Contents

Tensiometer

Pronunciation: /ˌtɛnsiˈɒmɪtə/

A tensiometer is an instrument that measures the surface tension of a liquid or the interfacial tension between two liquids. Force tensiometers weigh the pull of the liquid on a plate or ring, while optical tensiometers calculate tension from the shape of a drop.

Also called

Surface tensiometer Surface tension meter

Related terms

Pendant drop method Drop shape analysis (DSA) Goniometer

Topics

Surface tension Instruments Interfacial tension
Book a 15 minute demo Read the surface tension measurement guide

At a glance

What a tensiometer measures, the main types, and how to tell them apart.

What it is

Every tensiometer turns a measurable quantity into surface tension. Force types measure the pull of a meniscus on a probe. Optical types measure the outline of a drop. Bubble types measure the pressure needed to blow a bubble.

  • Force: Wilhelmy plate, du Noüy ring
  • Optical: pendant drop shape analysis
  • Pressure: maximum bubble pressure

Why it matters

The type decides what you can measure. Plates and rings suit routine checks on larger volumes. Optical instruments need only microlitres, do not touch the interface and handle liquid liquid pairs. Bubble types reach very young surfaces. Our comparison of optical drop shape and force tensiometry sets out the trade offs.

Worked example

A Wilhelmy plate 20 mm wide and 0.2 mm thick touches water at 20 °C, which wets it fully (θ = 0).

  • Wetted length L = 2 × (20 + 0.2) = 40.4 mm = 0.0404 m
  • F = γ L cos θ = 0.0728 × 0.0404 × 1 ≈ 0.00294 N
  • The balance reads about 2.9 mN of extra pull

Reference liquids for this kind of check are in our Droplet Lab reference measurement dataset.

Expert view

“Plot surface tension against surfactant concentration and the interface tells you when it is saturated: the curve bends at the point where the surface can hold no more.”

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

What the terms mean

The three families of tensiometer and what each one actually senses.

Force tensiometer (plate or ring)

A probe touches the surface and a sensitive balance records the pull of the meniscus. Needs millilitres of liquid and a clean, wettable probe.

Optical tensiometer (drop shape)

A camera images a pendant drop and software fits its outline. Needs microlitres, does not touch the interface, and suits liquid liquid pairs.

Bubble pressure tensiometer

Gas is blown through a capillary and the peak pressure is recorded. Reaches surface ages of milliseconds for dynamic work.

All three families report the same quantity in mN/m. They differ in sample size, surface age, contact with the interface and how they cope with two liquid systems.

γ Surface or interfacial tension, mN/m
F Measured force on the probe, N
L Wetted length of the plate, m
R Ring radius, m
θ Contact angle of the liquid on the probe, degrees
f Ring correction factor, dimensionless

Key equations

The working equation behind each main tensiometer type.

Equation: γ = F / (L cos θ)

Variable legend:

  • F: extra force from the meniscus, after buoyancy is removed (N)
  • L: wetted length, twice width plus twice thickness (m)
  • θ: contact angle on the plate, assumed 0 for a clean roughened platinum plate

Interpretation: The meniscus pulls along the whole wetted edge. With full wetting, force divided by length is the surface tension.

When to use: Use it for static surface tension of clean liquids and for slow surfactant ageing.

Assumptions:

  • The liquid fully wets the plate
  • The plate is clean and its edge is level with the surface

Common mistakes:

  • Using a contaminated plate, so θ is no longer 0
  • Forgetting the plate thickness in L

Equation: γ = f × F / (4 π R)

Variable legend:

  • F: maximum pull as the ring is drawn through the surface (N)
  • R: mean ring radius (m)
  • f: correction factor from published tables, typically a little below 1

Interpretation: The ring lifts a lamella of liquid with two edges, inside and outside, so the length is about 4πR. The correction accounts for the true shape of the lifted liquid.

When to use: Use it for routine surface and interfacial tension checks where a standard calls for the ring.

Assumptions:

  • Ring kept horizontal and fully wetted
  • Correct tables for the ring geometry and liquid densities

Common mistakes:

  • Leaving out the correction factor
  • Using it on surfactant solutions, where stretching the surface skews the reading

Equation: γ = Δρ g R0² / Bo

Variable legend:

  • Δρ: density difference between drop and surroundings (kg/m³)
  • g: 9.81 m/s²
  • R0: apex radius of curvature from the fit (m)
  • Bo: Bond number from the fit of the Young-Laplace equation

Interpretation: The drop outline fixes Bo and R0. With the density difference known, they give γ directly.

