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Water Contact Angle (WCA)
Water contact angle (WCA) provides a measure of wettability, hydrophobicity (if water is used), and the strength of contact of a solid surface.
It is often used to measure the degree of cleanliness of a surface.
- Quick, straightforward measurement of liquid-solid interface
- Indirect measurement of surface free energy
- Expertise required to make accurate measurements
Water Contact Angle measurement on PEDOT:PSS showing high hydrophilicity in the substrate.
From Ossila (https://www.ossila.com/pages/contact-angle-measurements-surface-wetting)
Contact Angle measurement of glycerol droplet over 4 seconds, showing relaxation behavior indicative of high surface wettability.
From Ossila (https://www.ossila.com/products/contact-angle-goniometer?variant=2856241430557)
Image shows liquid droplet and water contact angle measurements underway.
To make this measurement, a drop of water (or other liquid as needed) is cast on the surface of a solid sample. The angle between the face of the solid surface and the tangent line to the contacting edge of the droplet is recorded using a high-resolution video camera. This allows for characterization of the angle over time, starting with its initial spread upon contact with the surface. This relaxation process can be computationally analyzed using the Young-Dupre Equation to determine the wettability and interaction strength at the solid-liquid interface.
If the angle is greater than 90 degrees (i.e. the drop does not spread), the affinity between the solid and liquid is very low. If the droplet is water, then the material is considered to be hydrophobic. As the drop spreads and the angle becomes more acute, the wettability and hydrophilicity of the material proportionately increase.
A variation of this method can be used to measure the surface tension of liquids by analyzing the drop before contact using the pendant method.