Contents
Partially compliant

ASTM F21: atomizer test for hydrophobic surface films, and the water contact angle alternative

ASTM F21 sprays a fine water mist onto a dry surface and checks whether the droplets merge into a continuous film or stay as separate beads, a pass or fail sign of an oily or organic film. The Dropometer gets the same end result by a different route: it measures the water contact angle at chosen spots, from 10° to 175° with ±0.35° accuracy (product specification). A sessile drop contact angle is an alternative method, not the atomizer test the standard specifies; if a contract or customer requires an ASTM F21 result, run a correlation study on your own samples first.

What brings you here?

Who this is for
Aerospace and space hardware makers, electronics and semiconductor plants, metal finishers, and anyone who checks parts or clean areas for oily or organic films.
Compliance tier
Partially compliant: the Dropometer detects hydrophobic films by water contact angle, an alternative to the visual atomizer test the standard specifies.
Measurement
Sessile drop water contact angle, 10° to 175° range, ±0.35° accuracy (Dropometer datasheet).
Setup
About 2 minutes. Battery powered, 1 kg, runs at the line, in the clean area or in the lab.
Last updated
October 18, 2026
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Free one page checklist for water contact angle next to ASTM F21: what to prepare, the settings to lock, where to place drops, how to log witness surfaces and how to record readings below 10°.

Talk to an expert

Book a 15 minute demo on your parts or witness surfaces

Tell us your parts, your clean area and how you run the atomizer test today. We will measure live and tell you plainly which part of the job the Dropometer covers and which 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 F21
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

Standard

ASTM F21-20(2026), Standard Test Method for Hydrophobic Surface Films by the Atomizer Test, from ASTM Subcommittee E21.05. It is the 2020 edition, reapproved in 2026, and the active edition. Earlier editions include F21-14 and the original F21-65.

Compliance tier

Partially compliant. The Dropometer detects the same end result, a hydrophobic film, by measuring the water contact angle at chosen spots. It does not run the visual atomizer test over the whole surface, and very clean surfaces can read below its 10° range. A sessile drop contact angle is an alternative method, not the atomizer test the standard specifies; if a contract or customer requires an ASTM F21 result, run a correlation study on your own samples first.

Instrument performance

Contact angle range 10° to 175°, accuracy ±0.35°, display resolution 0.01°, from the Dropometer product specification.

Peer reviewed method

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

Customer evidence

We have no published client study run to ASTM F21 yet. A university shared facility uses the Dropometer for water contact angle on silicone elastomers and saline contact angle on titanium discs before and after plasma treatment. Read the case study. A 2023 Plant Science paper used it for static water contact angle on superhydrophobic aspen leaves. Read our analysis.

What it does not tell you

Beaded mist or a raised angle shows that a hydrophobic film is present. Neither identifies the contaminant. Use a chemical method such as FTIR or XPS when you need to know what it is.

Executive summary

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

ASTM F21 detects hydrophobic films with the atomizer test: a fine water mist on a dry, clean surface spreads into a thin continuous film, while an oily or organic film leaves the droplets standing as separate beads. Aerospace and space hardware makers, electronics and semiconductor plants and metal finishers use it where a part should not be immersed or doused with water. It also checks a processing area: a clean witness surface is left in the area, then tested for films that settled on it. The Dropometer is partially compliant: it detects the same films by measuring the water contact angle, but it does not run the visual spray test, and it reads angles from 10° upward. A sessile drop contact angle is an alternative method, not the atomizer test the standard specifies; if a contract or customer requires an ASTM F21 result, run a correlation study on your own samples first.

ASTM states the atomizer test can detect fractional molecular layers of hydrophobic contaminants, that it is not quantitative, and that rough or porous surfaces make it less sensitive. It only works on surfaces that water wets when clean, such as most metals; gold and many plastics wet poorly, so films are hard to see on them. For a number, the standard itself points to sessile drop contact angle methods, and notes that a drop covers only a small area. That is where the Dropometer fits: a measured angle at the spots that matter, next to the visual check over the whole surface. On rough parts, try our classroom experiment on Wenzel and Cassie Baxter wetting on rough surfaces.