When to use: Use it for small samples, liquid liquid interfacial tension and time resolved readings from about a second upward.

Assumptions:

  • Axisymmetric drop at rest, accurately scaled image

Common mistakes:

  • Fitting a drop that is too small to be well deformed

How to read it

Choosing and checking a tensiometer for the job.

Start from what you need to measure, not from the instrument. If wetting is part of the question too, an optical instrument that also reads contact angle saves a second setup; our free contact angle tool shows what that analysis looks like.

Matching the tensiometer to the task

If you need A good fit is Check next
Static surface tension of a clean liquid Wilhelmy plate, ring or pendant drop Water reference value at your temperature
Oil and water interfacial tension Pendant drop or ring Density difference and phase saturation
Microlitre samples or precious liquids Pendant drop Drop size large enough for a good fit
Surface ages of milliseconds Maximum bubble pressure Bubble frequency range of the instrument
Ultralow interfacial tension Spinning drop Whether the expected range is below about 0.1 mN/m

How it is measured

An optical tensiometer reading in four short steps. The surface tension measurement guide has the full procedure for every type.

1

Check with water

Measure pure water first and compare with the reference value at your temperature.

2

Form the sample drop

Hang a large, steady drop of the test liquid from a clean needle.

3

Capture and fit

Image the profile and let the software fit it to the Young-Laplace equation.

4

Report with conditions

Record γ with temperature, density difference and, for solutions, drop age.

Common misconceptions

Where tensiometers are most often misunderstood.

All tensiometers give the same number on every sample

On clean liquids, well run methods agree. On surfactant solutions they can differ, because each method samples a different surface age and some stretch the surface. Compare like with like.

A tensiometer and a goniometer are different instruments

Often, but not always. Optical instruments use the same camera and drop fitting for both. A pendant drop gives surface tension, a sessile drop gives contact angle. The Dropometer smartphone drop shape analysis kit measures both, plus surface free energy and sliding angle, with automatic edge and baseline detection.

Optical tensiometers are less accurate than force types

A well imaged, well deformed drop gives results that match force methods on reference liquids. Peer reviewed work has even shown a smartphone camera can do it, as our article on measuring surface tension with a smartphone describes.

Any tensiometer can measure ultralow IFT

Below roughly 0.1 mN/m, drops and menisci barely deform and force signals become tiny. Ultralow interfacial tension is normally measured with a spinning drop instrument.

Related terms

Where the tensiometer sits among other surface tension concepts.

Broader term

  • Surface tension
  • Interfacial tension (IFT)

Methods a tensiometer uses

  • Pendant drop method
  • Drop shape analysis (DSA)
  • Young-Laplace equation

Often confused with

  • Goniometer

FAQ

Short answers to the questions people ask most about tensiometers.

What does a tensiometer measure?

A tensiometer measures surface tension, the energy per unit area of a liquid surface in air, or interfacial tension between two liquids that do not mix. Results are reported in mN/m. Some optical instruments also measure contact angle, which describes how a liquid wets a solid rather than the liquid itself.

What is the difference between an optical and a force tensiometer?

A force tensiometer hangs a plate or ring from a precision balance and measures the pull of the liquid on it. An optical tensiometer images a drop and calculates tension from its shape. Optical types need far less sample and do not touch the interface; force types are traditional for many standards.

Which tensiometer is best for surfactant solutions?

It depends on the time scale. For equilibrium values and CMC curves, a Wilhelmy plate or a pendant drop held until it plateaus works well. For fast processes such as spraying or printing, you need dynamic data, from a bubble pressure instrument for milliseconds or a time resolved pendant drop for seconds and longer.

References

Primary sources and further reading.

1. Harkins, W. D., and Jordan, H. F. (1930). A method for the determination of surface and interfacial tension from the maximum pull on a ring. Journal of the American Chemical Society, 52(5), 1751 to 1772. https://doi.org/10.1021/ja01368a004
2. Berry, J. D., Neeson, M. J., Dagastine, R. R., Chan, D. Y. C., and Tabor, R. F. (2015). Measurement of surface and interfacial tension using pendant drop tensiometry. Journal of Colloid and Interface Science, 454, 226 to 237. https://doi.org/10.1016/j.jcis.2015.05.012
3. Chen, H., Muros-Cobos, J. L., Holgado-Terriza, J. A., and Amirfazli, A. (2017). Surface tension measurement with a smartphone using a pendant drop. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 533, 213 to 217. https://doi.org/10.1016/j.colsurfa.2017.08.019
4. Surface tension measurement guide
5. Pendant drop method
6. Drop shape analysis (DSA)