Contents
Partially compliant

ASTM D1331-20: standard test methods for surface and interfacial tension of solutions of paints, solvents, surface active agents and related materials

ASTM D1331 measures surface tension, and the tension between two liquids, with a du Noüy ring or a Wilhelmy plate. The Dropometer measures surface tension by pendant drop with ±0.03 mN/m accuracy (Dropometer datasheet), so it gets the same end result by an alternative method. It measures interfacial tension by pendant drop with the optional cuvette. If a contract or customer requires an ASTM D1331 result, run a correlation study on your own samples first.

What brings you here?

Who this is for
Surfactant makers, paint, ink and coatings labs, formulators and university surface chemistry groups.
Compliance tier
Partially compliant: the Dropometer gets the same end results by pendant drop, surface tension on the base kit and interfacial tension with the optional cuvette. That is an alternative to the ring or plate method the standard specifies.
Measurement
Pendant drop surface tension up to 75 mN/m, ±0.03 mN/m accuracy, 0.01 mN/m resolution, static and dynamic (Dropometer datasheet).
Setup
About 2 minutes. Battery powered, 1 kg, runs on the bench or at the plant.
Last updated
October 20, 2026
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Free one page checklist for pendant drop surface tension next to ASTM D1331: what to prepare, the settings to lock, how to build a CMC curve and what to record. Made for surfactant and coatings labs.

Talk to an expert

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Tell us your surfactant, solvent or paint and how you test it today. We will measure it live by pendant drop and tell you plainly where the Dropometer fits next to your ring or plate tensiometer and where it does not.

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View the official standard
Gurdeep-Saini-Photo
Written by
Gurdeep Singh Saini
Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.
COO at Droplet Lab
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Droplet-Lab logo
Technical Review by
Droplet Lab Team
Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.
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Verified against ASTM D1331
Gurdeep-Saini-Photo
Written By

Gurdeep Singh Saini

COO at Droplet Lab

Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.

Droplet-Lab logo
Reviewed By

Droplet Lab Team

Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.

Featured Studies & Publications

Research Paper Contact angle measurement with a smartphone
Research Paper Surface tension measurement with a smartphone using a pendant drop

Evidence box: ASTM D1331 standard test methods and the Dropometer

Standard

ASTM D1331-20(2024), the ASTM test methods for surface tension and for the tension between two liquids in paints, solvents and surfactant solutions, from ASTM International Subcommittee D01.24. It covers four ring and plate methods: A and C for surface tension, B and D for the tension between two liquid phases.

Compliance tier

Partially compliant. The Dropometer gets the same end results the standard is after: surface tension by pendant drop, and interfacial tension by pendant drop with the optional cuvette. Pendant drop is an alternative method, not the du Noüy ring or Wilhelmy plate the standard specifies. If a contract or customer requires an ASTM D1331 result, run a correlation study on your own samples first.

Instrument performance

Pendant drop surface tension up to 75 mN/m, accuracy ±0.03 mN/m, resolution 0.01 mN/m, static and dynamic, from the Dropometer product specification. The tension between two liquids runs with the optional cuvette; no accuracy figure or range is published for it.

Peer reviewed method

The drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers. See our validation publications.

Customer evidence

Researchers studying polymer and surfactant mixtures used the Dropometer to measure pendant drop surface tension of aqueous solutions across a range of surfactant concentrations (Polymers, 2025). Read our analysis of the paper.

What it does not tell you

Surface tension shows how well a surfactant lowers the tension of a liquid. It does not predict foam, emulsion or coating performance on its own. Pair it with the test your product needs.

Executive summary: surface tension of solutions of surface active agents

What this standard measures, who uses it and how the Dropometer fits

ASTM D1331 measures the surface tension of surfactant solutions, paints and solvents with a platinum du Noüy ring or a Wilhelmy plate, and the tension between two liquids with the same tools. Surfactant makers, paint and coatings labs and formulators use it to compare how well surface active agents lower surface tension. The Dropometer is partially compliant: it gets the same end results by pendant drop, surface tension on the base kit and interfacial tension with the optional cuvette. Pendant drop is an alternative method, not the ring or plate method the standard specifies; if a contract or customer requires an ASTM D1331 result, run a correlation study on your own samples first.

