Contact angle

Contact angle

File-Water droplet at DWR-coated surface1

Diagram droplets.

Video contact angle

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Contact angle is a quantitative measure of the wetting of a solid surface by a liquid, which reflects the wettability of the surface. The contact angle is defined geometrically as the angle formed at the junction where the interfaces of the liquid, solid, and gas (usually air) meet. It is a crucial parameter in the fields of surface science, material science, and fluid mechanics, playing a significant role in processes such as coating, painting, adhesion, and inkjet printing.

Definition

The contact angle (\(\theta\)) is measured within the liquid, where a liquid droplet contacts a solid surface. A low contact angle, less than 90°, indicates that the liquid spreads, or wets, the surface well, which is characteristic of hydrophilic surfaces. Conversely, a high contact angle, greater than 90°, suggests poor wetting, characteristic of hydrophobic surfaces. A contact angle of 0° means the liquid perfectly wets the surface, spreading out to form a thin layer.

Measurement

Contact angle measurement can be performed using several methods, including the sessile drop method, the captive bubble method, and the Wilhelmy plate method. The sessile drop method, where a droplet is placed on the surface and the angle is measured either through side view imaging or using a goniometer, is the most common.

Factors Influencing Contact Angle

Several factors can influence the contact angle, including the surface roughness, surface chemistry, and the presence of contaminants. Modifications to the surface, such as plasma treatment or chemical coating, can significantly alter the wettability and, consequently, the contact angle.

Applications

Contact angle measurements are essential in various applications: - In material science, they help in characterizing surface properties and developing materials with specific wetting properties. - In the pharmaceutical industry, they are used to optimize tablet coatings to control dissolution rates. - In electronics, they guide the development of water-repellent surfaces for corrosion protection. - In biomedical engineering, understanding and controlling contact angles on biomaterials can influence cell adhesion and protein adsorption, critical for implantable devices and tissue engineering.

Theoretical Models

Several theoretical models describe the relationship between contact angle and surface tension, including Young's equation, which relates the contact angle to the interfacial tensions between the solid, liquid, and gas phases. More complex models, like the Wenzel and Cassie-Baxter models, account for surface roughness and heterogeneity.

See Also

• Surface tension
• Wetting
• Hydrophobic effect
• Hydrophilic surface