The Dropometer contact angle and surface energy kit runs on battery, so you can check parts and witness surfaces at the line or in the lab. A plasma clean is a common way to strip these films before bonding; for that workflow, see plasma treatment verification for adhesion. An oils, fats and personal care ingredients maker uses the Dropometer to measure contact angle as a check of hydrophobicity. In a 2023 study of superhydrophobic aspen leaves, researchers used the Dropometer to measure static water contact angles of about 140° to 150° on a naturally water repellent surface.

For background, read the electrical and electronics guide and the contact angle measurement guide.

Further reading: sample preparation and contamination control, an open dataset of client collected wetting measurements, the captive bubble method for samples that can sit under water (it runs on the Dropometer with the optional cuvette) and choosing a contact angle goniometer for industrial QA.

How the Dropometer fits the workflow

A water contact angle check next to the ASTM F21 atomizer test, step by step, in our own words

From a cleaned part or witness surface to a recorded angle

These steps describe a water contact angle measurement on the Dropometer, run next to the atomizer test. They are not the ASTM F21 procedure and do not replace it. Buy the current edition and follow it for any result you report to the standard.

±0.35°

Contact angle accuracy

From the Dropometer product specification. Display resolution is 0.01°.

10° to 175°

Measuring range

Very clean metal or glass can read below 10°. Record those drops as below range.

About 2 min

Setup

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

1

Take the parts or witness surfaces

Take production parts, coupons or clean witness surfaces from the line or the clean area. Handle them by the edges with clean gloves or tongs. Note the cleaning step, the exposure place and time, and the atomizer result if you ran it. See sample preparation and contamination control.

2

Set up the Dropometer

Place the base plate on a level bench, mount the phone and LED, and fill a clean syringe with water of the purity your procedure names. If the surface was sprayed for the atomizer test, dry it as your procedure describes first. Lay it flat on the sessile stage, test side up.

3

Place drops where films collect

Dispense the same drop volume at set positions: the centre, near edges and holes, at fixture contact points and wherever the mist beaded. Do not touch the surface with the needle, or you may leave a film yourself.

4

Capture and read the angle

Capture each drop at a fixed time after it lands. Automatic edge and baseline detection reads the left and right angles, so two operators get the same reading from the same image. If the drop spreads below 10°, record it as below range.

5

Repeat across parts and exposures

Read several drops per position, several parts per batch and, for a clean area, several witness surfaces per place. Films from handling, fixtures or the air often sit in one area, so one drop is not enough.

6

Record and correlate

Record the mean, spread, positions, cleaning step, exposure place and time, atomizer result and room conditions. Compare the mean with your accept limit. State that these are contact angle results, not ASTM F21 atomizer test results. If the film may be silicone, see silicone and invisible residue detection.

7

Watch the full measurement

This video shows a complete Dropometer contact angle measurement, from setup to result. It covers the steps above in one run.

Dropometer contact angle measurement, 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 contact angle accuracy is ±0.35° (Dropometer product specification).

Read the validation papers

Customer evidence

We have no published client study on the atomizer test yet. In a 2023 Plant Science paper, researchers used the Dropometer to measure static water contact angles from 139.7° to 150.1° on superhydrophobic aspen leaves.

Read our analysis

Set your angle limit from your own atomizer test results

ASTM F21 · hydrophobic films · cleaning and clean area control

ASTM F21 is a pass or fail test and its public summary gives no contact angle that matches a pass (source: ASTM F21-20(2026) listing). So set your accept limit from your own correlation study. Run the atomizer test and the contact angle on the same parts or witness surfaces, clean and deliberately soiled, then pick the angle that separates passing surfaces from failing ones.

Check where your limit sits against the Dropometer range of 10° to 175°. If your limit is above 10°, a drop that spreads below 10° still sits under the limit: record it as below range and accept. If you must report exact values on very clean metal or glass below 10°, use a goniometer rated for low angles for that part of the work.

For a processing area, fix the witness material, its cleaning, its place and its exposure time, and test it the same way each time. ASTM notes that a contact angle drop covers a small area, so keep the atomizer test for the whole surface. Repeat the study when you change the cleaner, the rinse water, the material, the surface finish or the exposure time.

Example output

Example data for illustration only. These are not measured results. Your own surfaces, processing area and limits decide the numbers.

