10 Essential Steps for Achieving Reproducible Contact Angle Measurements (2025)

Contact angle measurement is a cornerstone technique in surface science, providing insights into wettability, adhesion, and surface energy. However, achieving reproducible measurements is critical for meaningful interpretation and application.

Importance of Reproducibility

Reproducibility ensures reliability across different laboratories and experimental conditions. Challenges in reproducibility often arise from variations in surface preparation, droplet size, and environmental factors. Non-reproducible data can lead to incorrect conclusions about a material’s surface properties and undermine the credibility of the research.

Applications of Reproducible Contact Angle Measurements

Material Selection

Contact angle measurements guide material selection for applications requiring specific wettability characteristics, such as waterproof coatings or biomedical implants.

Surface Treatment Evaluation

Evaluate the effectiveness of surface treatments like plasma cleaning, chemical etching, or coating deposition by comparing pre- and post-treatment contact angles.

Adhesion Testing

Explore practical applications of superhydrophobic materials, particularly using a superhydrophobic aluminium mesh to observe and understand the behaviour of oil and water droplets, including the unique phenomenon of mixing these liquids on the coated surface.

To have an in-depth understanding of Contact-Angle measurements and their applications please refer to our Contact Angle Measurement Definitive Guide. Here, we outline 10 essential steps to help you achieve reproducible contact angle measurements, incorporating findings from literature and practical insights.

Step #1. Understand Contact Angle and its Types

Contact angle is the angle formed at the intersection of the liquid, solid, and vapor phases. It is determined by the balance of interfacial tensions and provides a quantitative measure of surface wettability. The most common methods include:

Sessile Drop Technique: A droplet is placed on a solid surface, and the contact angle is measured.
Dynamic Measurements: Advancing and receding angles are measured by adding or removing liquid from a droplet

Contact angles are classified into:

Static Contact Angle

Static Contact Angle: Measured when the droplet is stationary. Static contact angle measurements are a starting point for surface characterization. They involve placing a liquid droplet on the surface and capturing its profile. However, these measurements can be influenced by the droplet size, deposition method, and substrate preparation:

Droplet Size: Use droplets with diameters >5 mm to minimize edge effects and variability.
Deposition Method: Employ consistent techniques to ensure uniform droplet shapes and volumes.

Advancing Contact Angle

Determined by slowly adding liquid to the droplet as the liquid front advances over a dry surface.

Advancing Contact Angle

Receding Contact Angle

Measured by gradually removing liquid as the liquid front recedes from a wetted surface.

Contact Angle Hysteresis

Contact angle hysteresis is the difference between advancing and receding angles. It can indicate surface roughness or chemical heterogeneity.

Learn how you can perform Advancing and Receding contact Angle measurements:

Step #2. Prepare the Solid Surface Meticulously

Surface preparation significantly affects contact angle reproducibility. Surface imperfections, such as roughness or chemical heterogeneity, can cause significant contact angle hysteresis. Employ larger droplets to average out local variations. For rough surfaces, consider surface modification techniques to achieve smoother profiles.

Follow these guidelines:

Cleaning: Use solvents, de-ionized water, or UV-ozone treatment to remove organic and inorganic contaminants. Ensure cleaning steps are consistent across samples.
Polishing: Achieve a smooth surface with minimal roughness to reduce variability. Use techniques like fine grinding or polishing with diamond abrasives for metals, or specific cleaning agents for polymers.
Storage: Avoid contamination by storing surfaces in clean, dust-free environments. Samples should be measured immediately after preparation whenever possible.
Rough/Porous Surfaces: Either apply a smoothing pre-coat or switch to the captive-bubble or tilt-plate method if absorption is unavoidable.

Step #3. Ensure Environmental Stability

Environmental factors like temperature, humidity, and vibrations can significantly impact measurements. Place the setup in an environment free from air currents, temperature fluctuations, and contaminant exposure. A closed chamber or hood can provide additional isolation, especially for sensitive experiments.

Some best practices include:

Temperature: Maintain a stable ambient temperature to avoid fluctuations in liquid properties.
Humidity: Keep relative humidity consistent to prevent changes in surface hydration.
Lighting: Use fixed LED ring light (e.g. built-in Godox® light on Droplet Lab Flagship) to prevent glare changes
Vibrations: Vibrations can cause the contact line to move over energy barriers, impacting the advancing and receding contact angles. This can lead to reduced contact angle hysteresis under vibrational environments. Even ambient vibrations can relax contact angles toward equilibrium, depending on the system’s metastable states. Use vibration isolation tables to minimize disruptions during droplet deposition and analysis.

Step #4. Calibrate Your Equipment

Accurate calibration ensures reliable data. Regular calibration of contact angle instruments is essential for accuracy. Calibration can be performed using standard reference materials with known contact angles. Advanced setups may also include self-check mechanisms to validate system performance. Regularly verify:

Syringe and needle cleanliness: Residues can alter droplet volume and shape.
Camera and optics alignment: Ensure sharp and distortion-free imaging.
Measurement software: Validate algorithms using known standards

Step #5. Choose the Right Liquid

Selecting a liquid with well-characterized properties enhances reproducibility:

Surface tension consistency: Verify the liquid’s surface tension before use.
Compatibility: Avoid reactions between the liquid and the substrate.
Examples: Common liquids include water, diiodomethane, and glycerol.

Step #6. Standardise Droplet Deposition

During droplet deposition, ensure smooth and controlled delivery to minimize oscillations or shape distortions. A syringe with a hydrophobic needle tip prevents liquid climbing on the needle surface, reducing inconsistencies.

