To bring it in equilibrium position a resultant force will acts on its surface and creates tension at the free surface side of the molecule. This tension force per unit length is known as Surface Tension and measured in N/m. 9/30/2014 Dr. M.M.J. Kumar 19
Properties of Fluids Surface Tension : Surface Tension is the force required to maintain unit length of a liquid surface film in equilibrium. Unit of surface tension is N/m. When a droplet of liquid is separated from a liquid jet it experiences external pressure force from all sides on its surface. This external force increases the internal pressure of the droplet. There will be an equilibrium condition arises at which the external force is balanced by the surface tension and Droplet takes the shape of a sphere as its surface area is minimum. 9/30/2014 Dr. M.M.J. Kumar 20
Properties of Fluids Surface Tension : The pressure difference (surface tension force) across a liquid droplet between its inner and outer surface is given by, When the liquid surface coexists with another immiscible fluid, usually gas, on both the sides, then 9/30/2014 Dr. M.M.J. Kumar 21 1 2 1 1 i o p p p r r 1 2 1 1 2 p r r
Properties of Fluids Surface Tension : Spherical liquid drop: ( r 1 = r 2 = r ) Cylindrical liquid jet: ( r 1 → ∞ and r 2 = r ) Spherical bubble: ( r 1 = r 2 = r ) 9/30/2014 Dr. M.M.J. Kumar 22 2 p r p r 4 p r
Properties of Fluids Surface Tension : Pressure intensity inside a Spherical liquid drop: Cylindrical liquid jet: ( r 1 → ∞ and r 2 = r ) Spherical bubble: ( r 1 = r 2 = r ) 9/30/2014 Dr. M.M.J. Kumar 23 Pr intensity on the Tensile force on the surface = projected area due to surface tension 2 2 2 p r r p r 2 2 2 4 p r r p r 2 2 p rl r l p r
Properties of Fluids Capillary : It is the phenomenon of rise or fall of a liquid level relative to the adjacent general level of the liquid. It can be either capillary rise or capillary depression. Whenever the liquid molecules have a greater affinity to solid particles of the surrounding surface with which it is in contact, the liquid tend to rise at the point of contact known as “Capillary Rise”. On the other hand if the liquid cohesion is more than its adhesion then the liquid tend to depress at the point of contact known as “Capillary Depression”. 9/30/2014 Dr. M.M.J. Kumar 24
Properties of Fluids Capillary : Consider, h is height to which the liquid is raised or depressed wrt the free surface level, when a tube of radius, r is inserted in to the liquid, then It can be either capillary rise or capillary depression. With force balance in vertical direction, we can write. 9/30/2014 Dr. M.M.J. Kumar 25 Force due to weight of Force at the surface due to the liquid column surface tension 2 2 cos 2 cos w wat at e r r e h Sw r r Sw r h
Properties of Fluids Capillary : The following assumptions are made for this derivation.
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