Answer: Less than 90° (acute, water wets glass).
- A Less than 90° (acute, water wets glass)
- B Greater than 90°, as for mercury on glass
- C Exactly 90°, where the meniscus is perfectly flat
- D Exactly 0°, where the surface is completely non-wetting
Correct answer: A. Less than 90° (acute, water wets glass)
Explanation: For water on glass, the contact angle is about 0° to 20° (acute). Water wets glass, so the meniscus is concave and capillary rise occurs. For mercury on glass, contact angle > 90° (capillary depression).
Since the same volume of fluid must pass every cross-section per second (continuity), the fluid speeds up where the pipe narrows; Bernoulli's equation then says this faster-moving fluid has lower pressure - the principle behind a venturi meter and an aircraft wing's lift.
Concept context
Pressure, buoyancy, Bernoulli equation, viscosity, and surface tension.