Proper droplet size and placement are crucial:

Drop Size: Use droplets with diameters >5 mm (~4–6 µL for water) to minimize edge effects and variability. Maintain diameters of 5-7 mm for better reproducibility.
Placement Technique: Employ consistent techniques to ensure uniform droplet shapes and volumes. Avoid distortion by depositing droplets slowly and carefully.

Step #7. Use Advanced Measurement Techniques

Manual measurements are prone to operator bias. If manual methods are used, thorough training and adherence to protocols are essential.

Modern contact angle instruments employ image analysis to calculate angles by fitting the droplet profile to the Young-Laplace equation. This method reduces human error and enhances precision. Ensure software settings are optimized for droplet size, lens magnification, and lighting conditions. Automated systems with image analysis software can enhance accuracy and consistency.

Leverage modern tools for precision:

Sessile Drop Method: A standard approach using image analysis.
Dynamic Methods: Measure advancing and receding angles to assess hysteresis.
Surface Analyst Tools: Devices with automated image analysis minimize operator bias.

Step #8. Repeat Measurements for Reliability

Perform measurements at multiple points to account for surface variability:

Sampling Frequency: Test at least 3-5 locations on the substrate.
Statistical Analysis: Use averages and standard deviations to interpret results.

Gage Repeatability and Reproducibility (GR&R)

Conduct GR&R studies to evaluate the precision and reliability of both the measurement instrument and the operator. GR&R studies involve:

Multiple operators performing measurements on identical samples.
Repeated trials to assess intra-operator and inter-operator variability.
Statistical analysis to determine the percentage of variation attributable to equipment, operator, or sample.

Step #9. Document the Experimental Setup

Transparency in reporting enables reproducibility by others:

Detailed Protocols: Include information on surface preparation, liquid properties, and environmental conditions.

Example Workflow

Step	Description
1	Prepare substrate: Clean, polish, and store in a controlled environment.
2	Calibrate the instrument using a standard reference.
3	Deposit a droplet using a clean syringe and measure the static contact angle.
4	Add liquid to measure the advancing angle.
5	Remove liquid to measure the receding angle.
6	Repeat measurements at multiple locations on the sample for statistical validation.

Photographs or Diagrams: Show the experimental setup to eliminate ambiguity

Step #10. Account for Contact Angle Hysteresis

Contact angle hysteresis can provide insights into material properties:

Identify Causes: Hysteresis may result from surface roughness or contamination.
Quantify Effects: Use hysteresis measurements to infer surface heterogeneity.

Tools for Reproducible Contact Angle Measurements

Instrument

Consider investing in Droplet Lab’s Smartphone-based Goniometer that offers precise drop generation using Hamilton Syringes in both manual and Automatic models.

Software

Image Analysis Algorithms: Utilize software like Young-Laplace fitting to analyze droplet profiles accurately.

Droplet Lab’s Goniometer uses Young-Laplace equation and integrates Custom-Built machine learning model trained on 24,000+ images offers consistent, effortless measurements.

Accessories

Controlled Humidity Chambers: For ultra-precise measurements, advanced environmental chambers can control temperature, humidity, and atmospheric composition. These chambers are especially useful for sensitive materials prone to rapid oxidation or contamination.

High-Purity Liquids: Ensure consistency in droplet properties.

Advanced Considerations in Contact Angle Measurement

1. Macroscopic vs. Microscopic Contact Angles

Macroscopic Contact Angles: Defined as a boundary condition for stress equations governing liquid-vapor interfaces. They are essential for predicting the shapes of fluid bodies and depend on precise measurement techniques.

Microscopic Contact Angles: Derived from molecular forces at the contact line and linked to interfacial tensions through the Young-Dupre equation.

2. Heterogeneous Surfaces

Natural surfaces often have spatial variations in surface chemistry and roughness, leading to distorted contact lines and contact angle hysteresis.
The interplay between surface heterogeneity and contact line distortion significantly impacts the measurement process.

3. Length Scales Relevant to Contact Angles

The Decker paper explores how thermal and vibrational fluctuations affect the contact line over microscopic and macroscopic scales.
Length scales near the contact line are critical for defining the Young-Dupre force balance and understanding surface wettability barriers.

4. Metastability and Pinning Effects

Metastable states caused by surface defects pin the contact line, resulting in hysteresis.
The cooperative nature of pinning on heterogeneous surfaces creates complex contact line distortions that current theoretical models struggle to quantify.

5. Impact of Surface Heterogeneity on Measurement Techniques

Distorted liquid-vapor interfaces due to surface heterogeneity require sophisticated measurement techniques to ensure accurate contact angle readings.
Averaging macroscopic angles over heterogeneous surfaces often lacks a quantitative framework for understanding chemical and physical surface properties.

Conclusion

Reproducible contact angle measurements are essential for reliable surface characterization. Achieving reproducible contact angle measurements requires a combination of meticulous preparation, precise techniques, and advanced tools. By following these 10 steps, researchers and professionals can ensure reliable, accurate, and meaningful results, contributing to advancements in surface science and material characterization.

Leveraging advanced techniques such as image analysis and GR&R studies further enhances confidence in results. With careful attention to detail, contact angle measurements can provide valuable insights into material properties, aiding in innovation across various industries.

For more insights and guides, visit our Surface Science Knowledge Hub.

References

1. Drelich, J. (2013). Guidelines to measurements of reproducible contact angles using a sessile-drop technique. Surface Innovations, 1(SI4), 248-254. DOI: 10.1680/si.13.00010.
2. Brighton Science. (2022). Gage Repeatability and Reproducibility of the Surface Analyst. Available at: Brighton Science Website.
3. Decker, E. L., et al. (1999). Physics of contact angle measurement. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 156,
177-189. DOI: 10.1016/S0927-7757(99)00069-2.