Contents
Complements workflow

ISO 25178 geometrical product specification, surface texture: areal roughness parameters (Sa, Sdr) and contact angle

ISO 25178 defines how to describe areal surface texture with parameters such as Sa and Sdr, measured on a profilometer or optical instrument. The Dropometer does not measure roughness. It complements the workflow: pair your ISO 25178 values with a water contact angle to see how much of a wetting change comes from texture and how much from surface chemistry.

What brings you here?

Who this is for
Surface finishing, machining, automotive, semiconductor and coating labs that measure areal roughness and need to know how it affects wetting.
Compliance tier
Complements workflow: the Dropometer does not measure Sa, Sdr or any ISO 25178 parameter. It adds the contact angle that you pair with them.
Measurement
Sessile drop contact angle, static, advancing and receding, 10° to 175°, ±0.35° accuracy (Dropometer datasheet). Roughness comes from your ISO 25178 instrument.
Setup
About 2 minutes. Battery powered, 1 kg, runs next to the profilometer or on the shop floor.
Last updated
October 20, 2026
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Free one page checklist for pairing ISO 25178 roughness with contact angle: what to prepare, the settings to lock, how to run a Wenzel check and what to record. Made for surface finishing and coating labs.

Talk to an expert

Book a 15 minute demo on your textured parts

Tell us your material, finish and the Sa and Sdr values you already measure. We will read contact angles live and tell you plainly what the Dropometer adds to your roughness data and what it does not.

Book a 15 minute demo
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 ISO 25178
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: ISO 25178-2 surface texture parameters and the Dropometer

Standard

ISO 25178-2:2021, Geometrical product specifications (GPS), Surface texture: Areal, Part 2: Terms, definitions and surface texture parameters, from ISO/TC 213, the technical committee of the International Organization for Standardization for geometrical product specifications. It replaces the 2012 edition. Part 1 covers the indication of surface texture on drawings and Part 3 the specification operators. Part 6 classifies methods for measuring surface texture, Part 600 and the 6xx parts cover the metrological characteristics of instruments, such as Part 601 for contact stylus and Part 603 for phase shifting interferometric microscopy, and Part 700 covers calibration.

Compliance tier

Complements workflow. ISO 25178 parameters are measured with stylus or optical instruments. The Dropometer measures none of them. It reads the contact angle that, paired with Sdr, shows how much of a wetting change comes from texture.

Instrument performance

Contact angle range 10° to 175°, accuracy ±0.35°, display resolution 0.01°, static, advancing and receding, 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 that pairs ISO 25178 parameters with Dropometer contact angles yet. Our Wenzel vs Cassie Baxter experiment shows the approach on the bench.

What it does not tell you

A contact angle does not give Sa, Sdr or any roughness value. A Wenzel correction is an estimate: it can fail when air is trapped in the texture or when texture is close in size to the drop.

Executive summary: areal surface texture in ISO 25178

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

ISO 25178 is the ISO series for areal surface texture: it defines 3D roughness parameters such as Sa, the arithmetical mean height, and Sdr, the developed interfacial area ratio, plus the filters and instruments used to measure them. Surface metrology labs in machining, surface finishing, automotive, additive manufacturing and semiconductors use it to specify and check surface finish. The Dropometer does not measure any ISO 25178 parameter, so this page is tiered complements workflow. It adds a water contact angle next to your roughness data, which shows whether a change in wetting comes from texture or from surface roughness and chemical heterogeneity.

Part 2 (ISO 25178-2:2021) holds the parameter definitions. Areal parameters extend the profile method of ISO 4287, and its successor ISO 21920-2, from a single line to an area. Sdr is the extra surface area that texture adds over a flat area, given in percent. The Wenzel model links it to wetting: cos θ* = r cos θY, where θ* is the measured angle, θY the angle on a smooth surface of the same material and r the true area divided by the projected area. With Sdr in percent, r is about 1 + Sdr/100. Our Wenzel vs Cassie Baxter experiment shows both models on the bench.