The standard has four methods. Methods A and C read surface tension with a ring or a plate, and ASTM recommends the plate for paints and resin solutions. Methods B and D read the tension between two liquid phases. A common use is a critical micelle concentration (CMC) curve: surface tension falls as surfactant is added, then levels off. For typical values, see our CMC reference values for common surfactants.

The Dropometer pendant drop tensiometer reads surface tension up to 75 mN/m with ±0.03 mN/m accuracy, static or dynamic, from one hanging drop. Use it to screen surfactants and build CMC curves, as in CMC assessment and surfactant efficiency. A university rheology lab uses the Dropometer for pendant drop surface tension, and researchers used it to measure the surface activity of polymer and surfactant mixtures. For background, read the chemicals guide, the surface tension measurement guide and our guide to understanding surfactants.

Further reading: reproducible surface tension measurement, choosing a tensiometer for surfactant and CMC research and a classroom experiment on how surfactants change surface tension.

How the Dropometer fits next to a du Noüy ring tensiometer

Pendant drop surface tension of a surfactant series, and two liquid tests in the optional cuvette, step by step

From a solution series to a CMC curve

These steps describe a pendant drop measurement on the Dropometer. They are not the ring or plate procedure of ASTM D1331 and do not replace it. Buy the current edition and follow it for any result you report to the standard.

±0.03 mN/m

Surface tension accuracy

From the Dropometer product specification. Resolution is 0.01 mN/m, range up to 75 mN/m.

10 fps

Camera frame rate

Dynamic readings from about 0.1 s of drop age. Fine for surfactant ageing, not millisecond studies.

About 2 min

Setup

Base plate, phone holder, LED and syringe. No PC needed.

1

Make up the solution series

Prepare each concentration in clean glassware with the water or solvent your method names. For the tension between two liquids, also prepare the second liquid. Let the solutions settle, and bring them to room temperature or the temperature the standard specifies. Note the density of each liquid for the fit.

2

Set up and check with water

Level the base plate, mount the phone and LED, and fit a clean needle. Hang a drop of pure water and read it. Compare it with the reference value for your room temperature before you test any surfactant.

3

Hang the drop and let it age

Form a pendant drop large enough to show a clear neck. For two liquid tests, as in Methods B and D, use the optional cuvette: fill it with the lighter, clear liquid and hang a drop of the denser liquid inside it. Surfactant molecules take time to reach the surface, so tension falls with drop age. Read every drop at the same age, or follow it in dynamic mode until it settles.

4

Read the surface tension

The software fits the Young-Laplace equation to the drop profile, using the density difference between the drop and the air, or the liquid in the optional cuvette for interfacial tension. Automatic edge detection gives the same reading from the same image. Reject drops with bubbles, a poor fit or a near round shape.

5

Repeat and build the curve

Read several drops at each concentration, working from the weakest solution to the strongest. Plot surface tension against log concentration. The break point is the CMC. See CMC reference values to sense check it.

6

Record and report the method

Record the method as pendant drop, with or without the optional cuvette, plus drop age, densities, room temperature, water quality and mean with spread. State that these are pendant drop results, not ring or plate results to ASTM D1331. If a customer requires a D1331 value, run a correlation study on your own samples first.

7

Watch the full measurement

This video shows a complete Dropometer pendant drop surface tension measurement, from setup to result. It covers the steps above in one run.

Dropometer pendant drop surface tension, start to finish.

Validated measurement approach

Independent benchmarking and publication-based validation references.

Peer reviewed method

The Dropometer drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers, one in Review of Scientific Instruments and one in Colloids and Surfaces A. Published surface tension accuracy is ±0.03 mN/m (Dropometer product specification).

