Contents
Hydrophobicity and Self Cleaning

Windshield Rain Repellent Testing: Rank Each Rain Repellent Treatment and Windshield Coating by How Water Beads, Rolls Off and Lasts on Car Windshield Glass

A rain repellent coating that beads water on day one can stop clearing the windshield after weeks of wipers, washer fluid and sun. Measure water contact angle and sliding angle on coated glass, before and after durability cycles, to find the best formulation and release each batch with numbers.

Who this is for: R&D formulators, automotive glass makers and QC teams who develop or produce rain repellent products and glass coatings.

Where it fits: Formulation screening, batch release, and durability testing after wiper, washer fluid, UV and cold cycles.

What it does not do: It does not replace road testing in heavy rain, wiper chatter checks or optical haze measurement.

Method
Sessile drop contact angle and tilt stage sliding angle
Standard
Partially compliant with ISO 19403-7, roll off up to 60° tilt
Setup
About 2 minutes
Book a 15 minute demo Request a quote
Car windshield in rain split down the middle: water smeared across the untreated side, water beads and a clearer road view on the treated side
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
Read More
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.
Read More
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
Why it matters

Why rain repellent performance matters for driving safety

574,047

crashes a year on US roads happen in rain or mist, about 10% of all crashes, with 2,810 deaths a year.

FHWA Road Weather Management Program, NHTSA data, 2019 to 2023 annual averages

Almost 34%

smaller minimum visual angle that drivers could resolve through a newly applied hydrophobic windshield coating, with response time more than one second faster.

Sayer et al., UMTRI-97-31, University of Michigan Transportation Research Institute (1997)

99° to 91°

fall in water contact angle on a cold snow ski wax between room temperature and 2 °C, measured with the Dropometer. Temperature changes repellency.

Kramer, RANGE: Journal of Undergraduate Research (2024), University of Utah

Sources: US Federal Highway Administration, Road Weather Management Program, How do weather events impact roads (NHTSA data, 2019 to 2023 averages); J. R. Sayer, M. L. Mefford, M. J. Flannagan, M. Sivak, S. Kojima, The influence of hydrophobic windshield coating on driver visual performance, UMTRI-97-31, 1997; Kramer, RANGE: Journal of Undergraduate Research, 2024, Fig. 6 (a customer publication that used the Dropometer). Figures come from these sources, not from Droplet Lab measurements.

Quick reference

What this rain repellent check does and what it does not

A quick reference for formulation and quality teams checking fit before reading further.

Evidence Box

Problem this solves

Rain repellent coatings that bead water when new but lose water repellency after wiper, washer fluid and sun exposure.

Standards

ISO 19403-7 covers roll off on a tilt stage; the Dropometer is partially compliant, up to 60° tilt. ISO 19403-6 covers dynamic contact angle.

Peer reviewed method

The Dropometer drop shape method was validated against a KRUSS DSA100E in two peer reviewed papers, in Review of Scientific Instruments and Colloids and Surfaces A.

Customer evidence

University of Utah researchers used the Dropometer to show commercial ski glide waxes were less water repellent at 2 °C than at room temperature.

Visibility evidence

A 1997 University of Michigan study found a new hydrophobic windshield coating improved driver visual performance; the authors expected the benefit to fall with wear.

Honest limit

Lab readings rank coatings and catch drift. Road, wiper and weather testing still decide final performance.

Who this is for

What rain repellent question are you trying to answer?

Pick the card closest to your role. Each one points to the part of this page that answers it.

R&D formulator

You need to find the best rain repellent formulation without a road test for every change. Contact angle and sliding angle on coated coupons rank candidates and show which ones keep repelling rain after durability cycles.

Rank against a reference coating tested the same day, with the same drop volume.

QC manager

Customers complain that a batch stopped working early. A contact angle and sliding angle gate on every batch catches weak cure, thin application or a changed raw material before the product ships.

Set gates from your own reference batches and field results, not a value from another product.

Automotive glass engineer

You apply a glass water repellent on the line or qualify a supplier's glass treatment. Measuring coated glass from each lot shows whether the coating is applied to the glass evenly and bonded well.

Measure flat coupons of the production glass; a full curved windshield does not fit the tilt stage.
Fit check

Is this the right test for your windshield water repellent?

It helps when you need numbers to compare coatings and track wear. It helps less when the question is optics, wiper noise or full vehicle behaviour.

Good fit if

You compare rain repellent products, ceramic coating options or glass coating formulations.
You need a release check for each batch of coating or coated glass.
Water repellency falls too fast in the field and you need to know why.
You test durability after wiper, washer fluid, UV or cold cycles.
Drops bead but cling to the glass, and you need to measure roll off.

