Absorption of vapour on a clean surface is spontaneous because
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Vapor Deposition (PVD) Processing (Second Edition), 2010
2.4.7 Surface Energy and Surface Tension
Surface energy and surface tension result from non-symmetric bonding of the surface atoms/molecules in contact with a vapor, and are measured as energy per unit area. Surface energy and surface tension differ slightly thermodynamically but the terms and values quoted are often used interchangeably. Surface tension is often used to define fluid surfaces (e.g. Table 13.4) while surface energy is used to define solid surfaces. Surface energy is an important indicator of surface contamination and the composition of a polymer surface. Surface energy has the dimension of force per unit length (dyne/cm – cgs units) or of energy per unit area (mN/m – SI units). Surfaces with a high surface energy will try to lower their energy by adsorbing low energy materials such as hydrocarbons.
Surface energy and interfacial energy are measured by the “contact angle” of a fluid droplet on the solid. The contact angle is measured from the tangent to the droplet surface at the point of contact, through the droplet to the solid surface. Figure 2.15 shows the contact angle of a water drop on a surface with a high surface energy and on a surface with a low surface energy. The surface tension of a liquid can also be measured by the Wilhelmy pin test, in which the downward pull on a clean metal pin being withdrawn from the fluid is measured by a microbalance with an accuracy of about 1 mg. It can also be measured by the fluid rise in a capillary tube.
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Figure 2.15. Contact Angle of a Water Drop on a Surface with a High Surface Energy (Left) and on a Surface with a Low Surface Energy (Right)
To measure the contact angle, a fluid droplet is applied to the surface, using a microsyringe to give a constant volume of fluid. Deionized (DI) water is a commonly used contacting fluid. The contact angle is then measured with a “contact angle goniometer.” There are three types of goniometer. The projection design projects an image of the drop; the operator establishes the tangent by rotating a fiducial filar in a long-focus microscope. The microscope-based design uses a low power microscope with an internal protractor scale to look at the image of the drop. The computerized, automated system uses a video camera to observe the image of the drop and digitize the image, and a computer program establishes the tangent and calculates the contact angle. Clean metal and oxide surfaces have a high surface free energy.