Surface Tension

 Surface Tension

Introduction: Surface Tension is very interesting property of the liquid. We come across various things like insect moving easily on the surface of the liquid, day to day activity such as removal of oil and dirt from the clothes with the help of detergent powder, cleaning of teeth with toothpaste, etc. The secret behind all this day to day activity lies behind surface tension of a liquid. Let us understand it in detail.

Photo by Colin Lloyd on Unsplash
Water drop due to Surface tension

Molecular Theory of Surface Tension

Figure A, Molecular theory

Intermolecular Force: The force between the molecules are called as intermolecular force. There exist a force of attraction between molecules which is due to intermolecular force.

Range of molecular force: The maximum distance upto which the molecular force exist between the molecules is called as range of intermolecular forces. The length of red arrow from center to the surface of sphere shown in the above diagram indicates range of molecular force.

Sphere of influence : An imaginary sphere whose radius is equal to range of intermolecular force is called as sphere of influence. All the sphere shown in diagram above is sphere of influence.

Free surface : The surface XY is called as free surface of the liquid.

Theory of surface tension : As shown in fig A, every  molecules which is inside the liquid is surrounded by large number of same molecules. This molecule within the liquid experiences an equal  molecular force due to neighbouring molecule from all direction, thus net force acting on the molecule is zero. On the other hand, the molecule which is on the surface  has most of the part surrounded by water molecules from downward side but it is surrounded by very less air molecules from upper side. Thus cohesive force is stronger than adhesive force and hence net cohesive force act on the surface molecule in downward direction. This net cohesive force results in downward movement of the molecule from surface layer which results in very less molecules on surface, due to this the remaining molecule stretch themselves to complete the surface and remains in tension. This tension on the surface of liquid is called as surface tension.

Surface Tension : The tangential force acting on the given length of liquid surface is called as surface tension.

Surface tension = Tangential force / Length

T = F / l

SI unit = Newton / Meter (N/m)

Dimension : [ L⁰ M¹ T^-2 ]

│<<< Manometer ││ Surface Energy>>>│

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 Related Topic :  Mechanical Properties of fluid      Pressure due to liquid column     Absolute and gauge pressure  Hydrostatic Paradox       Pascal's Law        Measurement of pressure         Surface Tension Surface Energy         Angle of contact          Capillary Action            Motion in a fluid                    Critical velocity and Reynold's Number                      Viscosity                 Stokes' Law            Terminal velocity                 Equation of continuity                  Bernoulli Equation          Speed of Efflux    Ventury Tube