Capillary tension

KapiUarititt, /. capillarity. Kapillaritittsanziehung./. capillary attraction. Kapillar-kraft,/, capillary force, -kreislauf, n. capillary circulation, -rohr, -rohrchen, n, -rohre, /. capillary tube, -spaonung, /. capillary tension, -stromung, /. capillary flow, -versuch, m. capillary test or experiment. -wirkung./. capillary action. 

Piezo drop-on-demand print head s principle of operation is relatively simple in comparison to CIJ. The jetted fluid is held in balance between a shght negative pressure in the ink system, and capillary tension in small nozzles. Each nozzle is attached to a small chamber from which ink is ejected on demand when a piezo crystal coupled to that chamber flexes on apphcation of voltage. While the principle of operation is simple, producing a viable print head is not. The print heads themselves are complex constructions and have specific requirements on the fluid to enable rehable operation. 

Figure 8 Hydrologic parameters defining pore-water flow in the Luquillo regolith (a) moisture saturation, (b) matric potential describing capillary tension, and (c) experimental hydrologic conductivities as functions of moisture saturation at several regolith depths (after White et aL, 1998).

One might anticipate that if the water in the cell wall were to be frozen and then the ice were to be subliminated off there should be no liquid capillary tension, no cell wall shrinkage and it should be possible to create a porous cell wall. However, sublimating the water molecules from the cell wall at -20°C does not prevent collapse of the internal pore structure (Merchant, 1957). This implies that the cell wall water is not actually frozen at this temperature the cell wall still shrinks and very little internal surface is created. Indeed there is evidence (Tarkow, 1971) that at least some adsorbed water does not lose the mobility characteristic of the liquid phase until very low temperatures (<-80°C). Of course water in the lumens behaves like bulk water and freezes at a temperature between -0.1°C and -2.0°C, depending on the concentration of dissolved sugars in the sap. 

Figure 8.4. Water and diethylamine have both hydrogen bond donor and acceptor properties through the -OH or = NH groups. They cross-link through hydrogen bonds and can withstand considerable capillary tension. Pyridine has only hydrogen bond acceptor properties and cannot cross-link with itself. Pyridine cannot withstand large capillary forces the energy storage capacity of the pyridine-saturated system is small (Thomas and Krmgstad, 1971).

Mechanical pulp mills are normally on the same site and integrated with paper production, e.g. of newsprint. Chemical pulp mills can be integrated or sell market pulp that is flash dried and baled. Here, the capillary tension on drying pulls baek the loose fibrillated wall elements to re-bond to the external surface of the fibre, and... 

Solvent can be associated with the solid in different chemical and physical states. It can be part of the crystalline structure, chemically adsorbed in solid interior or physically penetrated/adsorbed inside the interstices of the solid. The interstices of the solid are subjected to a large capillary tension upon drying. The force exerted to solid can be estimated by Laplace s law/ ... 

Figure 10. Illustration of drying process (24). Capillary tension develops in liquid as it stretches to prevent exposure of the solid phase by evaporation, and the network is drawn back into liquid (a). The network is initially so compliant that little stress is needed to keep it submerged, so the tension in the liquid is low, and the radius of the meniscus (rc) is large (b). As the network stiffens, the tension rises as rc decreases. At the critical point, the radius of the meniscus becomes equal to the pore radius the constant rate period ends and the liquid recedes into the gel (c). (Reproduced with permission from reference...

Polar solvent drying by using acetone and alcohols involves the slow replacement of the waterlogging water with increasing concentration of the solvents. The low surface tension of the solvents lowers the capillary tension forces that cause collapse during the evaporation of the waterlogging water from the wood. 

It was supposed, that molecules of alcohol with relatively long hydrocarbonaceous radicals and one functional OH-group would be adsorbed on an oxyhydroxides surface and in result of its hydrophobization reduced forces of capillary tension at removal of a dispersion medium. 

Water held by capillary tension in pores of diameters smaller than 50 nm wiU evaporate at lower values of the relative humidity as the diameter of the pores decreases. Indicatively, values of relative humidity from 95 % to 60 % are required when the diameter of capillary pores decreases from 50 nm to 5 nm [17]. In this case, evaporation can produce significant shrinkage of the cement paste. In addition, the mobihty of ions (thus the electrical conductivity of the solution in these micropores) is affected by chemical and physical interactions between the Hquid and the solid and is therefore lower than that of a solution of the same composition. 

The maximum capillary tension in the liquid occurs when the radius of the meniscus is sufficiently small to fit into the pore, that is, if a = 0°, then the moistening of solid surface is perfect and r = -R. If a = 90°, then the liquid does not wet the solid and the liquid/gas interface is fiat (r -oo) and p = 0. 

As one can conclude from formulas (62.2) and (62.3), capillary tension is expected to be smaller for the larger than for smaller porosities. 

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