The Dropometer contact angle and surface energy kit reads static, advancing and receding angles from 10° to 175° with ±0.35° accuracy (Dropometer datasheet). Read the contact angle measurement guide for method basics and reproducible contact angle measurement for bench practice on textured parts. Compare your smooth surface values with our contact angle of water on common surfaces. Texture also drives wetting of porous battery parts, as in electrolyte and separator wetting optimization. For industry background, read the mechanical and industrial guide.

Further reading: choosing a contact angle goniometer for ISO and GLP regulated QC and the Leidenfrost effect, where surface texture changes how drops behave on a hot surface.

How the Dropometer fits next to areal surface measurement

Pairing ISO 25178 roughness values with contact angle, step by step, in our own words

From a roughness map to a texture corrected angle

These steps describe a combined workflow. The roughness steps run on your ISO 25178 instrument, not on the Dropometer. They do not replace the standard. Buy the current editions and follow them for any roughness value you report.

±0.35°

Contact angle accuracy

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

r = 1 + Sdr/100

Wenzel roughness ratio

Area ratio from ISO 25178 Sdr in percent (source: Digital Surf areal parameter guide).

About 2 min

Setup

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

1

Plan matched coupons

Pick coupons of the same material and cleaning with different finishes, or the same finish with different treatments. Only one thing should change at a time, or texture and chemistry stay mixed.

2

Measure areal roughness to ISO 25178

On your stylus or optical instrument, measure Sa and Sdr over the area where the drops will sit. Record the instrument, the S filter, the L filter or F operator and the sampling spacing. Sdr depends strongly on these choices, so keep them the same for every coupon.

3

Set up the Dropometer

Level the base plate, mount the phone and LED, and fill the syringe with deionised water. Lay the coupon flat on the sessile stage. Handle it by the edges, since skin oils change the angle.

4

Read the contact angles

Place several drops of the same volume on each coupon and read the static angle. On rough parts, also read advancing and receding angles by adding and withdrawing liquid through the needle, because texture widens the gap between them. Automatic edge and baseline detection gives the same reading from the same image.

5

Run the Wenzel check

Work out r = 1 + Sdr/100, then the smooth surface estimate from cos θY = cos θ* / r. See the Wenzel model for the assumptions. If the result is far from your other coupons, the model may not hold on that surface.

6

Separate texture from chemistry

If coupons with different Sdr give the same estimate, texture explains the angle change. If one does not match, look for a chemistry change such as oil, oxide or release agent. Record roughness settings, angles, drop volume and room conditions together.

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

Roughness and wetting on the bench

We have no published client study pairing ISO 25178 roughness with Dropometer angles yet. Our classroom experiment walks through Wenzel and Cassie Baxter wetting on textured surfaces.

See the experiment

Set limits from your own areal surface texture parameters

ISO 25178 · Sa and Sdr · wetting of textured surfaces

ISO 25178 defines roughness parameters and how to measure them. It sets no contact angle limits, and no published table links Sa or Sdr to a pass or fail angle. Set limits from your own correlation study.

Measure several coupons of the same material and cleaning with different finishes. Record Sa and Sdr on your profilometer, then the water contact angle on the Dropometer. Plot the angle against r = 1 + Sdr/100 (Sdr in percent; a 3.5% Sdr equals an area ratio of 1.035, source: Digital Surf areal parameter guide). Extrapolate to r = 1 to estimate the smooth surface angle.

Do not rely on the Wenzel equation alone. On etched silicon coupons with the same chemistry, KRUSS found the water angle rose with roughness, the opposite of the Wenzel prediction, and advises an empirical fit instead (source: KRUSS application report AR295). Repeat the study when you change material, finishing process or roughness filters.

Example output

Example data for illustration only. These are not measured results. Your own material, finish, filters and cleaning decide the numbers.

Example: aluminium coupons, three finishes, Sa and Sdr from a profilometer

Coupon Sa (µm) Sdr (%) Measured angle (°) Wenzel estimate (°) Reading
Polished0.050.378.078.0Baseline
Ground0.44.077.578.0Texture explains the change
Bead blasted1.825.074.978.0Texture explains the change
Bead blasted, batch B1.825.092.091.6Chemistry changed: check cleaning

How to read this table

Sa and Sdr come from an ISO 25178 instrument, not the Dropometer. Each angle is the mean of five water drops on the Dropometer. The Wenzel estimate uses cos θY = cos θ* / r with r = 1 + Sdr/100. Three coupons give the same estimate, 78.0°, so texture explains their angle differences. The fourth does not, which points to a chemistry change such as oil left after blasting.

