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Angle of contact
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Angle between liquid surface and solid wall.
The angle between the liquid surface and the solid at the line of contact. Water on glass is below 90° (wets), mercury is above 90° (doesn't wet).
Water forms a meniscus curved up; mercury curved down.
It depends on the solid, liquid and gas.
In more detail
The angle of contact is the angle a liquid surface makes with a solid at the edge of contact. A small angle means the liquid wets the surface, while a large angle means it does not.
Real-world uses
- Waterproof fabrics are designed for large contact angles.
- Non-stick coatings cause water to bead up.
- Water forms a flat surface in a glass with a low contact angle, while mercury bulges out of the glass with a high angle.
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Frequently asked questions
What does a 90 degree contact angle mean?
The liquid is neither strongly attracted nor strongly repelled by the solid.
Does mercury wet glass?
No, mercury has a large contact angle with glass, so it forms beads.
What does a contact angle below 90 degrees mean?
The liquid wets the surface, spreading to cover it, which is the case for water on clean glass.
Related topics in Fluids & Elasticity
Surface tension · Surface energy · Excess pressure in drops and bubbles · Capillary rise · Cohesion and adhesion · Elasticity