Contact angle – Measuring surface tension

The contact angle serves as a gauge for a liquid’s capacity to adhere to the surface of a solid. The droplet’s shape on a given surface is contingent upon both the surface tension of the fluid and the characteristics of the surface itself. At the juncture where droplets meet the surrounding gas environment, surface tension induces a curved outline.

Explanation contact angle

At the periphery of the droplet, where the contour seamlessly blends into the supporting surface, the contact angle manifests between the liquid/solid interface and the tangent to the liquid/gaseous interface.

contact angle measurement with heating microscope

Meaning of the angle

Interpretation of the angle is as follows: Complete wetting of the solid surface occurs when the liquid spreads uniformly, resulting in a contact angle of 0°. If the angle falls within the range of 0° to 90°, the surface is considered wettable and is termed hydrophilic.

An angle spanning from 90° to 180° indicates that the surface is not wettable, classifying it as hydrophobic. When the angle approaches 180°, it signifies an ultrahydrophobic surface, exhibiting complete liquid repellency—a phenomenon commonly referred to as the lotus effect.

In summary:

  • Angle between 0° and 90°: Surface is wettable, hydrophilic.
  • Angle between 90° and 180°: Surface is not wettable, hydrophobic.
  • Angle close to 180°: Ultrahydrophobic surface, entirely liquid-repellent, demonstrating the lotus effect.

Contact angle as a basis for decision-making in practice

Understanding the contact angle is pivotal for evaluating the appropriateness of paints, varnishes, and coolants, aiming for optimal wetting of materials. Conversely, in enhancing certain textiles and water-repellent building facades, the objective is to minimize wetting as much as possible.

The wettability of surfaces can be manipulated through appropriate surface treatments. The measurement of the contact angle facilitates the deliberate development of effective methods in this domain.

Contact angle measurements are conducted as part of drop contour analysis, wherein the optical evaluation of the shadow image of the drop occurs. Additionally, the heating microscope from Linseis allows for precise determination of the temperature dependence of the contact angle. This comprehensive approach aids in refining techniques for optimizing wetting characteristics in various applications.

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