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Surface tension--Wetting Phenomena--Capillarity

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This presentation covers concepts such as surface tension, surface energy, liquid drops and bubbles, wetting, capillarity at the elementary school level. Comment down in a box for improvement.

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Surface tension--Wetting Phenomena--Capillarity

  1. 1. SURFACE TENSION
  2. 2. QUESTION 1 Why water droplets and soap bubbles are spherical in shape?
  3. 3. How water spiders and water striders can easily walk on water surface without sinking? QUESTION 2
  4. 4. Despite being denser than water, how steel needle and paperclip float on water? QUESTION 3
  5. 5. Jumping Sheet A n e x a m p l e o f s t r e t c h e d e l a s t i c m e m b r a n e
  6. 6. The property by virtue of which the free surface of a liquid behaves like a stretched elastic membrane tending to contract so as to occupy minimum surface area. Definition of Surface Tension
  7. 7. MATHEMATICAL DEFINITION 𝐹 ∝ 𝐿 𝐹𝐿 𝐹 = 𝑇 × 𝐿 𝑇 = 𝐹/𝐿 (𝑁/𝑚) The force acting per unit length of an imaginary line drawn on the free surface.
  8. 8. COHESION & ADHESION Cohesion is the force of attraction between the molecules of the same substance. Adhesion is the force of attraction between the molecules of two different substances. WATER Air BEAKER
  9. 9. EXAMPLES OF COHESION 1. Fixed shape and size of solids 2. Two liquid drops merge into one 3. Mercury does not wets the glass EXAMPLES OF ADHESION 1. Ink sticks to the paper 2. Sunmica sticks to the plywood 3. Water wets the glass
  10. 10. MOLECULAR RANGE Maximum distance upto which a molecule can exert a force of attraction on other molecules. For solids and liquids: R = 10 A°
  11. 11. SPHERE OF INFLUENCE Imaginary sphere drawn around a molecule as centre and molecular range as a radius R
  12. 12. 10 A° Surface film 𝑃 𝑃′ 𝑄 𝑄′ 𝐴 𝐵 𝐶 𝐷 MOLECULAR BASIS OF SURFACE TENSION
  13. 13. • PQ is the free surface of a liquid. • P’Q’ is an imaginary plane at a distance equal to molecular range and parallel to free surface. • The liquid enclosed between PQ and P’Q’ form a surface film. • A molecule below the surface film is surrounded by similar molecules from all sides. (A & B) • Therefore, the net force acting on such a molecule is zero. • But a molecule within a surface film is surrounded by air molecules from above and water molecules from below. (C & D) • Therefore, the net force is acting downward on such a molecule. • Due to the downward force, the molecules of surface film accelerate towards bulk liquid. • As a result, the density of surface film decreases, and pressure becomes negative in that region. • Negative pressure develops tension force in the surface film. • Due to which, free surface of liquid behaves like stretched elastic membrane and shows property of surface tension.
  14. 14. SURFACE ENERGY Liquid Air
  15. 15. • The interaction of a molecule with its nearest neighbors leads to a reduction of its potential energy. • A molecule at the surface region of a liquid has a smaller no of nearest neighbors. • Therefore, the potential energy of surface molecules is not decreased as much as the interior molecules. • Clearly, the surface molecules possess extra potential energy as compared to the molecules inside the liquid. • The potential energy of surface molecules per unit area of the surface is called surface energy. 𝑆𝐸 = 𝑃𝐸 𝐴 𝐽 𝑚2
  16. 16. SURFACE ENERGY
  17. 17. • Consider a liquid drop having some surface area. • If we distort a liquid drop its surface area increases. • This increase in surface area is caused by the rise of molecules from the interior to the surface. • As these molecules reach the surface film, work has to be done against inward cohesive force. • This work is stored as the potential energy of the molecules on the surface. 𝑆𝐸 = 𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎
  18. 18. RELATION BETWEEN T & SE 𝑙 𝑥 𝐹𝑇 𝑇𝑙 𝑇𝑙 𝐹 = 2 × 𝑇𝑙 𝑇 = 𝑊 2𝑙𝑥 = 𝑊 ∆𝐴 𝑊 = 𝐹 × 𝑥 𝑊 = 2 × 𝑇𝑙 × 𝑥 𝑊 = 𝑇 × 2𝑙𝑥 𝑇 = 𝑆𝐸
  19. 19. EFFECT OF TEMPERATURE ON SURFACE TENSION Temperature Kinetic energy Intermolecular distance Intermolecular force Surface tension Increases Increases Increases Decreases Decreases Decreases Decreases Decreases Increases Increases  The variation of surface tension with temperature is given by 𝑇 = 𝑇𝑜(1 − 𝛼𝑡)  The temperature at which surface tension of liquid becomes zero is called the critical temperature of the liquid.
