Surface Tension

      
      Surface tension is basically a property of liquid. The liquid surface behave like a stretched elastic membrane which has a natural tendency to contract and tends to have a minimum possible area. This property of liquid is called surface tension.

Inter molecular force

A. Cohesive force :-
       The force acting between the molecules of one type of molecules or molecules of different substance is called adhesive force.

B. Adhesive force :-
       The force acting between different type of molecules or molecules of different substance is called adhesive force.

 * Inter molecular force are different from the gravitational forces          not obey the inverse square low.

* The distance up to which these forces effective, is called molecular range. This distance is nearly 10^-9m. Within these limit this increases very rapidly as the distance decreases.

Property of surface tension :-

* surface tension is a scalar quantity.
* It acts tangential to liquid surface.
* surface tension is always produced due to cohesive force.
* More is the cohesive force, more is the surface tension.
* when surface area of liquid is increased, molecules from the interior of the liquid rise to the surface. For this , work is done against the downward cohesive force.

Dependency of surface tension :-

* On cohesive force :- those factor which increase the cohesive force between molecules increase the surface tension and those which decrease the cohesive force between molecules decrease the surface tension.

* On Impurities :- If the impurity is completely soluble then on mixing it in the liquid, its surface tension increases. E.g., on dissolving ionic salt in small quantities in a liquid, its surface tension increases. If the impurities is partially soluble in a liquid then its surface tension decreases because adhesive force between insoluble impurity molecules and liquid molecules decreases cohesive force effectively, e.g.

A. On mixing detergent in water its surface tension decreases.

B. Surface tension of water is more then ( alcohol + water ) mixture.

* On temperature
      On increasing temperature surface tension decreases. At critical temperature and boiling point it becomes zero.

NOTE :- Surface tension of water is maximum at four degree centigrade.

* On contamination
     The dust particles or lubricating materials on the liquid surface decreases its surface tension.

* On Electrification
     The surface tension of the liquid decreases due to electrification because a force starts acting due to it the outward direction normal to the free surface of liquid.

Definition of surface tension

    The force acting per unit lenght of an imaginary line drawn on the free liquid surface at right angle to the line and in the plane of liquid surface, its define as surface tension.
 For floating needle  2Tlsin Q= mg

* required excess force for lift
- wire  F ex = 2Tl
- for ring  F ex = 4πrt
- square frame F ex =8aT
- Hollow disc F ex = 2πT(r1 + r2 )
- Circular disc F ex = 2πrt
- Square plate F ex = 4at

*  Work = surface energy =4πr^2T
- liquid drop W = 4πr^2T
- Soap bubble W = 8πr^2T

* Excess pressure
P ex = P in - P out

- In liquid drop P ex = 2T / R

- In soap bubble P ex = 4T/R

* Angle of contact :-
    The angle enclosed between the tangent plane at the liquid surface and the tangent plane at the solid surface at the point of contact inside the liquid is defined as the angle of contact.

The angle of contact depends the nature of the solid and liquid in contact.
Angle of contact Q is less than 90° --> concave shape, liquid rise up.

Angle of contact Q is more than 90°--> convex shape, liquid falls.

Angle of contact Q is equal to the 90°--> plane shape, liquid neither rise nor falls.

* Effect of temperature on angle of contact :-

   On increasing temperature surface tension decreases, thus cosxc increases [ cosxc is inversely proportional to the temperature ] and xc decreases. So on increasing temperature, xc decreases.

* Effect of impurities on angle of contact :-

A. Solute impurities increase surface tension, so cosxc decreases and angle of contact xc increases.

B. Partially solute impurities decrease surface tension, so angle of contact xc decreases.

* Effect of water proofing agent =

     Angle of contact increases due to water proofing agent. It gets converted acute to obtuse angle.

* special thank you to ALLEN to provide me content for this blog.

* picture credit goes to google
https://www.researchgate.net/figure/The-ramifications-of-surface-energy-in-the-form-of-surface-tension-are-well-known-from_fig1_331025219

https://www.remet.com/en/insights/an-analysis-into-surface-tension/