Contents

Captive bubble method

Pronunciation: /ˈkæptɪv ˈbʌbəl ˈmɛθəd/

The captive bubble method measures the contact angle of a solid immersed in liquid, using an air bubble (or oil drop) pressed against its underside. It suits surfaces that must stay wet, such as membranes, hydrogels and contact lenses.

Also called

Captive bubble technique Inverted bubble method Underwater contact angle

Related terms

Cassie-Baxter model Hydrophobic vs hydrophilic Wenzel model

Topics

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

At a glance

What the captive bubble method measures, and when it beats a sessile drop.

What it is

The sample faces down in a cell of liquid, usually water. A bubble released from a needle below rises and sits against the surface. The bubble profile is imaged from the side, just like a sessile drop turned upside down.

  • The surface never dries out during the test
  • Angles are reported through the liquid phase
  • An oil drop can replace the bubble for underwater oil wetting

Worked example

A bubble under a wet membrane shows an angle of 140°, measured through the air inside the bubble.

  • By convention the contact angle is read through the water
  • θ = 180° − 140° = 40°
  • The membrane is hydrophilic in the wet state

You can fit the bubble outline from your own image with our free contact angle tool, then apply the conversion.

How it is measured

The bubble profile is fitted with the same drop shape tools used for sessile drops. Imaging, fitting and reporting practice are covered in the contact angle measurement guide.

Expert view

“What a liquid responds to is only the outermost few ångström of a surface: the skin, not the bulk. Change that skin and you change the wetting, even if the material underneath is identical.”

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

What the terms mean

How a captive bubble reads on different surfaces, and the symbols used.

Hydrophilic surface (θ below 90°)

Water holds onto the surface, so the bubble barely spreads. It looks nearly round and touches the solid over a small area.

Hydrophobic surface (θ above 90°)

Water is easily displaced, so the bubble flattens and spreads along the solid. The contact area is large.

Growing or shrinking bubble

A growing bubble pushes water back, giving the receding water angle. A shrinking bubble lets water return, giving the advancing angle.

The bubble image is a sessile drop turned upside down. The angle through the liquid is 180° minus the angle through the bubble.

θ Contact angle through the liquid, degrees
θb Angle measured through the bubble (air or oil), degrees
γLV Surface tension of the surrounding liquid, mN/m

Key equations

The conversion every captive bubble result needs, and how it links to the Young equation.

Equation: θ = 180° − θb

Variable legend:

  • θ: contact angle measured through the liquid (°)
  • θb: angle measured through the bubble (°)

Interpretation: The two angles add up to 180° at the contact line. Reporting θ keeps captive bubble results comparable with sessile drop values.

When to use: Use it every time you report a captive bubble result, and state which phase the angle was read through.

Assumptions:

  • Both angles are taken at the same contact point

Common mistakes:

  • Reporting the bubble side angle as the water contact angle
  • Mixing converted and unconverted values in one table

Equation: γSV = γSL + γLV cos θ

Variable legend:

  • γSV: solid vapor tension at the bubble (mN/m)
  • γSL: solid liquid tension (mN/m)
  • γLV: liquid surface tension (mN/m)

Interpretation: The same force balance applies, with the bubble as the vapor phase. The solid may now be swollen or hydrated, so its γ values differ from the dry state.

When to use: Use it to interpret captive bubble angles for wet surfaces in a surface energy context.

Assumptions:

  • Smooth, uniform surface at equilibrium
  • The bubble does not dissolve noticeably during the reading

Common mistakes:

  • Expecting captive bubble and dry sessile drop angles to match on hydrated surfaces

How to read it

What a captive bubble result tells you about a wet surface.

Captive bubble values describe the surface as it is in use, wet and possibly swollen. Compare them with sessile drop values only when you know both were taken the same way. Reference values like those in the Droplet Lab reference measurement dataset help you judge whether a reading is plausible.

From captive bubble result to meaning

If you see It usually means Check next
Captive bubble angle lower than the dry sessile drop angle The surface rearranges or swells when wet Soak time before the reading
Bubble will not stay against the surface Very hydrophilic surface, or bubble too small Bubble size, sample flatness
Angle drifts over minutes Bubble dissolving or surface still hydrating Degassed liquid, equilibration time
Large spread across a membrane Uneven coating or fouling Several positions, clean reference sample

Common misconceptions

Where captive bubble results are most often misread.

Captive bubble and sessile drop give the same angle

They often differ, because one measures a wet surface and the other a dry one. Bubble size and contact line motion also play a part. Report the method with every value, and do not average the two.

The angle through the bubble is the contact angle

By convention the contact angle is read through the liquid. Convert the bubble side angle with θ = 180° − θb before comparing results.

Related terms

Where the captive bubble method sits among other wetting methods.

Broader term

  • Contact angle
  • Drop shape analysis (DSA)

Related terms

  • Hydrophobic vs hydrophilic
  • Advancing and receding contact angle
  • Contact angle hysteresis
  • Wenzel model
  • Cassie-Baxter model

Often confused with

  • Sessile drop method
  • Pendant drop method

FAQ

Short answers to the questions people ask most about the captive bubble method.

When should I use the captive bubble method instead of a sessile drop?

Use it when the surface must stay wet to be representative, such as hydrogels, soft contact lenses, biomaterials and filtration membranes. It also avoids drop evaporation and absorption into porous samples. For dry, rigid surfaces such as glass, metals and coatings, the sessile drop method is simpler and more common.

Why are captive bubble angles often lower than sessile drop angles?

A surface kept in water can hydrate, swell or bring polar groups to its outer layer. That makes it more hydrophilic than the same surface measured dry. On membranes, published comparisons report differences between the two methods, so treat the method as part of the result.

Does the captive bubble method give advancing or receding angles?

Both, if the bubble volume is changed. Growing the bubble pushes the water line back and gives the receding water angle. Shrinking it lets water advance and gives the advancing angle. A bubble simply left at rest gives a static value somewhere in between.

References

Primary sources and further reading.

1. Drelich, J., Miller, J. D., and Good, R. J. (1996). The effect of drop (bubble) size on advancing and receding contact angles for heterogeneous and rough solid surfaces as observed with sessile-drop and captive-bubble techniques. Journal of Colloid and Interface Science, 179(1), 37 to 50. https://doi.org/10.1006/jcis.1996.0186
2. Baek, Y., Kang, J., Theato, P., and Yoon, J. (2012). Measuring hydrophilicity of RO membranes by contact angles via sessile drop and captive bubble method: a comparative study. Desalination, 303, 23 to 28. https://doi.org/10.1016/j.desal.2012.07.006
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. Hydrophobic vs hydrophilic
6. Wenzel model
7. Cassie-Baxter model