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Surface Tension

The property of a liquid due to which its free surface tries to have minimum surface area and behaves as if it were under tension some what like a stretched elastic membrane is called surface tension.
A small liquid drop has spherical shape, as due to surface tension the liquid surface tries to have minimum surface area and for a given volume, the sphere has minimum surface area.
Surface tension of a liquid is measured by the force acting per unit length on either side of an imaginary line drawn on the free surface of liquid (Fig. 16), the direction of this force being perpendicular to the line and tangential to the free surface of liquid. So if F is the force acting on one side of imaginary line of length L, then T = (F/L).
Fig. 16
  • Surface tension depends only on the nature of liquid and is independent of the area of surface or length of line considered.
  • It is a scalar quantity as it has a unique direction which is not to be specified.
  • Dimension: [MT –2] (similar to force constant).
  • Units: N/m (SI) and Dyne/cm [CGS]
  • It is a molecular phenomenon and its root cause is the electromagnetic forces.

Force due to surface tension

Suppose a body of weight W is placed on the liquid surface whose surface tension is T. If F is the minimum force required to pull it away from the water, then value of F for different bodies can be calculated as given in Table 3.
Body: Needle (Length = l )
Force: F = 2l TW

Body: Hollow disc (Inner radius =r1 outer radius =r2)
Force: F = 2π(r1r2)T +W

Body: Thin ring (Radius = r)
Force: F = 2π(rrT + W = 4πrTW
Body: Circular plate or disc (Radius = r)
Force: F = 2πrTW

Body: Square frame (Side =l )
Force: F = 8lTW

Body: Square plate
Force: F = 4lTW

Factors affecting surface tension

Temperature The surface tension of liquid decreases with rise of temperature. The surface tension of liquid is zero at its boiling point and it vanishes at critical temperature. At critical temperature, intermolecular forces for liquids become equal so they can expand without any restriction.
Force: The presence of impurities either on the liquid surface or dissolved in it considerably affect the force of surface tension. A highly soluble substance such as sodium chloride when dissolved in water increases the surface tension of water.

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