Example: glass witness slides left for set times at places in an assembly area

Witness slide Atomizer result Drops (n) Mean angle (°) Std dev (°) Decision
Freshly cleaned control, not exposedContinuous film5Below 10 (below range)n/aControl: under the 20° limit, value not given
Clean bench, 24 hContinuous film513.81.2Accept
Clean bench, 72 hSmall beaded patches521.61.7Above limit: shorten exposure or check air handling
Next to a bench using silicone grease, 24 hSeparate beads over the whole slide547.33.1Source found: contain or move the silicone
Next to the plastic packing station, 24 hBeads near one edge529.45.6Above limit: review packing materials

How to read this table

Each row is the mean of five drops of reagent water on one witness slide, read after the stated exposure. The atomizer column is the visual result on a twin slide from the same place. The decision column uses an example accept limit of 20° or lower, set from a correlation study against the atomizer test. The unexposed control read below 10°, outside the Dropometer range, so its value is not given.

Example output chart

The same example data as the table, plotted against the example accept limit. The unexposed control is left out because it read below 10°.

Bar chart of example water contact angles on glass witness slides after different exposure times and places in an assembly area, with a 20 degree accept limit and the 10 degree bottom of the Dropometer range marked

Example data, not measured results.

Business impact

What changes when a measured angle sits next to the atomizer test

Metric Before Dropometer With Dropometer Results
How the check is made Atomizer test, judged by eye Water contact angle read from the drop image, next to the atomizer test No published benchmark
Result format Pass or fail An angle per drop, 0.01° display, ±0.35° accuracy Dropometer product specification
Area covered The whole sprayed surface in view Small spots, one drop each, at positions you choose ASTM F21 notes contact angle devices cover a small area
Reported ASTM F21 result Atomizer test Still needs the atomizer test ASTM F21 scope

See the measurement on your own surfaces

Bring two parts or witness surfaces: one that passed the atomizer test and one that did not. We will measure both live.

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ROI estimate

Estimate the cost of the coating, bonding and contamination 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

Lock these before you compare angles between shifts, areas or sites

Parameter Recommended Setting Technical Rationale
Method Sessile drop water contact angle, reported as an alternative method ASTM F21 specifies the visual atomizer test. Its public summary names contact angle methods for quantitative wetting.
Test liquid Reagent water, the same grade as your atomizer test ASTM lists D1193 reagent water as a referenced document. Impure water carries its own films.
Drop volume Fixed for every drop A changing volume adds spread that is not about contamination.
Reading time Fixed for every drop Water keeps spreading on a clean surface, so the time must not vary.
Positions Set positions per part or witness surface, including edges and fixture points A drop covers a small area, and films are often uneven.
Measuring range 10° to 175°; record lower readings as below range Dropometer product specification. Very clean metal or glass can spread below 10°.
Fit model Young-Laplace or polynomial, the same for every run Both models are in the Dropometer software. Do not mix them within one study.
Time since cleaning or exposure time Fixed and recorded Clean surfaces pick up films from the air, so the angle rises with time.
Environment Record room temperature and humidity The Dropometer runs at 10 to 45 °C and has no temperature or humidity control.

Is ASTM F21 the right test?

Pick the method by the question you need answered

Start: What do you need to know about the surface?

A measured number at the spots that matter

Signals:

You want an angle per part, batch or witness surface to trend cleaning or a clean area, and your accept limit is above 10°.

Rule-out:

Use the water contact angle on the Dropometer. It is an alternative method, not the ASTM F21 atomizer test, so correlate it with your atomizer results first. For glass, see ASTM C813.

A pass or fail over a surface you cannot flood

Signals:

The part should not be immersed or doused, you can see the whole surface, and you need a fast check, or a customer names ASTM F21.

Rule-out:

Run the ASTM F21 atomizer test. It needs a water atomizer, clean water and a trained eye, not the Dropometer. ASTM says it does not suit assembled hardware that could trap water, or surfaces out of line of sight.

A pass or fail on parts you can immerse

Signals:

The part can be dipped in or doused with water and dried afterwards.

Rule-out:

Use the water break test, ASTM F22. Contact angle adds a number at chosen spots there too.

Why a coating or adhesive fails

Signals:

You need polar and dispersive surface free energy, not only whether a film is present.

Rule-out:

Use a two liquid method such as ASTM D7490. The Dropometer measures the angles for it.

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 water contact angle on the Dropometer, the ASTM F21 atomizer test, the ASTM F22 water break test and a two liquid surface energy method

Is ASTM F21 the right test?

