Thin rising/falling films

In this type the liquid flows as a thin film on the walls of a long, vertical, heated, tube. Both falling film and rising film types are used. They are high capacity units suitable for low viscosity solutions. 

The coefficient of friction between two unlubricated solids is generally in the range of 0.5-1.0, and it has therefore been a matter of considerable interest that very low values, around 0.03, pertain to objects sliding on ice or snow. The first explanation, proposed by Reynolds in 1901, was that the local pressure caused melting, so that a thin film of water was present. Qualitatively, this explanation is supported by the observation that the coefficient of friction rises rapidly as the remperarure falls, especially below about -10°C, if the sliding speed is small. Moreover, there is little doubt that formation of a water film is actually involved [3,4]. 

As discussed in Section 15.5.2, the separation of two or more sublimable substances by fractional sublimation is theoretically possible if the substances form true solid solutions. Gillot and Goldberger(10°) have reported the development of a laboratory-scale process known as thin-hlm fractional sublimation which has been applied successfully to the separation of volatile solid mixtures such as hafnium and zirconium tetrachlorides, 1,4-dibromobenzene and l-bromo-4-chlorobenzene, and anthracene and carbazole. A stream of inert, non-volatile solids fed to the top of a vertical fractionation column falls counter-currently to the rising supersaturated vapour which is mixed with an entrainer gas. The temperature of the incoming solids is maintained well below the snow-point temperature of the vapour, and thus the solids become coated with a thin film (10. im) of sublimate which acts as a reflux for the enriching section of the column above the feed entry point. 

This is the simplest experimental method for the determination of film pressure, which is defined as the difference in surface tension between the pure solvent and the film covered surface. The equipment consists of a thin glass, platinum or mica slide suspended from a balance and dipping into the film covered surface. Any change in surface tension causes the slide to rise or fall until buoyancy compensation is reached. 

Figure 6. Zoomed-in chronophotographs of the impact region, when a hydrophobic sphere (static contact angle 6>q 115°) is falling on an air-water interface at different impact velocities compared with the air entrainment threshold f/ (a) U = 2.4 m/s < f/ and (b) U = 5.0 m/s > f/. The thin liquid film that develops and rises along the sphere in both cases either gathers at the pole to encapsulate the sphere (low velocity), or is ejected from the sphere thus creating an air cavity behind it (high velocity).

Let us furthermore describe a circumstance which could be embarrassing in the course of an experiment, it happens rather often, either by a fall in temperature, or because one has introduced a too great quantity of mixture at 16°, that the mass used rises to the surface of the ambient liquid and flattens out more or less, but leaves above it a thin film of this liquid, a film which seems resistant, as between two juxtaposed masses ( 6) then, after some time, the mass presents a portion of plane surface at the level of that of the surrounding liquid and, which is strange, it has, so to speak, contracted an adherence with this last surface. For detaching it completely, the only means is to pour with care a little pure alcohol, which spreads on the set of two surfaces. 

A thin film of water spreads up the inside walls of the capillary because of strong adhesive forces between water and glass (water wets glass). The pressure below the meniscus falls slightly. Atmospheric pressure then pushes a column of water up the tube to eliminate the pressure difference. The smaller the diameter of the capillary, the higherthe liquid rises. Because its magnitude is also directly proportional to surface tension, capillary rise provides a simple experimental method of determining surface tension, described in Exercise 122. 

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