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1 STUCTUE AND POPETIES O LIQUIDS. Surface tension a) phenomenon The surface of a iquid behaves ike a stretched eastic membrane (proof pond skater, sma drops spheres Expanation: r range of attraction r nm, about 3 moecues The resutant force acting on the moecue is not equa to zero when the sphere is not fied by the same moecues the moecues near the surface are attracted into the iquid (the resutant force has a different size for different distances from the surface). b) physica quantity ( γ, in Czσ ) a oop of wire soap fim siding wire γ = γ = N [ ] m = const type of iquid, medium around materia property in the book of data, big vaue for mercury (- air) affected by temperature (how why?) additiona detergents in water ower the surface tension use? Expain the function. - - STUCTUE AND POPETIES O LIQUIDS

2 . Shape of iquid surfaces The shape of a iquid surface is perpendicuar to the resutant force acting on the particuar moecues (why?) A C A... adhesive force (attraction of the was of the container) C... cohesive force (attraction of the iquid moecues) force of the air moecues and force of gravity are negigibe in comparison with the two previous ones (what woud be their directions?) Sketch the shape of iquid surface near the wa of the container reated to the previous theory θ... ange of contact - between... a) b) c) θ θ θ θ 9 water in gass mercury in copper θ = 9 9 θ 8 mercury in gass inish the figures above! - - STUCTUE AND POPETIES O LIQUIDS

3 3. Capiarity Surface tension + different ange of contact + narrow capiary (reativey more moecues in contact with the container) iquid inside rises/fas a) c) equations h p c p c γ hρg = γ p c = 4γ p c =... radius of the capiary, drop, bubbe Experiment: Estimate which of the bubbes on the same straw wi disappear. Compare with reaity, discuss STUCTUE AND POPETIES O LIQUIDS

4 4. Measurement of surface tension capiarity rise method γ hρg = hρg γ = h drop method a drop fas just when its weight sighty exceeds the force hoding it together because of surface tension mg = γ mg = γπ γ = mg π This method can be improved using two iquids one of known surface tension and the same capiary. What is the advantage? m g = γπ m g = γ π when we divide the equations: m γ = more precise for e.g. 5 drops of each iquid m γ 5m 5m M = M γ = γ Measurement:... water, γ = 7mN m, 5 =... ethano, γ =?, 5 m = m STUCTUE AND POPETIES O LIQUIDS

5 5. Surface energy = energy needed to create unit area of the new surface (to raise the surface area by m ) A W x γ x σ = = = = γ A x x ( ) [ σ ] = J m = N m m = N m = [ γ ] x L3/33-35a, b, c, 38, 4-5 Questions:. A match with a ength of 4.4 cm foats on the surface of some water. If we genty pour a itte soap soution on one side of the surface divided by the match, it starts moving away from the soap soution towards the pure water. State the force (incuding direction of this force) acting on the match. γ water = 73 mn m - γ soap = 4 mn m -. Liquid fows from a vesse through a narrow capiary with a radius of.8 mm. One drop fas per second. How ong wi it fow if the mass of the iquid is 5 g? γ = -3 N m - g = 9.8 m s - 3. Stipuate the pressure of air in a spherica bubbe with a diameter of -3 mm at the depth of 8 cm beow the surface of water. Atmospheric pressure is hpa. γ = 73 mn m - ρ WATE = 3 kg m -3 g = m s - 4. Stipuate the mass of water that rises in the capiary of the inner diameter.7 mm due to capiary eevation. Assume that water perfecty wets the was of the capiary. Ange of contact θ = º. γ = 73 mn m - g = 9.8 m s - 5. A capiary of a diameter mm is verticay immersed in a vesse with iquid. The iquid rises to the height of. cm above the free surface of the iquid in the vesse. Which height does the same iquid reach if we immerse in it a capiary with the diameter of.5 mm? Assume that water perfecty wets the was of the capiary. 6. Two gass capiaries of radii mm and.5 mm are verticay immersed in ethano. Cacuate γ if the difference of heights of the surfaces is.9 mm due to capiary eevation. ρ = 789 kg m -3 g = 9.8 m s STUCTUE AND POPETIES O LIQUIDS

6 6. Voume expansion β... voume expansivity / cubic expansivity β V V t = { } = { V} V = m 3 t = K [ β ] = K β β 5 K water ethano gycero 8 5 L3/47-49, 5-53 Answers:..45 mn. 36 min 58 s 3. 4 kpa 4. 6 mg cm 6. mn m STUCTUE AND POPETIES O LIQUIDS