Controlling Wetting or Non-Wetting with Surface Tension

Key Note 

The ease of wetting (whether the contact angle is small or large) is determined by the relative surface tensions of the paint (solvent) and the object being painted. Good wetting is achieved when the surface tension of the object is higher than that of the paint. 

1. Wetting-defect Crawling and surface tension. 

When the compatibility between paint and the object being painted isn’t good, an even coat of paint isn’t achieved, and the object can show through. This is called ” Wetting-defect Crawling.” There are two types of Crawling: ” Wetting-defect Crawling ” and “foreign body Crawling” which will be discussed later. The phenomenon depends on the surface tension of the paint and the contact angle with the object. As the solvent evaporates and the volume of paint decreases, the surface tension causes the paint to contract into droplets that match the volume and the contact angle (θ), exposing the object. This is ” Wetting-defect Crawling” Since the solvent is a major factor in the paint’s surface tension, choosing the right solvent is important to prevent Crawling. 

2. Surface tension and contact angle 

Solvents with high surface tension tend to cause a problem known as “crawling,” where the paint does not spread smoothly. This is especially true for water-based paints. However, whether crawling occurs also depends on the nature of the surface being painted. If the surface tension of the surface (γS) is higher than that of the paint (γL), the contact angle (θ) becomes zero degrees. This means the paint does not shrink and covers the surface completely, preventing crawling. This basic principle holds true even when the effects of surface tension are complex: if γS is greater than γL, and θ = zero, crawling will not occur. 

3. Whether or not crawling occurs 

Most solids have higher surface tension than organic solvents, so normally, paints containing organic solvents don’t cause de-wetting. However, Teflon, with its high surface tension, is an exception. Even with organic solvents, Teflon tends to dewet because of its water-repellent and oil-repellent properties. On the other hand, water’s high surface tension can cause de-wetting with materials like plastic. But in paints, additives like surfactants are used to adjust the surface tension and prevent de-wetting. 

 

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