Fluid Properties and Dynamics

Although this chapter is concerned specifically with fluid flow resulting from surface and interfacial tension effects, it will be useful to introduce briefly some basic ideas about the nature and properties of fluids, especially hquids. In order to understand the concepts involved it is necessary to understand the language so that a number of terms must be introduced. 

Mechanics is a field of science concerned with the movement of bodies and the specific conditions controlhng that motion. It is commonly divided into two subsections (1) kinematics, concerned with the geometry of motion with no concern for the forces involved and (2) dynamics, which concerns itself with the forces acting to cause the motion. 

In a physical sense a fluid may be defined as a state of matter in which the atoms or molecules composing the phase have a spacing and mobility greater than that found in the sohd that allows a relatively free (compared to the solid) movement of the units when an external force is apphed that is, in a fluid the range of motion is greater than that in a solid. A useful definition 

FIGURE 6.1. Capillary flow can be cxjmpared to hydraulic flow. When there exists an unbalanced pressure in a hydrauUc system (a) flow occurs until the pressure is balanced. In a capillary system (b), where pressure difference is a result of different radii of curvature, surface tension, and related effects [Eq. (6.1)], flow also occurs until the pressure balance is achieved, or until no mechanism remains for reaching equilibrium. 

Fluids are divide into two subclasses liquids and gases. A hquid occupies a definite volume, independent of the total volume available in the container. It can have a free surface, such as that of a bowl of water, with the resulting surface properties introduced in Chapter 2. A gas, on the other hand, can have no free surface and will, in general, completely fill the volume of its container, although in some circumstances there may exist a density gradient due to external forces. While the terms gas and vapor are routinely used interchangeably, the usual convention is that a vapor is relatively easily condensed to a liquid while a gas would require significant effort for such a process. 

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