Surface Tension

This slides will give us a brief idea about the surface tension of a liquid. it will also describe about the importance and effect in our day to day life. determine the theory on surface tension and solve various problems on it.

1. SURFACE TENSION Aminul Islam

2. CONTENTS Surface Tension Angle of contact Determination theory of surface tension Problems SURFACE TENSION 2

3. DEFINITION: The tension of the surface film of liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area . SURFACE TENSION 3

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5. Surface tension is the property of the free Surface of a liquid at rest behave like a stretched membrane in order to acquire minimum surface area i.e contractive tendency. SURFACE TENSION

6. DEFINITION Surface tension can be defined as the force acting per unit length perpendicular on an imaginary line drawn on the liquid surface, tending to pull the surface apart along the line. If F is the force acting on the length l of the line AB, then surface tension is given by, Its unit is N m -1 Dimension is [MT -2 ] SURFACE TENSION 6

7. IMPORTANCE Separation of oil and water is caused by a tension in the surface between dissimilar liquids (‘ ’Interface Tension’’). A spider can walk on a surface of water but on the ethanol it will drown. Why? Because the surface Tension of water is high enough to ethanol.   SURFACE TENSION

8. 3. High surface tension of water is also the reason for spherical shape of rain drop. Actually the surface tension of water would be much lower, nothing would really float on top. Even the smallest particles would sink to the bottom which causes the failure of the ecosystem.  SURFACE TENSION 8 8

9. COHESIVE FORCE Cohesive force is the force of attraction between the molecules of the same substance. This cohesive force is very strong in solids, Weak in liquids and extremely weak in gases. 9 9 SURFACE TENSION

10. ADHESIVE FORCE Adhesive force is the force of attraction between the molecules of two different substances. For example: Fevicol , gum etc exhibit strong adhesive property. 10 10 SURFACE TENSION

11. Angle of Contact ( AoC ) When the liquid is in contact with solid, the angle between the solid surface and the tangent to the free surface of the liquid at the point of contact, measured from inside the liquid is called the angle of contact. SURFACE TENSION

12. CHARACTERISTICS The AoC is constant for a given liquid-solid pair. The AoC between the liquid and solid surface is small (acute), the liquid is said to wet the surface (Water-Glass) if the AoC is larger , the surface is not wetted. (Mercury-Glass) SURFACE TENSION 12 12

13. If there are impurities in the liquid, then they alter the values of the AoC The AoC decreases with an increase in temperature. 6. A liquid will completely wet the solid if the AoC is zero . SURFACE TENSION 13 13

14. Let us consider a capillary tube. Due to surface tension, water rises to a height ‘ h’ in the capillary tube . The surface tension T of the water acts inwards and the reaction of the tube R outwards where R = T but opposite in direction. 14 14 SURFACE TENSION Surface tension of a liquid by using a capillary tube

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16. The reaction R can be resolved into two rectangular components. Horizontal component R sin θ acting radially outwards Vertical component R cos θ acting upwards The horizontal components are canceled each other whereas the vertical component balances the weight of water column in the tube. 16 16 SURFACE TENSION

17. Total upward force = R cos θ × circumference of the tube F = 2 π r R cos θ F = 2 π r T cos θ ...(1) [∵ R = T ] This upward force is responsible for the capillary Rise which is equal to weight of the water column acting downwards. F = W ……………...(2) 17 SURFACE TENSION

18. volume of water in the tube : a. Volume of cylindrical water column = πr 2 h b. Volume of water in the meniscus = (Volume of cylinder of height r and radius r) – (Volume of hemisphere) 18 SURFACE TENSION

19. 19 19 SURFACE TENSION If ρ is the density of water, then weight of water in the tube is ……………… (3)

20. Substituting (1) and (3) in (2) 20 20 SURFACE TENSION Since r is very small, 3/r can be neglected compared to h

21. For water, θ is small, therefore cos θ ≈ 1 21 21 SURFACE TENSION

22. Problem: A capillary tube of 0.05cm bore stands vertically in a wide vessel containing a liquid of surface tension 30 dyne/cm. The liquid wets the tube and has a specific gravity 0.8. Calculate the rise of the liquid in the tube. Solution: Given that Ɵ = 0 T = 3.061 cm r = d/2 = (0.05/2) c = 0.025 cm T = 30 Dyne/cm ρ = 0.8 g = 980 cm/s 2 22 22 SURFACE TENSION

23. Problem: Calculate the height to which a liquid will rise in a capillary tube of radius 0.02 cm when surface tension is 26x10 -3 N/m and density 800 kg m -3 . take angle of contact is zero. Solution: Given that Ɵ = 0 r = 0.02 cm = 0.0002 m = 0.33 m T = 26x10 -3 N/m ρ = 800 kg m -3 g = 9.80 m/s 2 23 23 SURFACE TENSION

24. Problem: A liquid of density 1.05 gm/cc and angle of contact 20 ̊ has a vertical capillary tube of 2 mm diameter dipping into it. If the surface tension is 235 dynes/cm, find the rise of the liquid in the capillary tube. Solution: Given that Ɵ = 20 d = 2mm r = (d/2)=1mm=0.1 cm = 4.29 cm T = 235 dynes/cm ρ = 1.05 gm/cc g = 980 cm/s 2 24 24 SURFACE TENSION

25. THANK YOU 25 25