Contact Angle Measurement
FIG 1: Interactions before treatment
FIG 2: The closer the match of polar and dispersive components, the better it is for wetting and adhesion.
FIG 3. Evaluating the drop image to measure the contact angle.
FIG 4: Youngs Equation
Contact Angle Measurement (FIG 1 & 2)
Contact Angle is a measurement used to determine a surfaces hydrophilic or hydrophobic properties. The ability of a liquid to spread over a surface with out forming individual droplets determines its wettability or hydrophilic characteristics. In contrast to this a surface that does not allow the water to spread evenly and creates water droplets or beading is considered to be hydrophobic in nature.
The contact angles are precise measurements of surface energy of the material itself vs. the cohesive forces associated with water or other specific chemistries used for testing. Specific contact angles are well defined for most bulk materials and depending on the desired properties can be modified with specific surface treatments in order to improve the desired properties.
If the contact angle of water is less < 30°, the surface is designated hydrophilic and < 10°, the surface is commonly refer to as super-hydrophilic.
When water beads into distinct droplets it is a demonstration of hydrophobic properties, when the contact angle. Surfaces with contact angles greater than in the range of 90°- 120° they are designated as hydrophobic. Super-hydrophbic properties can be attained by creating Micro-textures or micro-patterns on the surface which allowing contact angles exceeding 150°.
There is a range of methods for activating the surface when treating materials before coating, bonding or printing. These include thermal or electrical methods, such as gas plasma, flame, atmospheric plasma or corona treatment, and other chemical treatments with oxidizing gases.
Equally important are the pretreatment and cleaning steps which remove hydrophobic substances from the surface. All these methods increase the surface free energy (SFE) and therefore improve wettability and adhesion.
Standards such as DIN 55660 for coating materials and DIN EN 828 for adhesives processes specify contact angle measurement as the method for determining the SFE of surfaces and for checking an activating or cleaning pre-treatment process.
The contact angle method (FIG 3)
With a contact angle measurement, a drop of a pure test liquid is dispensed onto the solid sample. The drop is recorded using a camera, and the video image is evaluated.
The contact angle ? is measured at the point where the contour line of the drop cuts the surface line. The better the wetting, the smaller this angle is; for complete wetting, it is 0°.
According to Young’s equation (FIG 4), which is proven for over 200 years, there is a fundamental relationship between the contact angle ?, the SFE of the solid ss, the surface tension (SFT) of the liquid sl and the interfacial tension (IFT) sls between liquid and solid.