Contact Angle Goniometer

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ramé-hart 210-U4 Contact Angle Goniometer [SOP]
WCA-052621.png
Sample Type: Liquids and Solutions

A Contact Angle Goniometer, or Water Contact Angle (WCA) Tensiometer, measures the static contact angle of a drop of various liquids and solutions and surface tension.

Introduction and Applications

A drop of water on hydrophilic (left) and hydrophobic (right) surfaces [1]

Contact angle, θ (theta), is a quantitative measure of wetting of a solid by a liquid. The behavior of a single drop on the surface of a solid tells you a lot about its properties. An obvious example is the application of different types of coatings and surface treatments. For example, if contact angle between the paint and the surface to be painted is high, that will lead to poor spreading and non-uniform coating. Contact angles can also be used to check if a surface treatment has been successful. Plasma treatment is often utilized to improve the adhesion between the substrate and the coating to be applied. The low contact angle between water and treated substrate indicate the success of the surface treatment.

To measure contact angle, place a drop of liquid is placed on the surface. Low contact angle values, i.e. below 90°, are related to good wetting whereas high contact angle values, above 90°, indicate poor wetting. When the contact angle is 0°, the surface is said to wet completely. If the liquid applied on the surface is water, terms hydrophilic and hydrophobic are used, respectively.

Three-phase contact point where solid, fluid and fluid meet

The contact angle is geometrically defined as the angle formed by a liquid at the three-phase boundary where a liquid, gas, and solid intersect. There are three different forces acting on this three-phase contact point between solid, fluid and fluid

The [math]\displaystyle{ \lambda_{lv} }[/math] is the surface tension of a liquid, [math]\displaystyle{ \lambda_{sl} }[/math] is the interfacial tension between the solid and liquid and [math]\displaystyle{ \lambda_{sv} }[/math] is the surface tension of the solid i.e. surface free energy. The well-known Young´s equation describes the balance at the three-phase contact.

[math]\displaystyle{ \lambda_{sv} = \lambda_{sl} + \lambda_{lv}cos(\theta_y) }[/math]

The interfacial tensions, [math]\displaystyle{ \lambda_{sv}, \lambda_{sl}, \lambda_{lv} }[/math] form the equilibrium contact angle of wetting, many times referred to Young´s contact angle [math]\displaystyle{ \theta_y }[/math]

References

  1. Susanna, L (n.d.). Contact angle – What is it and how do you measure it? [White Paper]