The angle of contact

When a liquid surface meets a solid surface the angle (f) between the two surfaces is important.

This angle is known as the angle of contact, and its size determines whether the liquid will spread over the surface or whether it will form droplets on it. If φ is less than 90^o the liquid will spread and wet the surface (Figure 1(a)), while if it is greater than 90^o it will form droplets (Figure 1(b)). This effect can be seen clearly with water on glass. For a clean glass-water boundary the angle of contact is nearly zero but this increases to well over 90^o if the surface is waxed.

In fact a surface will be wetted by water if the angle of contact is less than 90^o, and so waterproofing is designed to produce an angle of contact that is greater than 90^o. Addition of soap to water will reduce the angle of contact and is therefore useful in washing!

Student investigation 1
Using the apparatus shown in Figure 2, investigate the effect on the angle of contact between water and glass when measured amounts of concentrated soap solution are added to it. The microscope slide should be rotated until the liquid meets the glass at a zero angle.

If possible the angle of contact for different surfaces should be studied, by replacing the microscope slide with other materials.

 

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