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Two parallel glass plates, each of area 10⁻² m², are separated by a film of water of thickness 0.05 mm. Taking the surface tension of water as 70 x 10⁻³ N/m and the contact angle as 0 deg, find the force needed to pull the plates apart.
- 28 N
- 112 N
- 5.6 N
- 11.2 N
Correct answer: 28 N
Solution
For theta = 0, the film meniscus radius r = d/2, giving a suction pressure 2T/d. This acts over plate area A, so separation force F = (2T/d) A. Here 2T/d = 2 x 70e-3 / 5e-5 = 2800 Pa; F = 2800 x 10⁻² = 28 N.
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