Example output chart

The same example data as the table: the measured angle and the Wenzel estimate of the smooth surface angle for each coupon.

Grouped bar chart of example measured water contact angles and Wenzel estimates on four aluminium coupons, where three finishes share a 78 degree estimate and one batch reads about 92 degrees

Example data, not measured results.

Business impact

What changes when roughness data sits next to a contact angle

Metric Before Dropometer With Dropometer Results
What is measured Sa and Sdr only Sa and Sdr plus a contact angle per finish No published benchmark
Wetting change after finishing Texture and chemistry mixed in one judgement Wenzel estimate shows how much texture explains Wenzel model; check it with your own correlation
Angle result Not measured, or a water break test by eye An angle per drop, 0.01° display, ±0.35° accuracy Dropometer product specification
Reported roughness value ISO 25178 instrument Still needs the ISO 25178 instrument ISO 25178 scope

See the measurement on your own finishes

Bring two coupons of the same material with different finishes, and their Sa and Sdr values. We will read both live.

Book a 15 minute demo Request a quote
ROI estimate

Estimate the cost of the bonding and coating failures this check 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 parameters in ISO 25178 and contact angle

Lock these before you compare angles across finishes or sites

Parameter Recommended Setting Technical Rationale
Roughness instrument Your stylus or optical instrument, the same for every coupon The Dropometer does not measure roughness. Sdr values from different instruments are not directly comparable.
Filters and sampling S filter, L filter (such as a Gaussian filter) or F operator and spacing as your ISO 25178 specification sets Sdr depends strongly on filtering and lateral resolution, so fix them before you compare coupons.
Measured area Where the drops sit, and larger than the drop base The angle responds to the texture under the drop edge, not an average from elsewhere on the part.
Roughness ratio r = 1 + Sdr/100, with Sdr in percent Converts the ISO 25178 Sdr into the area ratio the Wenzel equation uses.
Test liquid Deionised water, or the liquid your process uses Keep one liquid for the whole study so only texture and chemistry vary.
Drop volume Fixed for every drop Wenzel holds best when the drop is much larger than the texture (Biolin Scientific).
Angle type Static, plus advancing and receding on rough parts Texture pins the drop edge and widens hysteresis, so one static angle can mislead.
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.
Environment Record room temperature and humidity The Dropometer runs at 10 to 45 °C and has no temperature control.

Is ISO 25178 the right test?

Pick the method by the question you need answered

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

Roughness values for a drawing or spec

Signals:

You need Sa, Sdr or other areal parameters to specify or accept a surface finish.

Rule-out:

Use ISO 25178 on a stylus or optical instrument. The Dropometer does not measure roughness.

How texture changes wetting

Signals:

You want to know how much a new finish changes the contact angle.

Rule-out:

Pair your Sdr values with Dropometer contact angles and run the Wenzel check. This page's workflow covers it.

Whether texture or chemistry changed

Signals:

Roughness is in spec but bonding or coating results moved.

Rule-out:

Compare contact angles on coupons with matched Sdr. For adhesive work, see DIN EN 828, which the Dropometer supports.

Coating adhesion after surface prep

Signals:

You need to rate how well a coating sticks to the prepared surface.

Rule-out:

Use a tape test such as ASTM D3359. Roughness and contact angle help explain the result; they do not replace 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 ISO 25178 roughness measurement, contact angle with a Wenzel check, contact angle on matched coupons and an ASTM D3359 tape test

Is ISO 25178 the right test?

Interpreting surface texture and contact angle together

What roughness and contact angle tell you together, and what they do not

Roughness amplifies the trend: In the Wenzel model, texture makes a surface below 90° wet more and a surface above 90° wet less.
Same estimate, same chemistry: If coupons with different Sdr give the same Wenzel estimate, texture alone explains their angle differences.
Sdr, not Sa, enters the model: The Wenzel ratio comes from area, so Sdr feeds it directly. Two surfaces with the same Sa can have different Sdr.
Very high angles need another model: Above about 150°, air is often trapped under the drop. See the Cassie Baxter model instead of Wenzel.
Expert perspective

Why the same material can wet two ways

Pick the measurement to match the question: the same surface can read one way as a sessile drop and another on a tilted plate, and both can be the right answer.

