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Surface Tension

•    The surface tension of a liquid is the property by which a fluid resists tensile stress.

•    It is due to the cohesion between the molecules at the surface of liquid. It is the force required to maintain unit length of the film in equilibrium. It is denoted by 'σ' (sigma). S.I. unit = N/m.

•    All liquid molecules have tendency to attract each other. A liquid molecule on the interior of the liquid body has other molecules on all its side, so that the forces of attraction are in equilibrium and the molecule is equally attracted on all sides, as a molecule at point A as shown in.

•    A liquid molecule at the free surface at point B, does not have liquid molecule above it to pull them outward, so that the free surface molecule try to move towards the centre by downward force.

•    This force on the liquid surface, is normal to the liquid surface. A film or layer occurs on the liquid surface which is in tension and resist small loads if placed gently over it.

•    It occurs at the interface of a  liquid and a gas or at the interface of two liquids and is essentially due to intermolecular forces of cohesion.

•    As the temperature increases, the surface tension decreases.

1)    A small needle placed gently upon the water surface will not sink but will be supported by the tension at the water surface.

2)    The effect of surface tension is to reduce the surface of a free body of liquid to a minimum. That is why falling drops of rain become spherical in shape.

•    Surface tension force can be reduced by the addition of detergents. Surface tension value of liquids when it is not with air

σ  = 0.073 N/m for air – water interface
σ  = 0.48 N/m for air – mercury interface

•    Surface tension leads to the phenomena of capillary waves on a liquid surface and capillary rise or depression.

Pressure Intensity Inside a Droplet:

Consider a spherical droplet of diameter ‘d’. Let the excess pressure developed inside the droplet ‘P’ and 'σ' is the surface tension in the surface of droplet as shown in.

For static equilibrium condition

Surface tension force = Force due to excess pressure
Surface tension  Circumference = Excess pressure x Normal area

σ (πd) = P (π/4d2)

P = 4σ / d

Thus pressure intensity inside a droplet varies inversely with the diameter. Pressure intensity decreases with an increase in the size of the droplet.

Pressure Intensity Inside a Soap Bubble:

Soap bubble has two surfaces in contact with air, inside and courtside.

Consider a soap bubble of external diameter d and thickness of bubble = ( )d as shown in.

σ  = Surface tension on inner surface
σ  = Surface tension on outer surface

For static equilibrium,

Force due to excess pressure = Force due to surface tension

Excess pressure x Normal area = Surface tension x Circumference

P x π/4 (d-σd)2 = (σπd) + σπ(d-σd) (Neglecting σd)

Pπ/4d2 = 2 σπd

P = 8σ /d

Thus excess pressure inside the soap bubble is twice as that of droplet.

Pressure Intensity Inside a Liquid Jet:

Consider a liquid jet as a cylinder of diameter ‘d’ and length ‘I’ as shown n.

Force due to excess pressure = Force due to surface tension
Excess pressure x Normal area = Surface tension x Circumference

P(dxL) = σ(2L)

P = 2σ/d

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