Cleanliness and Contamination

Surface Cleanliness Testing and Verification: Evaluate Clean Surfaces with Contact Angle Measurements Alongside the ATP Test

An ATP test shows biological residue. It does not show oil, silicone, cleaning agent or other films that stop a coating, adhesive or seal from working. A water contact angle check on the cleaned surface adds a fast, numeric verification of that second kind of surface cleanliness.

Who this is for: Quality assurance and process teams in medical device, pharmaceutical, biotechnology and food and beverage manufacturing.

Where it fits: After each cleaning step on product contact surfaces, before coating, bonding or assembly, and during cleaning validation.

What it does not do: It does not detect microorganisms or prove a surface is sterile. Keep ATP, cultures and your disinfection checks for that.

Method

Sessile drop water contact angle

Standard

Supports the ASTM F22 water break test

Setup

About 2 minutes

Book a 15 minute demo Request a quote

Gloved hand peeling a test strip from a grey panel surface

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

What the evidence says about ATP testing

r = 0.0018 to 0.29

correlation between ATP readings and microbial colony counts in eight hospital studies, judged no or poor correlation.

ARHAI Scotland, ATP bioluminescence and fluorescent markers literature review, v2.0 (2023)

20% to 78%

sensitivity of ATP bioluminescence across hospital studies, with specificity from 38% to 100%.

ARHAI Scotland, ATP bioluminescence and fluorescent markers literature review, v2.0 (2023)

83.1° to 24.1°

contact angle on titanium before and after plasma cleaning removed surface contaminants, measured with the Dropometer by Harvard researchers.

Ho et al., Biomimetics 10:571 (2025)

Sources: ARHAI Scotland, Literature review and practice recommendations: existing and emerging technologies used for monitoring the effectiveness of decontamination of the health and care environment, ATP bioluminescence and fluorescent markers, version 2.0, 2023; K. Ho, T. Shiba, C.-Y. Chen, D. M. Kim, Biomimetics 10:571, 2025 (a customer publication that used the Dropometer). Figures come from these sources, not from Droplet Lab measurements.

Quick reference

What this surface cleanliness check does and what it does not

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

Evidence Box

Problem this solves

Surfaces that pass visual inspection and an ATP test but still carry oil, silicone or cleaning agent films.

Standards

ASTM F22 is a visual water break test for hydrophobic films, and ASTM F21 an atomizer test. A contact angle reading supports both with a number.

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 case study

A US medical device manufacturer checks catheter surfaces are clean before a hydrophilic coating, and ordered a second Dropometer within seven months.

Customer publication

Harvard researchers used the Dropometer to show contact angle falling as plasma cleaning removed contaminants from titanium (Ho et al., Biomimetics, 2025).

Honest limit

Contact angle shows films that change wetting. It does not detect bacteria or ATP, and it does not prove a surface is sterile.

Who this is for

What surface cleanliness 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.

Medical device or pharma QA

Your cleaning procedures for product contact surfaces need verification beyond a visual check. Contact angle adds a direct reading of chemical residue on each part or batch, alongside your ATP and analytical methods.

Contact angle supports, and does not replace, the methods in your validated cleaning procedure.

Food and beverage hygiene manager

You use ATP for hygiene monitoring, but oily or fatty films can stay after cleaning. A contact angle check on clean metal surfaces shows whether that kind of residue is gone.

Use ATP for biological residue and contact angle for films; they answer different questions.

Process engineer

Surface preparation decides whether a coating or bond holds. Checking the level of cleanliness after the cleaning step shows whether the surface is ready, before an expensive failure.

Set gates from your own clean reference parts, not a value from another line.

Fit check

Is contact angle the right cleanliness test for your surfaces?

It helps when non-biological films are the risk. It helps less when the question is microbial contamination or the surface is rough or porous.

Good fit if

Coatings, adhesives or seals fail on surfaces that looked clean.

Oils, silicone, lubricants or cleaning products may stay after cleaning.

You clean smooth metal, glass or polymer product contact surfaces.

You want a number for quality control of each part, not only a pass from visual inspection.

