Capillary Driving Forces in Liquid-Fluid Systems

FIGURE 6.4. When a liquid is placed on a solid surface, it may form an even duplex film or form a drop with a finite contact angle. The angle, measured through the liquid, may be small (a) or large (fc), reflecting the extent of interaction between soUd and liquid. 

CAPILLARY DRIVING FORCES IN LIQUID-FLUID SYSTEMS 

Of the driving forces for capillary action mentioned above, the most fundamental are those of interfacial tension and related effects (e.g., contact angle). As pointed out in Chapter 2, a liquid-fluid interface behaves as if it is an elastic film stretched over (or between) the two phases and resisting any more stretching to produce greater interfacial area. The tension results fundamentally from the imbalance in the forces acting on the molecules at the interface, which tend to pull the molecules back into the bulk phases. At equilibrium, the surface 

Mathematically, an interface is a two-dimensional region. In reality, it will be three-dimensional, although the third dimension may have the thickness of only one or two molecules. Because it is three-dimensional, the interfadal region may be treated in the context of hydrostatics, or in terms of molecular forces and distribution functions. Alternatively, a thermodynamic approach may be taken to arrive at the same conclusions. 

The fact that a tension exists at a liquid-fluid interface imphes that, if it is curved, there will be a difference in hydrostatic pressure across the interface. Laplace derived an expression for the pressure difference across a curved interface in terms of surface tension and curvature. The equation, referred to as the Laplace equation, is 

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CAPILLARY DRIVING FORCES IN LIQUID-FLUID SYSTEMS 115... 

Capillary flow systems of most practical interest are those that involve a solid, a liquid, and a second fluid phase. In the absence of other external forces, the net driving force for capillary flow in such a system will be controlled by... 

The working portion of the viscometer resembles that of a conventional Ostwald-type viscometer. The complete apparatus consists of a liquid feed reservoir, a capillary, and efflux bulbs in series. The complete unit is submersed in a constant-temperature oil bath. The driving head which forces the liquid through the capillary consists of a controlled nitrogen gas pressure system which is not shown in Figure 1. To initiate an experiment, the fluid reservoir which has a capacity of 200 ml is filled with the solution to be studied, and the fluid reservoir cap with an 0-ring seal is put into place. Stirring is provided to facilitate heat transfer to the sample solution. 

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