Less relevant if

You need optical haze, distortion or glare measurements.
You need wiper chatter or noise testing.
You can only test a full curved windshield, not coupons.
Drops on your coating do not move by 60° tilt and you need the exact angle above that.
Summary

How to test a rain repellent coating

The answer in under a minute.

A rain repellent works when rain beads and rolls off the windshield before you need to use the wipers. Measure water contact angle for how strongly the coating repels water, and sliding angle for how easily drops roll off. Test fresh coupons and again after wiper, washer fluid and UV cycles, then release only batches inside your gates.

The Dropometer contact angle kit with its tilt stage measures both on flat glass; our sliding angle measurement guide and contact angle measurement guide explain the methods. The roll off test follows ISO 19403-7, where the Dropometer is partially compliant up to 60° tilt. University of Utah researchers used it to show water repellency falling in the cold; read our analysis of that paper. Our automotive guide and transportation guide cover the wider context.

Expert Quote

Expert perspective on repellency and roll off

“

Repellency and mobility are not the same thing: a high contact angle says a drop is repelled, but whether it rolls off is a separate question; the two can even be mutually exclusive.

Dr. Alidad Amirfazli

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

The problem

Why rain repellent products lose water repellency on the windshield

A hydrophobic rain repellent treatment makes water droplets bead up and roll off the glass, which improves driving visibility in wet weather. At highway speed, airflow pushes drops across the windshield, so drivers need to use the wipers less. In a University of Michigan study, acuity at night through a newly treated windshield came close to daytime acuity through an untreated one. Performance does not stay the same. Every pass of the windshield wiper abrades the coating, washer fluid and glass cleaner contain solvents and surfactants, and direct sunlight, heat and cold all act on the film. A coating applied to dirty glass, or not given time to cure, may not stick to the glass at all. Hard water, water spots and road film leave a contaminant layer that changes how water behaves. The result is a coating that beads water on a clean windshield in the showroom, while wear causes water to spread or cling a few weeks later. Spray tests and visual water beading checks cannot rank formulations or catch this drift with numbers. Contact angle and sliding angle can.

Water stops beading after a few weeks of wiper use.
Drops bead but cling to the glass instead of rolling off.
Patchy or streaky water across the windshield after application.
Performance varies between batches or glass suppliers.
Worse results in cold weather conditions or heavy rain.
Drivers report smeared water and glare at night in the rain.
Troubleshooting

Why a rain repellent coating fails to repel water

Why:

  • Road film, glass cleaner residue, hard water deposits and other contaminants stop the repellent coating bonding to the glass surface.

How to detect:

  • Bare glass after cleaning reads well above its normal low angle, and the coated glass reads low and patchy.

Corrective action:

  • Clean the glass with a fixed method, check bare glass first, then coat. Measure bare glass each time so you can tell glass problems from coating problems.

Why:

  • Too little product, missed areas or heavy buffing leave a thin or broken film.

How to detect:

  • High spot to spot spread in contact angle and sliding angle across one coupon.

Corrective action:

  • Fix the amount, pattern and buffing steps, and train operators. Note whether the formula is easy to apply evenly, and raise it with R&D if not.

Why:

  • Low temperature, high humidity or too short a time before rain or washing leave the film weak.

How to detect:

  • Readings start fine but fall sharply after the first wiper or wash cycles.

Corrective action:

  • Lock cure time and temperature, and test durability only on fully cured coupons.

Why:

  • Wiper blades abrade the film, and windshield washer fluid solvents and surfactants attack it.

How to detect:

Contact angle drops and sliding angle rises with each durability cycle. For reference data, see our client wetting measurements dataset.

Corrective action:

  • Compare binders and primers for wear and water resistance on the same wiper and washer fluid cycles, and set a retention gate.

Why:

A high contact angle with high contact angle hysteresis means drops bead but stay put, so the windshield does not clear.

How to detect:

  • Contact angle is high but sliding angle is high too, or drops do not move by 60°.

Corrective action:

  • Adjust the formulation or topcoat for lower hysteresis, and rank candidates on sliding angle as well as contact angle.

Why:

Some hydrophobic films repel water less in the cold. University of Utah researchers measured this on ski glide waxes.

How to detect:

  • Contact angle falls and sliding angle rises when coupons are tested cold.

Corrective action:

  • Add a cold test to development and release checks for products sold for winter.

Not sure why your coating stops working?

Send coated glass from a good batch and a weak one, and we will compare them on a call.