You need to compare cleaning procedures or cleaning equipment.

Less relevant if

You need to know whether bacteria or another microorganism is present.

Surfaces are porous or rough surfaces such as fabric, wood or grout.

You need to prove a surface is sterile.

Residues are hydrophilic and do not change how water wets the surface.

Summary

How to verify surface cleanliness beyond ATP

The answer in under a minute.

An ATP test detects biological residue, but it says nothing about oils, silicone or cleaning agent films, and its link to microbial counts is weak in hospital studies. Measure water contact angle on the cleaned surface and compare it with a clean reference. A higher contact angle means an organic film is still there: clean again before the next step.

The Dropometer contact angle kit runs the check at the bench or beside the line; our contact angle measurement guide explains the method. The reading adds a number to the ASTM F22 water break test. A US medical device manufacturer uses it to check catheter surfaces before coating, and Harvard researchers used it to track contaminant removal; read our analysis of that paper. Our medical device guide covers the wider context.

Expert Quote

Expert perspective on invisible films

“

Because only the top few ångström decide wetting, an invisible contaminant film, even a monolayer picked up from the air, is enough to change how a liquid behaves on the surface.

The problem

Why visually clean is not the same as clean

Most cleaning verification starts with visual inspection, and many sites add an ATP test. ATP bioluminescence measures adenosine triphosphate from a swab, read on a luminometer in relative light units. It is a fast tool for hygiene monitoring and for quantification of biological residue, and many sites use it to judge the efficacy of cleaning. It has limits. In hospital studies, ATP readings showed little correlation with microbial counts, disinfectant residues can quench the signal, and benchmarks vary from under 100 to under 500 RLU between studies and systems. ATP also cannot detect oil, silicone, lubricant, mold release or cleaning agent films, which carry no ATP. Those are exactly the contaminants that stop coatings, adhesives and seals working on medical device parts, pharmaceutical equipment and product contact surfaces. A surface can pass visual inspection and an ATP test and still fail at the next process step. A water contact angle check measures that kind of surface cleanliness directly.

Coatings or adhesives fail on surfaces that passed ATP.

Water beads on stainless steel or glass after cleaning.

Results change when the cleaning product or supplier changes.

Visual inspection passes but defects appear later.

Cleaning teams disagree on whether a surface is clean.

Troubleshooting

Why surfaces stay contaminated after cleaning

Why:

Surfactants and other cleaning products left after poor rinsing form a film that changes how water wets the surface.

How to detect:

Contact angle differs from the clean baseline across the whole part.

Corrective action:

Check rinse volume, rinse water quality and product concentration, then recheck. Our guide to sample prep and contamination control covers clean handling.

Why:

Process oils, lubricants and silicone carry no ATP, so an ATP test passes while the organic contamination stays.

How to detect:

A higher contact angle than the clean reference, often at contact points or edges.

Corrective action:

Add a degreasing step or solvent, then confirm the angle returns to the baseline.

Why:

Too little contact time, cool solutions or light wiping leave residue in corners and rough areas.

How to detect:

High readings at the same spots on every part.

Corrective action:

Increase contact time or agitation for effective cleaning of those areas, and update the cleaning procedures.

Why:

When you disinfect, some disinfectants leave films after drying, and they can also quench ATP readings, which makes results harder to read.

How to detect:

Contact angle shifts after disinfection compared with after cleaning alone. Our wettability glossary entry explains why a thin film changes the reading.

Corrective action:

Add a rinse after disinfection where your process allows, and set a separate baseline for disinfected surfaces.

Why:

Handling, packaging and air bring films back onto clean surfaces before they are used.

How to detect:

The angle rises with storage time. Our contact angle reference values for common surfaces give a sense check.

Corrective action:

Handle with clean gloves, cover cleaned parts, and set a maximum time before use.

Why:

Worn cleaning equipment, changed cleaning supplies or new cleaning staff change cleaning performance without anyone noticing.

How to detect:

Readings drift by shift, line or team. Our experiment on contact angle on treated and untreated surfaces shows the size of change to expect.