Read the validation papers

Customer evidence

A university research team used the Dropometer to measure pendant drop surface tension of polymer and surfactant mixtures, published in Polymers in 2025. We have no published client study run to ASTM D1331 itself.

Read our analysis

Check with water, then correlate with your ASTM D1331 test method

ASTM D1331 · surface tension · surfactant and coatings QC

Check the setup with pure water before each session. The reference value is 72.74 mN/m at 20 °C and 71.97 mN/m at 25 °C (source: IAPWS R1-76(2014)). Water reads about 0.15 mN/m lower for each degree warmer (same source), so record room temperature.

ASTM D1331 is a test method. Your product specification or your customer sets the limits. Ring, plate and pendant drop values on surfactant solutions can differ, because the ring reading needs a correction factor and each method reads the surface at a different age. Pendant drop is an alternative method, not the ring or plate method the standard specifies. The same applies to interfacial tension with the optional cuvette: no accuracy figure is published for it, so your correlation data sets the limits.

If you work to a D1331 limit, run a correlation study. Measure the same samples on your ring or plate tensiometer and on the Dropometer, at the same temperature, then set your pendant drop limits from that data. Repeat the study when you change surfactant family, solvent or test temperature. See reproducible surface tension measurement for bench practice.

Example output

Example data for illustration only. These are not measured results. Your own surfactant, water quality and temperature decide the numbers.

Example: anionic surfactant in water, pendant drop, six concentrations

Concentration (mM) Drops (n) Surface tension (mN/m) Std dev (mN/m) Reading
0.5562.00.2Below CMC: falling
1555.20.2Below CMC: falling
2548.40.3Below CMC: falling
4541.60.2Below CMC: falling
8536.10.1Above example CMC: plateau
16535.90.1Above example CMC: plateau

How to read this table

Each row is the mean of five drops read at the same drop age at 22 °C room temperature. The example CMC of about 7.1 mM is where the straight line through the first four points meets the plateau of 36.0 mN/m.

Example output chart

The same example data as the table, on a log concentration axis, with the plateau and the example CMC marked.

Line chart of example surface tension falling from 62.0 to 36.1 mN/m as an anionic surfactant concentration rises, then levelling off at about 36 mN/m above an example CMC of 7.1 mM

Example data, not measured results.

Business impact

What changes when pendant drop sits next to your ring or plate tensiometer

Metric Before Dropometer With Dropometer Results
Sample per reading A vessel of liquid for the ring or plate One hanging drop from the needle No published benchmark
Surface age One reading per immersion Dynamic reading over drop age from about 0.1 s Dropometer product specification, 10 frames per second
Result format Ring force with a correction factor Surface tension from the drop shape, 0.01 mN/m resolution, ±0.03 mN/m accuracy Dropometer product specification
Reported D1331 value Ring or plate tensiometer Still needs the ring or plate tensiometer ASTM D1331 scope

See the measurement on your own solutions

Bring a surfactant you know well and one you are screening. We will measure both live by pendant drop.

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ROI estimate

Estimate the cost of the formulation and wetting failures this test can catch

Enter your own numbers. The defaults are examples, not Droplet Lab results.

Quality cost snapshot

Estimate what wetting related failures cost you.

Example only. Use what you see in a trial on your own samples.
Your own figure.
Your own estimate.

Result

~0
Estimated annual cost today
~0
Estimated annual cost avoided
~0
Payback period

Annual exposure combines audit/CAPA cost, customer rejections, and recall probability; avoided exposure applies the risk reduction percentage.