Book a 15 minute demo
Quality records

What a rain repellent test record contains

Each test produces a record you can file with the batch or formulation. These records can feed into your existing quality records under IATF 16949, ISO 9001 or your own development process.

Coating and batch

Product, formulation, batch or lot, and the reference coating used.

Glass and preparation

Glass type, supplier, how the glass was cleaned, application method and cure.

Method settings

Drop volumes, reading time, tilt rate, spots per coupon and test temperature.

Readings

Contact angle median and spread, sliding angle, and any drop that did not slide by 60°.

Durability history

Wiper, washer fluid, UV and cold cycles completed, with readings after each one.

Drop images

Automatic edge and baseline detection gives the same reading from the same image, so a reviewer can recheck any result.

Measurements

What to measure on a windshield rain repellent coating

Repellency

Static water contact angle

Why it matters: It shows how strongly the coating repels water compared with bare glass.

How to interpret: Compare with your reference coating and with clean bare glass, measured the same day. There is no universal pass value; set gates from your own references and road results.

When it is not enough: It does not show whether drops roll off.

Roll off

Sliding angle on a tilt stage

Why it matters: It shows how easily drops start to move, which is what clears the glass.

How to interpret: Lower is easier roll off. Compare only at the same drop volume and tilt rate, and record no movement by 60° as 60° or more.

When it is not enough: Drop volume and dirt change the result, so lock both.

Pinning

Advancing and receding angles at the onset of motion

Why it matters: Their difference, the hysteresis, explains why some drops bead but do not move.

How to interpret: Lower hysteresis usually means drops slide at a lower tilt.

When it is not enough: On some surfaces the onset of motion is hard to capture reliably.

Durability

Retention after durability cycles

Why it matters: It shows how long the coating keeps working under wiper, washer fluid, UV and cold.

How to interpret: Track contact angle and sliding angle against cycles, and set retention gates from field results.

When it is not enough: Lab cycles only approximate real wear; confirm with road testing.

Uniformity

Spot to spot spread across a coupon

Why it matters: Patchy application gives streaks and uneven clearing across the windshield.

How to interpret: A good median with one weak spot still means part of the glass is poorly coated.

When it is not enough: Coupons may not show defects at the edges of a real windshield.

Validation

Validated measurement approach for rain repellent coatings

How the Dropometer itself has been validated, and where customers have used it.

Peer reviewed method

The Dropometer drop shape method is published in Review of Scientific Instruments and Colloids and Surfaces A. Contact angle was validated against a KRUSS DSA100E, with a published accuracy of 0.35°.

See the validation papers

Customer evidence on water repellency

University of Utah researchers measured four commercial low fluorinated ski glide waxes with the Dropometer at 2 °C and at room temperature. The two waxes sold for the coldest snow were less water repellent in the cold: about 91° versus 99°, and 93° versus 98°.

Read the case study
QC Protocol

How to test windshield water repellent products

Six steps for one coating and one glass type. Repeat for each new formulation, glass supplier or durability cycle.

2

Baseline bare glass and a reference

Clean the glass, measure it bare, then measure your reference coating. These anchor every later reading.

4

Run durability cycles

Apply wiper, washer fluid, UV and cold steps, rinse, dry and measure again after each set.

5

Set gates against road results

Compare readings with road or rig testing and set release, retest and reject bands.

6

Release each batch and trend

Measure every batch, release inside the gates, and trend readings by lot and glass supplier.

We completed our gage R&R study on the unit and it performed very well.

Brandon Barbee

Corporate Quality Engineer, Zeus Industries, Polymer Manufacturing

Download the rain repellent performance SOP

A two page SOP you can adapt for your coating and glass, with the method lock, gate setting steps, durability cycles, a troubleshooting table and a record sheet.

Sliding angle measurement with the Dropometer

Example output

Example rain repellent durability log

Example data only. Your values and gates will differ and must come from your own references.

How to read this log

Example gates after the durability set: contact angle at or above 100° and sliding angle at or below 25°. Formula B still beads water but drops cling, so it is retested. Formula C has lost most of its water repellency. Green rows are released, amber rows retested, red rows rejected.

Sessile drop contact angle measurement: DI Water on Glass, left contact angle 44.9°, right 45.7°

Example rain repellent durability log

Sample Wiper cycles Contact angle (°) Sliding angle (°) Versus gate Result
Clean bare glass 0 24.5 No slide by 60 Baseline Reference
Reference coating 0 104.8 19 Reference Reference
Formula A 0 107.6 15 Inside Release
Formula A 1,000 102.3 21 Inside Release
Formula B 1,000 99.1 34 Sliding angle outside Retest
Formula C 1,000 84.0 No slide by 60 Both outside Reject

Example data, not customer data. Water contact angle and sliding angle on flat coated glass coupons at room temperature, fixed drop volumes, 1,000 cycles on a wiper rig.