Interpreting results

What the angle tells you, and what it does not

Higher angle, more hydrophobic film: A clean metal or glass surface wets well. Oils, silicones and other organic films make water bead up, so the angle rises and the mist stays as separate droplets.
A rising trend on witness surfaces points to the air: If clean witness surfaces read higher with exposure time or near one station, films are settling from the area, not from your cleaning.
Spread across the part matters: A good mean with one high position points to a handling, fixture or drying problem in that area.
Angle is not identity: A raised angle shows a film is present. It does not say what the film is or where it came from.
Expert perspective

Why a film a few molecules thick keeps water from spreading

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

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

A sessile drop contact angle is an alternative method, not the atomizer test ASTM F21 specifies. Do not report angles as F21 results without a correlation study on your own samples.
Each drop covers a small spot. A film can sit between your drops, so keep the atomizer test for the whole surface.
Very clean metal or glass can read below 10°, outside the Dropometer range. Record it as below range, or use a goniometer rated for low angles.
ASTM notes the test only suits surfaces that water wets when clean. On gold and many plastics, water beads even without a film, so a film changes the angle less. Set limits per material.
ASTM notes that very rough or porous surfaces make the test much less sensitive. Roughness also changes the contact angle, so set limits per surface finish.
Water left from the atomizer spray spoils a contact angle reading. Measure first, or dry the surface as your procedure describes.
The Dropometer has no temperature or humidity control, and its camera runs at 10 frames per second. Read at a fixed time and record room conditions.
FAQ

Frequently asked questions

What does ASTM F21 measure?

It detects hydrophobic films on a surface with the atomizer test. A fine water mist goes onto the dry surface. On a clean surface the droplets spread and join into a thin continuous film; where an oily or organic film remains, they stay as separate beads. ASTM describes it as a fast, non destructive pass or fail check that is not quantitative.

Can the Dropometer run ASTM F21?

In part. It gets the same end result, whether a hydrophobic film is present, by measuring the water contact angle at chosen spots. It does not run the visual spray test over the whole surface. A sessile drop contact angle is an alternative method, not the atomizer test the standard specifies; if a contract or customer requires an ASTM F21 result, run a correlation study on your own samples first.

What water contact angle matches a pass in the ASTM F21 atomizer test?

No published angle does. ASTM F21 is a pass or fail test and its public summary gives no contact angle threshold. Run both tests on the same parts or witness surfaces, clean and deliberately soiled, and set your accept limit from that data. See silicone and invisible residue detection for a screening workflow.

When should I use ASTM F21 instead of ASTM F22?

When the surface should not or cannot be immersed or doused with water. The atomizer test uses a fine mist, where the water break test to ASTM F22 floods the part. ASTM notes that neither suits assembled hardware where water could be trapped, and F21 needs a clear line of sight to the surface.

Why does ASTM F21 mention contact angle methods?

Because the atomizer test gives no number. The public ASTM F21 summary says sessile drop contact angle methods, such as ASTM C813 and ASTM D7490, can be used where quantitative wetting data is needed. It also notes that a drop covers a small area, so a mix of spot readings and a whole surface check works best.

Can I use ASTM F21 to monitor a clean area?

Yes. ASTM describes leaving a surface that is free of hydrophobic films in the processing area, then testing it. Water contact angle on the Dropometer adds a number to the same idea: fix the witness material, place and exposure time, and trend the angle. Clean witness glass or metal can read below 10°, so record those drops as below range.

Does the atomizer test work on gold or plastic parts?

Poorly. ASTM notes the test only suits surfaces that water wets when clean, such as most metals. Gold and many plastics wet poorly even when clean, so a film is hard to see. The contact angle still reads on these surfaces, but the change a film causes is smaller, so set limits per material from your own data.

Legal note

This page summarises ASTM F21 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 detects hydrophobic films by water contact angle, an alternative to the atomizer test 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 F21-20(2026) listing (active); ASTM F21-20 listing (historical); ASTM F21-14 listing (historical); ANSI webstore: ASTM F21-20(2026), 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 F21-20(2026). store.astm.org/f0021-20r26.html
2. ASTM F21-20 listing (historical). store.astm.org/f0021-20.html
3. ASTM F21-14 listing (historical). store.astm.org/f0021-14.html
4. ANSI webstore: ASTM F21-20(2026). webstore.ansi.org/standards/astm/astmf21202026
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/