Method settings for surface and interfacial tension

Lock these before you compare pendant drop results between runs or sites

Parameter Recommended Setting Technical Rationale
Method Pendant drop, reported as an alternative method ASTM D1331 specifies the du Noüy ring or Wilhelmy plate.
Liquid density Entered for each liquid; both liquids when you use the optional cuvette The Young-Laplace fit needs the density difference between the drop and the air or liquid around it.
Drop age at reading Fixed for every drop, or read at a steady value Surfactant solutions lower their surface tension as the surface ages.
Fit model Young-Laplace The standard fit for a pendant drop profile in the Dropometer software.
Replicates Several drops per concentration, fresh drop each time Report the mean and spread, as you would for ring readings.
Concentration order Weakest to strongest; rinse the needle between solutions Surfactant left in the needle lowers the next reading.
Range Surface tension up to 75 mN/m Covers water, surfactant solutions and common solvents (Dropometer datasheet).
Two liquid tests (Methods B and D) Optional cuvette filled with the lighter, clear liquid Interfacial tension by pendant drop with the optional cuvette needs the drop to hang inside the second liquid.
Environment Record room temperature; condition samples first The Dropometer has no temperature control and runs at 10 to 45 °C.

Is ASTM D1331 the right test?

Pick the method by the question you need answered

Start: What do you need to know about your liquid?

Surfactant efficiency or a CMC curve

Signals:

You want to rank surfactants or find the CMC from surface tension against concentration, in R&D or screening.

Rule-out:

Use pendant drop on the Dropometer. For a standard CMC method, see ISO 4311, which uses a plate, stirrup or ring.

A reported ASTM D1331 value

Signals:

Your customer or specification asks for a result to ASTM D1331, any of Methods A to D.

Rule-out:

The standard specifies a du Noüy ring or Wilhelmy plate tensiometer, with the plate recommended for paints and resin solutions. Pendant drop on the Dropometer is an alternative method: run a correlation study on your own samples before you report against a D1331 limit.

Tension between two liquid phases

Signals:

You need the oil and water, or solvent and water, tension that Methods B and D cover.

Rule-out:

Use pendant drop with the optional cuvette: hang a drop of the denser liquid inside the lighter one and enter both densities. For a reported D1331 value, correlate with a ring or plate first. See ASTM D971 / IEC 62961 and EN 14210.

Surface tension over drop age

Signals:

You want to see how fast a surfactant lowers surface tension, from about 0.1 s of drop age.

Rule-out:

Use dynamic pendant drop on the Dropometer. For readings in the first milliseconds, use a bubble pressure tensiometer.

Decision tree at a glance

Start from the question you need answered. Blue boxes are measurements the Dropometer runs.

Decision tree for choosing between a pendant drop CMC series, ring or plate testing for a reported ASTM D1331 value, pendant drop with the optional cuvette for two liquid phases, and dynamic pendant drop

Is ASTM D1331 the right test?

Interpreting surface tension of liquid solutions

What a surface tension curve tells you, and what it does not

Falling line, then a plateau: Surface tension drops as surfactant is added, then levels off. The break is the CMC.
Lower plateau, more effective surfactant: The plateau value shows how far a surfactant can lower surface tension. An earlier break means less surfactant does the job.
Age changes the number: A fresh surface reads higher than an aged one. Compare values only at the same drop age.
Methods can disagree: Ring, plate and pendant drop values on the same solution, or the same pair of liquids, can differ. Correlate before you compare them.
Expert perspective

Why a surface tension curve shows what a surfactant does

Surface tension versus concentration is one of the few measurements that lets you watch a surfactant reorganise an interface in real time: read the curve, not just the endpoint.

Dr. Alidad Amirfazli

Department of Mechanical Engineering, York University, Co-founder and Scientific Advisor, Droplet Lab

Common pitfalls and limits

Honest limits of the method and of the Dropometer

Interfacial tension needs the optional cuvette, a clear outer liquid and a clear density difference. No accuracy figure is published for it, so set limits from your own correlation study.
Pendant drop is an alternative method, not the ring or plate method ASTM D1331 specifies. Do not swap results without a correlation study on your own samples.
The Dropometer has no temperature control. Condition samples and liquids, and record room temperature (operating 10 to 45 °C).
Thick or heavily pigmented paints can be hard to hang as a clean, steady drop. The standard recommends the plate for paints.
Trace surfactant on the needle or glassware lowers readings. Check with pure water first and rinse between solutions.
The camera runs at 10 frames per second. That suits readings from about 0.1 s of drop age, not millisecond adsorption studies.
FAQ

Frequently asked questions

What does ASTM D1331 measure?

