Contact angle is used to measure wettability; the solid is water-wet if the contact angle is below ___ and is oil-wet if the contact angle is more than ___

Contact angle is used to measure wettability; the solid is water-wet if the contact angle is below ___ and is oil-wet if the contact angle is more than ___

• A. 90, 90
• B. 60, 120
• C. 0, 180
• D. 45, 135

Answer: (D)

Wettability is determined by the contact angle between a liquid and a solid: small angles mean the liquid spreads well and the surface is water-wet, while large angles mean poor wetting and the surface behaves more oil-wet. The thresholds used to denote strong wetting are 45 degrees for water-wet and 135 degrees for oil-wet. So the solid is water-wet when the contact angle is below 45, and oil-wet when it is more than 135. Angles between these values indicate partial wetting. The other options don’t reflect these extremes in wetting behavior.

Wettability is determined by the contact angle between a liquid and a solid: small angles mean the liquid spreads well and the surface is water-wet, while large angles mean poor wetting and the surface behaves more oil-wet. The thresholds used to denote strong wetting are 45 degrees for water-wet and 135 degrees for oil-wet. So the solid is water-wet when the contact angle is below 45, and oil-wet when it is more than 135. Angles between these values indicate partial wetting. The other options don’t reflect these extremes in wetting behavior.