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

The Dropometer does not measure Sa, Sdr or any ISO 25178 parameter. Measure roughness on your own instrument.
Sdr changes with filters, sampling spacing and instrument, so the Wenzel estimate changes too. Fix them before you compare.
The Wenzel equation can fail. KRUSS saw water angles rise with roughness on hydrophilic silicon (application report AR295). Use an empirical fit.
On rough hydrophilic parts the angle can fall below 10°, outside the Dropometer range of 10° to 175°.
The Dropometer has no temperature or humidity control. Record room conditions and keep them steady.
The camera runs at 10 frames per second. That suits a reading at a fixed time, not millisecond spreading into texture.
FAQ

Frequently asked questions

What does ISO 25178 measure?

ISO 25178 is the international standard that describes 3D surface texture over an area rather than along a single line. Part 2 defines parameters such as Sa, the arithmetical mean height, and Sdr, the extra area texture adds. Other parts cover filters, specification operators and the stylus and optical instruments that measure them.

Can the Dropometer measure ISO 25178 roughness parameters?

No. The Dropometer does not measure Sa, Sdr or any roughness value, so it complements an ISO 25178 workflow. It reads the contact angle on the same surface. Paired with Sdr, that angle shows whether a wetting change comes from texture or from chemistry, which helps explain bonding and coating results.

How does ISO 25178 Sdr relate to the Wenzel roughness ratio?

The Wenzel ratio r is the true surface area divided by the projected area. Sdr is the extra area in percent, so r is about 1 + Sdr/100. A 3.5% Sdr gives r = 1.035 (source: Digital Surf areal parameter guide). The Wenzel equation is then cos θ* = r cos θY. See the Wenzel model.

Should I correct contact angles for roughness with ISO 25178 Sdr?

Use it as a check, not a rule. KRUSS found water angles rose with roughness on hydrophilic silicon, against the Wenzel prediction, and recommends fitting angle against r across several coupons, then extrapolating to r = 1 (source: KRUSS AR295).

Which matters more for wetting, Sa or Sdr?

Sdr enters the Wenzel equation directly, because the model works on area. Sa is a height average and says nothing about slope, so two surfaces with the same Sa can wet differently. Record both, with the filters you used.

What if my textured surface is superhydrophobic?

Above about 150°, air is often trapped under the drop and the Cassie Baxter model applies instead of Wenzel (source: Biolin Scientific). Read advancing and receding angles, or the sliding angle on the Dropometer tilt stage, since one static angle says little on these surfaces.

Legal note

This page summarises ISO 25178 in our own words and does not reproduce the standard. Buy the current editions from ISO before you write a procedure. The Dropometer does not measure surface roughness: it complements an ISO 25178 workflow with contact angle readings. Droplet Lab does not certify compliance. Your own method validation does.

View the official standard Book a 15 minute demo

How this page was created

Sources, review and tools

Transparency 4 checklist items
01

Sources

ISO 25178-2:2021 listing; ISO 25178-3:2012 listing; ANSI webstore preview: ISO 25178-2:2021 contents (Sa 4.2.8, Sdr 4.4.3); Digital Surf: areal field parameters guide (Sdr as percent excess area), 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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We work hard to keep this page accurate and up to date. If you spot an error (wrong revision/year, missing requirement, incorrect interpretation, or broken link), tell us and we'll review it.

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References

1. ISO 25178-2:2021. iso.org/standard/74591.html
2. ISO 25178-3:2012 listing. iso.org/standard/42895.html
3. ANSI webstore preview: ISO 25178-2:2021 contents (Sa 4.2.8, Sdr 4.4.3). webstore.ansi.org/preview-pages/ISO/preview_ISO+25178-2-2021.pdf
4. Digital Surf: areal field parameters guide (Sdr as percent excess area). guide.digitalsurf.com/en/guide-areal-field-parameters.html
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/