Signal order

The order to check signals on coated glass

Check these in order. Each one rules out a problem before you look at the next.

1

Visual check

No haze, streaks or missed areas on the coated glass.

2

Contact angle versus reference

Shows whether the coating repels water as strongly as it should.

3

Sliding angle

Shows whether drops roll off or stay pinned.

4

Spot to spot spread

Shows whether the coating is even across the glass.

5

Retention after cycles

Shows whether the coating lasts under wiper, washer fluid, UV and cold.

6

Road or rig test

Confirms visibility in real rain before a new formulation launches.

FAQ

Common questions about rain repellent testing

Questions formulators and quality teams ask before adding contact angle and sliding angle.

What is the best way to test a windshield rain repellent?

Measure water contact angle and sliding angle on flat coated glass, fresh and after wiper, washer fluid, UV and cold cycles, against a reference coating. Then confirm the best candidates on the road. ISO 19403-7 describes the roll off test on a tilt stage, and ISO 19403-6 the dynamic contact angle.

Why does water bead on the windshield but not roll off?

A bead shows the coating repels water, but roll off depends on how much the drop's edge is pinned. If the advancing and receding angles differ a lot, the drop sticks until the tilt or airflow is strong enough; Furmidge described this link in 1962. Measure sliding angle as well as contact angle.

Does a higher contact angle mean a better rain repellent?

Not always. Two coatings with the same contact angle can have very different sliding angles, so one clears the glass and the other leaves drops in place. Rank on both, and on how well each one holds up after durability cycles.

How long does a rain repellent last, and when should drivers reapply it?

There is no single answer: it depends on the coating, wiper use, washer fluid, sun and weather conditions. The 1997 University of Michigan study expected the benefit of a new coating to fall with wear. Measuring retention after durability cycles lets you give a reapply interval backed by your own data.

Can we test on a curved car windshield?

Measure flat coupons of the production glass, coated and cured the same way. Contact angle can be read on gently curved areas, but the tilt stage needs a flat coupon. Coupons also let you run durability cycles in a controlled way.

Can this compare a ceramic coating, a sealant and a rain repellent?

Yes. Any glass coating or glass treatment can be measured with the same method, so a ceramic coating, a sealant and a rain repellent coating can be ranked side by side. Our hydrophobic performance verification page covers other hydrophobic coatings.

What proof do rain repellent claims need?

Claims such as improving visibility during rainy drives, ease of cleaning, simplifying the cleaning process or helping keep your windshield cleaner should rest on data. Contact angle and sliding angle show whether water beads up and rolls off after wear. Road testing then confirms visibility and driving safety in wet conditions.

Which instrument do we need?

You need sessile drop contact angle and a tilt stage for sliding angle, with repeatable drop volumes. Our measurement method selector helps choose the method, and our guide to the best contact angle goniometer for industrial QA compares options.

Business impact

What changes when you test water repellent products with numbers

Typical changes in how teams work. Benchmarks are given only where a published source exists.

Before and with contact angle and sliding angle testing

Metric Before Dropometer With Dropometer Indicative Benchmark
Formulation ranking Spray and look at water beading Contact angle and sliding angle on every candidate No published benchmark; track on your own formulations
Durability Found from customer complaints Retention measured after wiper and washer fluid cycles Benefit of a new hydrophobic coating expected to fall with wear (UMTRI, 1997)
Visibility case for the product Claims without lab data Lab data plus road testing Minimum visual angle almost 34% smaller with a new hydrophobic coating (UMTRI, 1997)
Batch release Visual check only Contact angle and sliding angle gates No published benchmark; track on your own batches

See the test on your own coating

Send coated glass from two formulations or batches. We will show the difference on a call.

Book a 15 minute demo

Estimate saved development iterations

Enter your own numbers. The result is an estimate, not a Droplet Lab claim.

Rain repellent R&D cost estimate

Estimate saved iterations and lab cost.

Each Dropometer unit is $5,000; the default models 1 unit.
Your own coating and glass cost per test batch.
Your own hours to apply, cure, measure and analyse one iteration.
No published benchmark. Use your own estimate and start low if unsure.

Result

~0
Iterations saved / month
~0
Monthly savings
~0
Payback period
~0
First year net benefit

Monthly savings = materials saved + technician time saved from reduced iterations.

