ 2.7 mm
  • Critical thickness ec = 2 κ⁻¹ sin(θ/2)
  • θ = 60°: ec = 2 × 2.7 × 0.5 ≈ 2.7 mm
  • θ = 20°: ec = 2 × 2.7 × 0.174 ≈ 0.95 mm

Thinner films are metastable and dewet once a hole forms.

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

When a film stays, when it waits for a defect, and when it breaks up by itself.

Stable film (complete wetting, θ = 0°)

The liquid wants to cover the surface. The film thins but does not open holes on its own.

Metastable film (partial wetting, thicker films)

The film is stable to small ripples but dewets once a hole forms at a dust particle, defect or low energy spot. Holes appear at random sites and grow.

Unstable film (partial wetting, very thin films)

Below a few tens of nanometres, attractive molecular forces amplify thickness ripples. Holes appear everywhere at a typical spacing, a process called spinodal dewetting.

A dewetting hole grows with a rim of collected liquid at its edge. The contact line at the rim recedes over the solid.

θ Equilibrium contact angle of the liquid on the solid, degrees
S Spreading coefficient, mN/m; dewetting needs S < 0
κ⁻¹ Capillary length √(γLV / ρg), mm
ec Critical film thickness below which a film on a partially wetting solid is not stable, mm

Key equations

The two numbers that decide whether a film can dewet.

Equation: S = γSV − (γSL + γLV) = γLV (cos θY − 1)

Variable legend:

  • S: spreading coefficient (mN/m)
  • θY: Young contact angle (°)

Interpretation: If S ≥ 0 the film is stable and spreads. If S < 0 the liquid has a finite contact angle and a film can dewet.

When to use: Use it to judge whether dewetting is possible at all for a given liquid and surface.

Assumptions:

  • Uniform, smooth solid
  • No reaction or absorption

Common mistakes:

  • Assuming a low contact angle means no risk: any finite angle allows dewetting of thin enough films

Equation: ec = 2 κ⁻¹ sin(θ / 2), κ⁻¹ = √(γLV / ρg)

Variable legend:

  • ec: thickness of a large puddle at equilibrium (m)
  • κ⁻¹: capillary length (m)
  • ρ: liquid density (kg/m³)
  • g: 9.81 m/s²

Interpretation: A film thinner than ec has more energy than a puddle of thickness ec, so it tends to retract.

When to use: Use it to estimate when millimetre scale films on partially wetting surfaces will pull back.

Assumptions:

  • Gravity dominated films, well above molecular range
  • θ is the equilibrium angle; in practice the receding angle governs whether the line moves

Common mistakes:

  • Applying it to nanometre films, where molecular forces dominate

How to read it

What a dewetting pattern usually tells you.

The pattern of holes says a lot about the cause. Compare contact angles on the problem substrate with typical values in our contact angle reference values for water, and read angles from images with our free contact angle tool.

From defect to likely cause

If you see It usually means Check next
Round craters with a central speck (fisheyes) Local low energy contamination, such as silicone or oil Contact angle at and away from the defect
Coating pulls back from edges or across a whole panel (crawling) Substrate surface energy too low for the coating Substrate cleaning or treatment, and receding angle of the coating liquid
Random holes of different sizes Nucleated dewetting at dust or defects in a metastable film Cleanliness of substrate and coating environment
Holes at a regular spacing in a very thin film Spinodal dewetting driven by molecular forces Film thickness and substrate chemistry

Common misconceptions

Where dewetting is most often misread.

If a liquid spreads when applied, it will stay spread

Applying a film forces it to cover the surface. Whether it stays depends on the contact angle, the film thickness and the presence of defects. A coating can look perfect when wet and crawl minutes later.

The advancing angle predicts dewetting

Dewetting is a receding process, so the receding angle matters more. A liquid that wets well as it is applied, with a low advancing angle, can still dewet if its receding angle is clearly above zero. Measure both when diagnosing coating defects.

Dewetting is always bad

Controlled dewetting is used to pattern films, make arrays of drops or particles, and test film stability. Classes can study it with simple films through our teaching curriculum hub.

Related terms

Where dewetting sits among other wetting concepts.

Opposite process

  • Spreading
  • Wettability

Controlled by

  • Three-phase contact line
  • Dynamic contact angles
  • Contact angle hysteresis

Related terms

  • Surface free energy
  • Critical surface tension
  • Surface roughness and heterogeneity

FAQ

Short answers to the questions people ask most about dewetting.

What causes dewetting?

A film covering a surface that the liquid only partially wets. The spreading coefficient is negative, so the surface would rather be dry and the liquid gathered into drops. Low energy contamination, thin films and high receding angles all make dewetting more likely. Defects or thermal fluctuations start the holes.

What is the difference between dewetting and crawling?

Crawling is the coatings industry name for dewetting of a wet paint or ink film. The coating pulls back from areas of low surface energy, leaving bare substrate. Fisheyes are a local version, where a small contaminated spot opens a round hole in an otherwise wetting film.

How do you prevent dewetting of a coating?

Make the coating wet the substrate: raise the substrate surface energy by cleaning or treatment, or lower the coating surface tension with formulation changes. Checking the contact angle of the coating liquid on the substrate, especially the receding angle, shows whether the margin is enough.

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. Redon, C., Brochard-Wyart, F., and Rondelez, F. (1991). Dynamics of dewetting. Physical Review Letters, 66(6), 715 to 718. https://doi.org/10.1103/PhysRevLett.66.715
3. Reiter, G. (1992). Dewetting of thin polymer films. Physical Review Letters, 68(1), 75 to 78. https://doi.org/10.1103/PhysRevLett.68.75
4. Contact angle measurement guide
5. Three-phase contact line
6. Dynamic contact angles
7. Wettability