Contents

Goniometer

Pronunciation: /ˌɡəʊniˈɒmɪtə/

A contact angle goniometer is an instrument that images a liquid drop on a solid and measures the contact angle where the drop meets the surface. Modern goniometers use a camera and drop shape software rather than a protractor eyepiece.

Also called

Contact angle goniometer Contact angle meter Optical tensiometer Drop shape analyzer

Related terms

Sessile drop method

Topics

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

At a glance

What a goniometer is made of, and what it can and cannot tell you.

What it is

The name comes from the Greek gonia, angle, and metron, measure. In surface science it means an instrument for wetting angles.

  • A level sample stage
  • A syringe or pipette to place drops
  • Even backlighting behind the drop
  • A camera and lens on the drop's horizon
  • Software that finds the edge and baseline and fits the profile

Why it matters

The goniometer is the standard tool behind wettability checks in coatings, adhesion, cleaning validation, biomaterials and teaching labs. Most goniometers also measure surface tension from a pendant drop. Buyers comparing setups for research, teaching or production can start at our contact angle goniometer buying guide hub.

Worked example

A drop image reads 71.6° on the left and 73.2° on the right.

  • Mean angle = (71.6 + 73.2) / 2 = 72.4°
  • Left to right difference = 1.6°
  • A small difference suggests a level stage and a uniform spot

You can check a fit on your own drop image with our free contact angle tool.

How it is measured

A goniometer usually runs the sessile drop method, and many also run pendant drop, captive bubble and tilting tests. The full procedure is in the contact angle measurement guide.

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

The three generations of goniometer you will meet in labs today.

Manual (protractor) goniometer

The operator lines up a crosshair with the drop tangent in an eyepiece and reads the angle off a scale. Results depend strongly on the operator.

Camera goniometer with software

A camera captures the drop and software fits the profile. Edge detection and fitting make readings repeatable and allow video of changing drops.

Smartphone based goniometer

A phone camera with a macro lens, backlight and stage replaces the dedicated camera. The analysis follows the same drop shape principles.

The camera sits level with the sample surface so the contact points and baseline are seen edge on.

θ Contact angle, through the liquid, degrees
θL, θR Left and right contact angles of the same drop, degrees
θA, θR (dynamic) Advancing and receding angles, degrees

How to read it

What goniometer readings usually tell you, and what to check before you trust them.

A goniometer reading reflects both the surface and the setup. Comparing your results with published values, such as the dataset of client collected wetting measurements, helps you judge whether a reading is plausible.

From goniometer reading to meaning

If you see It usually means Check next
Left and right angles differ a lot Stage not level, or a non uniform spot Stage level, a new position on the sample
Readings change between operators Manual baseline or tangent placement Automated edge and baseline detection
A reference surface reads differently from last month Calibration, lighting or lens drift Calibration check with a reference
Angle falls steadily during the reading Evaporation or liquid soaking into the sample Time of reading, sample porosity

How it is measured

What a goniometer does in four short steps. The contact angle measurement guide has the full procedure.

1

Level and focus

Level the stage and focus the camera on the sample surface line.

2

Place the drop

Dispense a drop gently onto the sample.

3

Capture

Record a sharp image or video of the backlit drop.

4

Analyze

The software detects the edge and baseline, fits the profile and reports the angles.

Common misconceptions

Where goniometer results are most often misread.

A more expensive goniometer gives the true contact angle

Hardware sets image quality, but the surface sets the angle. Hysteresis, roughness and contamination change the reading on any instrument. Good sample handling matters as much as the instrument.

A goniometer measures surface energy directly

It measures contact angles. Surface energy is calculated from angles of two or three test liquids with a model such as OWRK. The instrument provides the inputs, not the answer.

Once set up, it never needs checking

Lens position, camera tilt and lighting drift over time and shift the measured angle. A periodic check against a reference surface keeps results comparable.

Goniometers are only for research labs

The same measurement supports production quality checks and teaching. Students learn wetting with simple setups in labs from the surface science teaching curriculum hub.

Related terms

Where the goniometer sits among wetting methods and instruments.

Broader term

  • Contact angle
  • Drop shape analysis (DSA)

Related terms

  • Sessile drop method
  • Captive bubble method
  • Tilting plate
  • Polynomial fitting method

Often confused with

  • Tensiometer
  • X ray goniometer (crystallography)

FAQ

Short answers to the questions people ask most about contact angle goniometers.

What is the difference between a goniometer and a tensiometer?

A goniometer measures the contact angle of a drop on a solid. A tensiometer measures the surface or interfacial tension of a liquid. Optical instruments often do both: sessile drops for angles and pendant drops for surface tension. Force tensiometers use a ring or plate instead of an image.

Can a smartphone work as a contact angle goniometer?

Yes, with the right optics, lighting and software. The Dropometer smartphone drop shape analysis kit turns a phone into a goniometer with automatic edge and baseline detection. Its drop shape method is validated in peer review against a KRUSS DSA100E, with a published accuracy of 0.35°.

What should a goniometer measure besides static contact angle?

For many surfaces, advancing and receding angles and the sliding angle tell you more than one static angle. Look for the ability to add and withdraw liquid during recording, to tilt the sample, and to measure pendant drop surface tension if you also test liquids.

References

Primary sources and further reading.

1. 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
2. Huhtamäki, T., Tian, X., Korhonen, J. T., and Ras, R. H. A. (2018). Surface-wetting characterization using contact-angle measurements. Nature Protocols, 13(7), 1521 to 1538. https://doi.org/10.1038/s41596-018-0003-z
3. Chen, H., Muros-Cobos, J. L., and Amirfazli, A. (2018). Contact angle measurement with a smartphone. Review of Scientific Instruments, 89(3), 035117. https://doi.org/10.1063/1.5022370
4. Contact angle measurement guide
5. Sessile drop method