It measures the surface tension of liquids such as surfactant solutions, paints and solvents, and the tension between two liquids. Methods A and C read surface tension with a du Noüy ring or Wilhelmy plate. Methods B and D read the tension between two liquid phases with the same tools. The methods were written mainly for aqueous solutions of surface active agents, but are also applicable to nonaqueous solutions and mixed solvent solutions.

Can the Dropometer run ASTM D1331?

It gets the same end results by a different method, so it is partially compliant. It measures surface tension by pendant drop, and interfacial tension by pendant drop with the optional cuvette. Pendant drop is an alternative method, not the ring or plate method the standard specifies. If a contract or customer requires an ASTM D1331 result, run a correlation study on your own samples first.

Can the Dropometer measure the tension between two liquids, as in ASTM D1331 Methods B and D?

Yes, with the optional cuvette. Fill the cuvette with the lighter, clear liquid, hang a drop of the denser liquid from the needle inside it and enter both densities. This is interfacial tension by pendant drop with the optional cuvette, not the ring or plate of Methods B and D, so correlate with your ring or plate results before you compare values.

Are pendant drop and ASTM D1331 ring results the same?

Not automatically. The ring reading needs a correction factor, and each method reads the surface at a different age. On surfactant solutions that matters. Run a correlation study on your own samples before you compare pendant drop values with D1331 limits.

Which ASTM D1331 method suits paints?

ASTM recommends Method C, the Wilhelmy plate, for paints and resin solutions. It needs no buoyancy correction and copes with moderate viscosity. Thick or pigmented paints can also be hard to hang as a steady pendant drop.

How do I find the CMC from surface tension?

Measure a series of concentrations and plot surface tension against log concentration. Fit a straight line to the falling part and another to the plateau. The CMC is where they meet. See ISO 4311 and our CMC reference values.

Does the Dropometer control test temperature?

No. The Dropometer has no temperature control and runs at 10 to 45 °C. Condition samples and liquids to the temperature your method sets, and record room temperature with each result. Water surface tension falls about 0.15 mN/m per degree (source: IAPWS R1-76(2014)).

Legal note

This page summarises ASTM D1331 in our own words and does not reproduce the standard. Buy the current edition from ASTM before you write a procedure. The Dropometer is partially compliant: it measures surface tension by pendant drop, and interfacial tension with the optional cuvette, an alternative to the ring or plate method the standard specifies. Droplet Lab does not certify compliance. Your own method validation does.

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How this page was created

Sources, review and tools

Transparency 4 checklist items
01

Sources

ASTM D1331-20(2024) listing; ASTM D1331-20 listing; ANSI webstore: ASTM D1331-20(2024); Nimonik: ASTM D1331-20(2024) summary (June 2024, 6 pages), the Dropometer product specification and Droplet Lab validation papers.

02

Drafting assistance

Drafted with Claude (Anthropic) from the sources listed, then reviewed by Droplet Lab.

03

What we did not do

We did not copy text from the standard. Buy the current edition for the full procedure.

04

Last checked

October 2026

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References

1. ASTM D1331-20(2024). store.astm.org/d1331-20r24.html
2. ASTM D1331-20 listing. store.astm.org/d1331-20.html
3. ANSI webstore: ASTM D1331-20(2024). webstore.ansi.org/standards/astm/astmd1331202024
4. Nimonik: ASTM D1331-20(2024) summary (June 2024, 6 pages). standards.nimonik.com/products/astm/d1331/
5. Droplet Lab. Peer reviewed validation papers in Review of Scientific Instruments and Colloids and Surfaces A. dropletlab.com/validation/publications/
6. Droplet Lab. Dropometer product specification, 2026. dropletlab.com/products/dropometer/