Corrective action:

Trend readings by team and equipment, retrain, and service the equipment.

Not sure whether your surfaces are clean?

Bring a clean reference part and one you suspect to a call, and we will compare them on screen.

Book a 15 minute demo

Quality records

What a surface cleanliness record contains

Each check produces a record you can file with the cleaning batch or part. These records can feed into your existing quality records under ISO 13485, GMP, ISO 9001 or your site's quality system, alongside ATP results.

Surface identity

Part, equipment or area, material and the types of surfaces measured.

Cleaning details

Cleaning step, cleaning products, contact time, operator and time.

Method settings

Probe liquid, drop volume, spots and the reading time after each drop lands.

Readings

Contact angle at each spot, the median and the spread, with any ATP result in relative light units.

Decision

Release, additional cleaning or hold, with the operator and date.

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 to check the cleanliness of a surface

Primary check

Water contact angle on the cleaned, dry surface

Why it matters: Organic films raise the contact angle on clean metal surfaces, glass and many polymers.

How to interpret: Compare with a clean reference of the same material and finish. There is no universal pass value; set your benchmark from your own clean parts and process results.

When it is not enough: It does not detect microorganisms or hydrophilic residues.

Biological residue

ATP test in relative light units

Why it matters: It shows biological residue that contact angle does not.

How to interpret: Use your system's benchmark, validated locally; thresholds differ between systems and studies.

When it is not enough: Correlation with microbial counts is weak, and it misses oils and films.

Uniformity

Spot to spot spread across the surface area

Why it matters: Contamination on surfaces is often patchy, at corners and contact points.

How to interpret: A median inside the gate with one high spot still means part of the surface is not clean.

When it is not enough: Very small features may not hold a drop.

Trend

Readings over time by line and team

Why it matters: Routine monitoring of routine cleaning shows drift in cleaning performance before failures.

How to interpret: Set a risk-based frequency: more checks where the risk of contamination is highest.

When it is not enough: A trend shows a change, not its cause.

Validation

Validated measurement approach for surface cleanliness

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 cleaning

A US medical device manufacturer measured a water drop on a mandrel about 1 mm across during a live demo, bought a Dropometer to check catheter surfaces before coating, and ordered a second unit within seven months.

Read the case study

QC Protocol

How to add contact angle to your cleaning verification

Six steps for one surface type and one surface cleaning process. Repeat for each new material or cleaning procedure.

1

Lock the method

Fix the probe liquid, drop volume, reading time, spots per surface and drying time after cleaning.

2

Baseline clean surfaces

Measure test surfaces you know are clean and that pass your next process step.

3

Set gates from your risks

Use a risk-based approach: measure deliberately soiled references and set release, additional cleaning and hold bands where problems start.

4

Check after each cleaning step

Measure the cleaned, dry surface at the set spots, alongside your ATP test where you use one.

5

Release, clean again or hold

Release surfaces inside the gates. Send others for additional cleaning, and hold repeat failures.

6

Trend and review

Trend readings by line, team and cleaning product, and review the cleaning process when they drift.

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 surface cleanliness verification SOP

A two page SOP you can adapt for your surfaces and cleaning process, with the method lock, gate setting steps, a troubleshooting table and a record sheet.

Contact angle measurement with the Dropometer

Example output

Example cleaning verification log: ATP and contact angle together

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

How to read this log

Example gates: ATP under the site benchmark, and contact angle within 8° of the clean baseline. Each test catches something the other misses. Green rows are released, amber rows get additional cleaning, red rows are held.

Sessile drop contact angle measurement: DI Water on PMMA, left contact angle 58.4°, right 61.5°

Example cleaning verification log

Surface	Soil	ATP (RLU)	Contact angle (°)	Versus baseline	Result
Clean reference	None	12	32.0	Baseline	Reference
Panel 1	Cleaned, rinsed	18	34.5	+2.5	Release
Panel 2	Light machine oil	21	71.8	+39.8	Additional cleaning
Panel 3	Protein soil	640	41.0	+9.0	Hold

Example data, not customer data. Water contact angle on cleaned stainless steel, read at a fixed time after deposition; ATP in relative light units from one luminometer.