  20. 20. EFFECT OF IMPURITIES ON SURFACE TENSION Type Impurity Intermolecular Force Surface Tension Soluble Table Sugar Increases Increases Alcohol & Phenol Decreases Decreases Common Salt Increases Increases Soaps & Detergents Decreases Decreases Insoluble Oil & Grease (𝜌 < 𝜌 𝑤) Decreases Decreases Tar & Mercury (𝜌 > 𝜌 𝑤) Unaffected Unaffected
  21. 21. WATER O H H δ− δ+ δ+
  22. 22. TABLE SUGAR
  23. 23. ALCOHOL & PHENOL
  24. 24. COMMON SALT Na+ Cl− Na+ Cl−
  25. 25. SOAPS & DETERGENTS Na+ Na+
  26. 26. REDUCTION OF SURFACE TENSION BY OIL
  27. 27. WETTING Wetting refers to the study of how a liquid deposited on a solid substrate spreads out. Understanding of wetting enables us to explain why liquids spread readily on some solids but not on others.
  28. 28. CONTACT LINE The location where the three phases (solid, liquid and air) meet.
  29. 29. 𝜃 CONTACT ANGLE The angle between tangent to the liquid-air interface at contact line and the solid-liquid interface.
  30. 30. YOUNG’S EQUATION 𝑇𝑠 𝑇𝑙 𝑇𝑠𝑙 𝑇𝑠 = 𝑇𝑠𝑙 + 𝑇𝑙 cos 𝜃 cos 𝜃 = 𝑇𝑠 − 𝑇𝑠𝑙 𝑇𝑙 𝜃
  31. 31. CASE-1 OF WETTING cos 𝜃 = 𝑇𝑠 − 𝑇𝑠𝑙 𝑇𝑙 If 𝑇𝑠 > 𝑇𝑠𝑙 and 𝑇𝑠 − 𝑇𝑠𝑙 < 𝑇𝑙 0 < cos 𝜃 < 1 𝜃 is acute (𝜃 < 90° ) 𝜃 Wetting condition (Normal water on glass)
  32. 32. CASE-2 OF WETTING cos 𝜃 = 𝑇𝑠 − 𝑇𝑠𝑙 𝑇𝑙 If 𝑇𝑠 < 𝑇𝑠𝑙 and 𝑇𝑠 − 𝑇𝑠𝑙 < 𝑇𝑙 −1 < cos 𝜃 < 0 𝜃 is obtuse (𝜃 > 90° ) 𝜃 Non-wetting condition (Normal water on wax)
  33. 33. CASE-3 OF WETTINGcos 𝜃 = 𝑇𝑠 − 𝑇𝑠𝑙 𝑇𝑙 If 𝑇𝑠 > 𝑇𝑠𝑙 and 𝑇𝑠 − 𝑇𝑠𝑙 > 𝑇𝑙 cos 𝜃 > 1 Contact angle is impossible and liquid spread over the surface Perfect-wetting condition (Distilled water on clean glass)
  34. 34.  For given solid-liquid pair, the angle of contact is constant.  The value of angle of contact depends upon nature of liquid and solid in contact.  It depends upon the medium which exists above the free surface of liquid.  The angle of contact changes due to impurity.  The angle of contact changes with temperature. Characteristics of Contact Angle
  35. 35. APPLICATIONS OF WETTING
  36. 36. APPLICATIONS OF WETTING
  37. 37. APPLICATIONS OF WETTING
  38. 38. APPLICATIONS OF WETTING
  39. 39. APPLICATIONS OF WETTING
  40. 40. EXCESS PRESSURE INSIDE LIQUID DROP 𝑃𝑖 𝑃𝑜 𝑟 𝑟 ∆𝑟 ∆𝑃 = 𝑃𝑖 − 𝑃𝑜 𝐴1 = 4𝜋𝑟2 𝐴2 = 4𝜋 𝑟 + ∆𝑟 2 𝐴2 = 4𝜋 𝑟2 + 2𝑟∆𝑟 + ∆𝑟2 𝐴2 = 4𝜋𝑟2 + 8𝜋𝑟∆𝑟 ∆𝐴 = 8𝜋𝑟∆𝑟 𝑊 = ∆𝑃 × 4𝜋𝑟2 × ∆𝑟 𝑊 = 𝑇 × 8𝜋𝑟∆𝑟 ∆𝑃 × 4𝜋𝑟2 ∆𝑟 = 𝑇 × 8𝜋𝑟∆𝑟 ∆𝑃 × 𝑟 = 2𝑇 ∆𝑃 = 2𝑇 𝑟
  41. 41. EXCESS PRESSURE INSIDE LIQUID DROP 𝑇 × 2𝜋𝑅 = ∆𝑃 × 𝜋𝑅2 𝑇 × 2 = ∆𝑃 × 𝑅 ∆𝑃 = 2𝑇 𝑅
  42. 42. EXCESS PRESSURE INSIDE SOAP BUBBLE 𝑇 × 2(2𝜋𝑅) = ∆𝑃 × 𝜋𝑅2 𝑇 × 4 = ∆𝑃 × 𝑅 ∆𝑃 = 4𝑇 𝑅
  43. 43. PRESSURE ACROSS FREE SURFACE The pressure on concave side is greater than pressure on convex side.
