First slide

Surface tension

Question

A small drop of water of surface tension T is squeezed between two clean glass plates so that a thin layer of thickness d and area A is formed between them. If the angle of contact is zero, the force required to pull the plate apart is

Easy

A

$(TA / d)$
B

$(2 TA / d)$
C

$(4 TA / d)$
D

$(8 TA / d)$

Solution

An extremely thin layer of liquid can be regarded as a collection of large number of semi-spherical drops. Excess of pressure across a thin layer = T/r
(where r=d /2) $∴ p = \frac{T}{r} = \frac{T}{(d / 2)} = \frac{2 T}{d}$ Force due to surface tension pushing the two plates together
F = excess of pressure x area of layer
$= 2 TA / d$

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