Signal order

The order to check signals after cleaning

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

1

Visual inspection

No visible soil, streaks or wet areas.

2

Contact angle versus baseline

Shows whether an oil, silicone or cleaning product film is left.

3

ATP test

Shows whether biological residue is left.

4

Spot to spot spread

Shows whether contamination is patchy.

5

Microbial testing

Cultures or other microbial methods where your risk assessment calls for them.

FAQ

Common questions about surface cleanliness testing

Questions quality, hygiene and process teams ask before adding contact angle.

What are the limitations of ATP testing?

ATP shows biological residue, but a 2023 review by ARHAI Scotland found poor correlation with microbial counts in most hospital studies, sensitivity from 20% to 78%, and no shared benchmark between systems. Disinfectants can quench readings. ATP also cannot detect oils, silicone or cleaning product films, which carry no ATP.

How do contact angle measurements check surface cleanliness?

Clean metal, glass and many polymer surfaces have high surface energy, so water spreads. Organic films lower surface energy and give a higher contact angle. Measuring the angle on the cleaned surface and comparing it with a clean reference shows whether a film is still there. Our contact angle guide explains the method.

Should we use ATP or contact angle?

Use both testing methods where both risks matter. ATP is a tool for hygiene verification of biological contamination; contact angle verifies chemical cleanliness for coating, bonding and product quality. Neither proves a surface is sterile. Choose based on the contamination risk: what would hurt your product or patient most.

Which standards apply?

ASTM F22 is a water break test for hydrophobic surface films, and ASTM F21 is an atomizer test for the same purpose. Both are visual. A contact angle reading supports them with a number you can trend and set gates on.

Does this work for environmental cleaning and commercial cleaning?

For smooth, hard surfaces, yes. Cleaning companies and professional cleaning teams cleaning surfaces such as stainless steel can use it to compare cleaning products and methods on test surfaces. For environmental cleaning in healthcare, where the question is microbial contamination and cross-contamination, ATP, fluorescent markers and cultures remain the main monitoring methods.

Can contact angle prove a medical device surface is clean?

It can show that a medical device surface is free of films that change wetting, which matters before coating or bonding. It is not full proof of cleanliness: it does not detect microorganisms, endotoxin or specific residues, and it does not prove a part is sterile. Use it alongside the methods in your validated cleaning and sterilization process.

What should we look for in an instrument for this check?

It should measure small parts and curved surfaces, give repeatable results across operators, keep drop images and work beside the line. Our guide to the best contact angle goniometer for industrial QA compares options.

Business impact

What changes when you add contact angle to cleaning verification

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

Before and with a contact angle cleanliness check

Metric	Before Dropometer	With Dropometer	Indicative Benchmark
Chemical residue check	Visual inspection only	Contact angle on each part or batch	No published benchmark; track on your own parts
Reliance on ATP alone	ATP pass taken as clean	ATP plus a film check	ATP sensitivity 20% to 78% in hospital studies (ARHAI Scotland, 2023)
Benchmarks	Borrowed thresholds	Set from your own clean references	ATP cut offs ranged from under 100 to under 500 RLU between studies (ARHAI Scotland, 2023)
Cleaning drift	Found after failures	Trended by line and team	No published benchmark; track on your own lines

See the check on your own surfaces

Bring a clean reference part and one you suspect. We will show the difference on a call.

Book a 15 minute demo

Estimate the cost of contamination related rework

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

Instrument cost ($) Each Dropometer unit is $5,000; the default models 1 unit.

Rework events / month Rework events per month that your own records trace to surface contamination.

Avg rework cost per event ($) Your own average cost to clean again, rework or replace one part or batch.

Failures preventable (%) Use your own failure analysis; start low if unsure.

Scrap cost per event ($) Scrap cost per event from surface contamination only, from your own records.

Other monthly savings ($) Other monthly savings you can show from your own records. Set to 0 if unsure.

