What is Contact Angle?
Contact angle is the angle formed at the three-phase contact line where a liquid drop rests on a solid surface. It is the most direct measure of surface wettability — the tendency of a surface to be wetted by a liquid. A contact angle below 90° indicates a hydrophilic (water-attracting) surface, while an angle above 90° indicates a hydrophobic (water-repelling) surface. Superhydrophobic surfaces exhibit contact angles above 150°.
Why Wettability Matters
Surface wettability is a critical parameter in a surprisingly wide range of industries and processes:
- Coating and painting — poor wettability causes coating dewetting, orange peel and adhesion failure
- Pharmaceuticals — tablet coating uniformity and drug dissolution depend on surface energy
- Electronics manufacturing — PCB cleaning validation and solder wettability assessment
- Biomedical devices — implant surface hydrophilicity affects protein adsorption and cell attachment
- Textiles — water-repellent finishing verification on fabrics and membranes
- Food packaging — barrier coating performance and food release from surfaces
How Contact Angle is Measured
An optical tensiometer captures an image of a sessile drop (a drop resting on a horizontal surface) and fits the drop profile to calculate the contact angle using Young-Laplace or polynomial fitting algorithms. The measurement is operator-independent when automated drop shape analysis is used. Static contact angle measures equilibrium wettability, while advancing and receding contact angles characterize dynamic wettability and contact angle hysteresis, which correlates with surface heterogeneity and roughness.
Surface Energy Calculation
By measuring contact angles with multiple liquids of known surface tension (water, diiodomethane, ethylene glycol), surface energy components can be calculated using models such as Owens-Wendt, Wu or van Oss-Chaudhury-Good. Surface energy values enable prediction of adhesion strength and selection of optimal primers or adhesion promoters.
Surface Tension Measurement
Force tensiometers measure the surface tension of liquids rather than the contact angle of surfaces. Using Wilhelmy plate or Du Noüy ring methods, they determine equilibrium and dynamic surface tension of surfactant solutions, enabling calculation of the critical micelle concentration (CMC) — an essential parameter in formulation chemistry and detergent performance testing.
Choosing Between Optical and Force Tensiometers
Optical tensiometers (such as the Attension Theta Flex) are the right choice when surface wettability, contact angle on solid materials and surface energy are the primary interests. Force tensiometers (such as the Attension Sigma 700) are preferred for liquid surface tension, CMC determination and Wilhelmy plate measurements on fibers and membranes. Many laboratories benefit from both instruments.
NewRoad supplies Attension tensiometers from Biolin Scientific. Contact us to discuss which wettability analysis solution fits your application.