  44. 44. PRESSURE ACROSS SURFACE 𝑃𝐴 𝑃𝐵 𝑃𝐴 𝑃𝑩 𝑅 𝑅 ∆𝑃 = 𝑃𝐴 − 𝑃𝐵 = 2𝑇 𝑅 ∆𝑃 = 𝑃𝐵 − 𝑃𝐴 = 2𝑇 𝑅
  45. 45. Capillarity is the tendency of a liquid to flow in narrow spaces as a result of interfacial tensions. CAPILLARITY
  46. 46. Wa t e r M e r c u r y LIQUID SURFACE NEAR THE CONTACT IS CURVED (𝜃 < 90° ) (𝜃 > 90° ) (Concave) (Convex) 𝜃 𝜃
  47. 47. CONCAVE SURFACE
  48. 48. CONVEX SURFACE
  49. 49. Capillary Tube A g l a s s t u b e h a v i n g a v e r y f i n e b o r e
  50. 50. RISE & FALL OF LIQUID IN A CAPILLARY TUBE Wa t e r M e r c u r y
  51. 51. REASON FOR CAPILLARY ACTION . ... C A BD . ... C A BD 𝑃𝐴 = 𝑃𝐶 = 𝑃𝐵 𝑃 𝐷 < 𝑃𝐵 𝑃𝐴 = 𝑃𝐶 = 𝑃𝐵 𝑃 𝐷 > 𝑃𝐵
  52. 52. FORMULA FOR HEIGHT OF CAPILLARY ACTION 𝜃 𝑅 𝑟 𝐴 𝐵 ℎ 𝑃𝐴 + 𝜌𝑔ℎ = 𝑃𝐵 𝑃𝐴 = 𝑃𝑎𝑡𝑚 − 2𝑇 𝑅 𝑃𝐵 = 𝑃𝑎𝑡𝑚 𝜌𝑔ℎ + 𝑃𝑎𝑡𝑚 − 2𝑇 𝑅 = 𝑃𝑎𝑡𝑚 𝜌𝑔ℎ = 2𝑇 𝑅 ⇒ ℎ = 2𝑇 𝜌𝑔𝑅 ℎ = 2𝑇 cos 𝜃 𝜌𝑔𝑟 &
  53. 53. JURIN’S LAW ℎ ∝ 1 𝑟
  54. 54. WHO BEARS THE WEIGHT OF THE RAISED LIQUID? 𝑇𝑆 𝑇𝑆𝑙 𝑚𝑔 𝑇𝑆 × 2𝜋𝑟 − 𝑇𝑆𝑙 × 2𝜋𝑟 = 𝑚𝑔 (𝑇𝑆 − 𝑇𝑆𝑙) × 2𝜋𝑟 = 𝜌 × 𝜋𝑟2 ℎ × 𝑔 (𝑇𝑆 − 𝑇𝑆𝑙) × 2 = 𝜌𝑟ℎ𝑔 ℎ = 2(𝑇𝑆 − 𝑇𝑆𝑙) 𝜌𝑟𝑔 ℎ = 2𝑇𝑙 cos 𝜃 𝜌𝑟𝑔 𝑇𝑆 = 𝑇𝑆𝑙 + 𝑇𝑙 cos 𝜃
  55. 55. APPLICATIONS OF CAPILLARITY
  56. 56. APPLICATIONS OF CAPILLARITY
  57. 57. APPLICATIONS OF CAPILLARITY
  58. 58. THANK YOU
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This presentation covers concepts such as surface tension, surface energy, liquid drops and bubbles, wetting, capillarity at the elementary school level. Comment down in a box for improvement.

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