Liquid, effect forces

In the body of a liquid, intermolecular forces pull the molecules in all directions. At the surface of the liquid, the molecules pull down into the body of the liquid and from the sides. There are no molecules above the surface to pull in that direction. The effect of this unequal attraction is that the liquid tries to minimize its surface area. The minimum surface area for a given quantity of matter is a sphere. In a large pool of liquid, where sphere formation is not possible, the surface behaves as if it had a thin stretched elastic membrane or skin over it. The surface tension is the resistance of a liquid to an increase in its surface area. It requires force to break the attractive forces at the surface. The greater the intermolecular force, the greater the surface tension. Polar liquids, especially those that utilize hydrogen bonding, have a much higher surface tension than nonpolar liquids. 

Results from studies using calcium carbonate showed that, relative to dry powder, the effect of conditioning at 55% relative humidity (at 20 °C) increased agglomerate strength threefold, which was attributed to the development of liquid-bridge forces [86]. 

Although, in a vacuum, only the Van Dcr Waals Inrccs arc important in liquids, all forces may operate simultaneously. In liquids, it is extremely difficult to separate the effects of each of the aforementioned forces. 

Another problem encountered in processing of filled composites by the RIM-process is sedimentation of the filler in tanks, transport tubes, etc. In order to overcome this problem the tanks are fitted with stirrers, so that the filled liquid is forced to circulate in the intervals between shots. Finally, it is necessary to consider the possibility of breakdown of the filler structure if this occurs, there is usually an adverse effect on the properties of a final product. 

When a liquid is forced through a channel or over a surface maintained at a temperature greater than the saturation temperature of the liquid, forced-convection boiling may result. For forced-convection boiling in smooth tubes Rohsenow and Griffith [6] recommended that the forced-convection effect be computed with the Dittus-Boelter relation of Chap. 6 [Eq. (6-4)] and that this effect be added to the boiling heat flux computed from Eq. (9-33). Thus... 

In the case of a liquid-gas interface, molecules of the liquid in the boundary can only develop attractive cohesive forces with molecules situated below and adjacent to them. They can develop attractive adhesive forces with molecules of the gaseous phase. However at the gas-liquid interface, these adhesive forces are quite small. The net effect is that molecules at the surface of the liquid have potential energies greater than those of similar molecules in the interior of the liquid and experience an inward force toward the bulk of liquid. This force pulls the molecules of the interface together and the surface contracts. 

In a sedimenting centrifuge, a continuous liquid phase moves through the rotor. To accomplish a useful separation, the discontinuous phase, either the insoluble solids or the immiscible liquid drops (or both), must move in a direction different from the flow of the continuous liquid. Stokes law is usually applied to describe this relationship. The effective force accelerating the particle in a centrifugal field is then given as ... 

FIGURE 11.44 Effect of surface vibration on heat transfer to liquids with forced flow. 

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