When it’s raining, have you ever noticed how water forms beads on a freshly waxed car, while it spreads out on a car that hasn’t been waxed in a long time? This simple observation is a great example of surface wettability, which can be quantitatively measured by something called the contact angle.
The contact angle is the angle formed at the interface where a liquid droplet meets a solid surface. If the contact angle is low, it means the liquid spreads easily, indicating high wettability and strong adhesion to the solid. Conversely, a high contact angle means the droplet retains its shape, pointing to low wettability and weak adhesion. Contact angles can range from 0° (complete wetting) up to 180° (no wetting at all). When the contact angle with water is lower than 90 degrees, the surface is hydrophilic. On the other hand, when the contact angle is over 90 degrees, the surface is called hydrophobic. The surrounding phase in contact angle measurements is usually a gas (like air), but can also be another immiscible liquid.
What determines the contact angle between a droplet and a surface? At the interface of solid, liquid, and gas, several forces come into play. These include:
Cohesion: The attraction between like molecules (e.g., water to water).
Adhesion: The attraction between unlike molecules (e.g., water to glass).
The balance of these forces is described by the thermodynamic definition of contact angle, known as the Young equation. This equation incorporates the surface tensions at the interfaces between solid, liquid, and gas, and helps define why a particular angle forms. In real-world measurements, factors such as surface roughness and chemical heterogeneity also affect the observed contact angle.
Contact angle measurements are critical in various industries and research fields. They help evaluate surface cleanliness, coating performance, and adhesion properties, and even play a key role in developing new materials and formulations. For instance, in the electronics industry, understanding and optimizing surface wettability is crucial for processes like photoresist adhesion and wafer cleaning.
Explore More: Related Topics on Our Blog
• Optimization of Photoresist Adhesion through Contact Angle Measurement
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• Wafer Cleaning Process – RCA Cleaning and Contact Angle
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• The Role of Contact Angle in Superhydrophilic Antifog Surfaces
Explore how contact angle measurements are used to evaluate and develop antifog coatings.
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Modern tensiometers, like the Attension Theta or Sigma series, provide automated and accurate measurement of contact angles. Depending on the application, you might use static or advancing/receding contact angle measurements. Special techniques are also available for challenging surfaces, such as fibers or superhydrophilic coatings.
Contact angle is a foundational concept in surface science, offering insight into wettability, adhesion, and surface energy. Whether you’re developing new materials, ensuring quality in manufacturing, or researching biointerfaces, understanding and measuring contact angle is essential.
For a more in-depth discussion and practical tips, download our comprehensive contact angle white paper or explore our blog for more specialized articles.
Susanna is an Application Scientist at Biolin Scientific. In her PhD thesis, she developed fabrication methods for a new type of inorganic-organic polymers. Microfabricated polymer chips were utilized as tool for biomolecule separation in analytical chemistry.