Limits

What lab readings cannot tell you about a rain repellent

Honest limits, so you know when to reach for another test.

Sliding angle changes with drop volume; never compare different volumes.
Dirt or residue on the glass changes both readings; clean the glass the same way every time.
The tilt stage reads up to 60°; drops that do not move are recorded as 60° or more.
Lab tests do not reproduce airflow at speed or wiper chatter.
Full curved windshields need flat coupons for sliding angle.
One test temperature can hide poor cold weather performance.

Contact angle and sliding angle rank coatings and catch drift. Visibility at speed, wiper chatter, optical haze and long term weathering still need road, rig and optical testing.

Book a 15 minute demo Request a quote
Related

Standards, evidence and related water repellency workflows

These pages cover the standards, the customer evidence and the other hydrophobic coating problems behind this test.

Standard

ISO 19403-7

Roll off angle on a tilt stage for coated surfaces.

View standard

Standard

ISO 19403-6

Dynamic contact angle for paints and coatings.

View standard

Case study

University of Utah

Ski wax water repellency at 2 °C and room temperature.

Read case study

Published paper

Kramer (2024): our analysis

Low fluorinated ski glide waxes measured with the Dropometer.

Read our analysis

Instrument

Dropometer

Contact angle and sliding angle with the tilt stage.

View product

Related use case

Hydrophobicity and self cleaning

All hydrophobic and anti soiling use cases in one place.

View Page

Related use case

Ski wax glide testing

Hydrophobicity checks on ski glide waxes.

View Page

Related use case

Self cleaning and anti soiling

Validation of self cleaning coatings, including solar panels.

View Page

How this page was created

Editorial and technical transparency notes for this page.

Transparency Details 4 checklist items
01

Drafting assistance

Drafted with Claude Opus 5.5 (Anthropic) using web search for sources, then edited by the Droplet Lab team.

02

Technical review

Reviewed and edited for technical accuracy by the Droplet Lab Team.

03

Verification steps

Standard identifiers, units, thresholds and key procedural claims are checked against cited sources before publication.

04

Updates

Reviewed every 12 months or when the underlying method changes.

Report a correction

Spotted an issue in this summary? Send a correction request and our team will review it.

Correction Request

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.

Contact us to report a correction
Sources

References

1.
US Federal Highway Administration, Road Weather Management Program. How do weather events impact roads? Weather related crash statistics, NHTSA data, 2019 to 2023 averages. https://ops.fhwa.dot.gov/weather/roadimpact.htm
2.
Sayer, J. R., Mefford, M. L., Flannagan, M. J., Sivak, M., Kojima, S. The influence of hydrophobic windshield coating on driver visual performance. UMTRI-97-31, University of Michigan Transportation Research Institute, 1997. https://deepblue.lib.umich.edu/items/9f124bf2-c43b-465f-bd92-841317949666/full
3.
Furmidge, C. G. L. Studies at phase interfaces. I. The sliding of liquid drops on solid surfaces and a theory for spray retention. Journal of Colloid Science 17:309, 1962. https://doi.org/10.1016/0095-8522(62)90011-9
4.
Kramer. Temperature dependent performance analysis of marketed low fluorinated Nordic ski glide waxes. RANGE: Journal of Undergraduate Research, University of Utah, 2024. Customer publication that used the Dropometer; the link goes to our analysis of the paper. https://dropletlab.com/validation/citations/analysis/temperature-dependent-performance-analysis-of-marketed-low-fluorinated-nordic-ski-glide-waxe/
5.
ISO 19403-7:2024. Paints and varnishes. Wettability. Part 7: Measurement of the contact angle on a tilt stage (roll off angle). https://dropletlab.com/industry-standards/iso-19403-7/
6.

ISO 19403-6. Paints and varnishes. Wettability. Part 6: Measurement of dynamic contact angle.

https://dropletlab.com/industry-standards/iso-19403-6/
7.

Chen, X. et al. Contact angle measurement with a smartphone. Review of Scientific Instruments 89(3):035117, 2018.

https://pubs.aip.org/aip/rsi/article-abstract/89/3/035117/368179/Contact-angle-measurement-with-a-smartphone
8.

Surface tension measurement with a smartphone using a pendant drop. Colloids and Surfaces A: Physicochemical and Engineering Aspects.

https://www.sciencedirect.com/science/article/abs/pii/S0927775717307744
9.
Droplet Lab validation papers
10.
Customer publications that cite the Dropometer
11.
ISO 19403-7 explained
12.
University of Utah case study
13.
Sliding angle measurement guide