Result

~0

Monthly savings

~0

Payback period

~0

First year net benefit

Monthly savings = preventable rework cost + preventable scrap cost + other monthly savings.

Limits

What contact angle cannot tell you about cleanliness

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

It does not detect bacteria, viruses or ATP.

It does not prove a surface is sterile or disinfected.

Hydrophilic residues may not change the reading.

Porous or rough surfaces give readings that are hard to compare.

Wet surfaces give wrong readings; measure only dry surfaces.

Gates do not transfer between materials, finishes or cleaning products.

The check flags films that change wetting. Microbial contamination, sterility, specific residues and their levels still need ATP, cultures, analytical methods and your quality process.

Book a 15 minute demo Request a quote

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.

ARHAI Scotland. Literature review and practice recommendations: existing and emerging technologies used for monitoring the effectiveness of decontamination of the health and care environment. ATP bioluminescence and fluorescent markers. Version 2.0, 2023. https://www.nss.nhs.scot/media/4227/atp-and-fluorescent-markers-literature-review-v2.pdf

2.

Ho, K., Shiba, T., Chen, C.-Y., Kim, D. M. Plasma treatment to remove titanium surface contaminants and improve implant biocompatibility: an in vitro study. Biomimetics 10:571, 2025. Customer publication that used the Dropometer; the link goes to our analysis of the paper. https://dropletlab.com/validation/citations/analysis/plasma-treatment-to-remove-titanium-surface-contaminants-and-improve-implant-biocompatibility-an-in-vitro-study/

3.

ASTM F22. Standard test method for hydrophobic surface films by the water break test. https://dropletlab.com/industry-standards/astm-f22/

4.

ASTM F21. Standard test method for hydrophobic surface films by the atomizer test. https://dropletlab.com/industry-standards/astm-f21/

5.

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

6.

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

7.

Droplet Lab validation papers

8.

Customer publications that cite the Dropometer

9.

ASTM F22 explained

10.

Medical device manufacturer case study

Surface Cleanliness Testing and Verification: Evaluate Clean Surfaces with Contact Angle Measurements Alongside the ATP Test

Featured Studies & Publications

What the evidence says about ATP testing

r = 0.0018 to 0.29

20% to 78%

83.1° to 24.1°

What this surface cleanliness check does and what it does not

Evidence Box

What surface cleanliness question are you trying to answer?

Medical device or pharma QA

Food and beverage hygiene manager

Process engineer

Is contact angle the right cleanliness test for your surfaces?

Good fit if

Less relevant if

How to verify surface cleanliness beyond ATP

Expert perspective on invisible films

Why visually clean is not the same as clean

Why surfaces stay contaminated after cleaning

Not sure whether your surfaces are clean?

What a surface cleanliness record contains

Surface identity

Cleaning details

Method settings

Readings

Decision

Drop images

What to measure to check the cleanliness of a surface

Water contact angle on the cleaned, dry surface

ATP test in relative light units

Spot to spot spread across the surface area

Readings over time by line and team

Validated measurement approach for surface cleanliness

How to add contact angle to your cleaning verification

Lock the method

Baseline clean surfaces

Set gates from your risks

Check after each cleaning step

Release, clean again or hold

Trend and review

Download the surface cleanliness verification SOP

Example cleaning verification log: ATP and contact angle together

Example cleaning verification log

The order to check signals after cleaning

Visual inspection

Contact angle versus baseline

ATP test

Spot to spot spread

Microbial testing

Common questions about surface cleanliness testing

What changes when you add contact angle to cleaning verification

Before and with a contact angle cleanliness check

See the check on your own surfaces

Estimate the cost of contamination related rework

Estimate avoided scrap and rework from undetected contamination.

What contact angle cannot tell you about cleanliness

Standards, evidence and related cleanliness workflows

ASTM F22

ASTM F21

Medical device manufacturer

Ho et al. (2025): our analysis

Dropometer

Cleanliness and contamination

Hydrophilic coating validation

Silicone and invisible residue detection

How this page was created

Drafting assistance

Technical review

Verification steps

Updates

Report a correction

References