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Substances that flow under shear stress and have a density range from essentially zero (gases) to solidlike values (liquids). Fluids are one of the two major forms of matter. Solids, the other form, generally deform very little when shear forces are applied, and their densities do not change significantly with pressure or temperature.
The distinction between solids and fluids is easily seen in substances and mixtures which show a well-defined melting process. For substances with large molecules, such as polymers, ceramics, and biologicals, this distinction is less clear. Instead, there is a slow evolution of structure and of resistance to flow as temperature or some other variable is changed. See Glass transition
Molecular density varies greatly in fluids and is their most important characteristic. The distinction between vapors (or gases) and liquids is most clear for substances and mixtures that show well-defined vaporizing (boiling) and condensing processes. The high-density liquid boils to make a low-density gaseous vapor. The illustration shows the pressures and temperatures for which pure substances are single phases. At the conditions of the lines between the single-phase regions, two phases can be observed to coexist. At the state of intersection of the lines (the triple point), three phases can coexist. For most substances, the triple-point pressure is well below atmospheric. However, for carbon dioxide, it is very high, so that dry ice sublimes rather than melts, as water ice does. Beyond the end of the liquid-vapor (saturation or vapor-pressure) line, vaporization and condensation cannot be observed. The state at the end of this line is called the critical point, where all the properties of the vapor and liquid become the same. There is no such end to the solid-liquid (or melting) line, because the solid and fluid structures cannot become the same. See Critical phenomena, Phase transitions
Mixtures of fluids show the same general density and multiphase behavior as pure fluids, but the composition is an extra variable to be considered. For example, the density differences between the vapor and the liquid phases cause them to have different relative amounts of the components. This difference in composition is the basis of the separation process of distillation, where the vapor will be richer in some components while the liquid will be richer in others. It is also possible for mixtures of liquids to be partially or nearly wholly immiscible, as are water and oil. The separation process of liquid extraction, used in some metal-purification systems and chemical-pollution-abatement processes, depends on different preferences of chemical solutes for one liquid phase or the other. See Phase rule
The usual observation of the presence of more than one fluid phase is the appearance of the boundary or interface between them. This is seen because the density or composition (or both) changes over a distance of a few molecular diameters, and this variation bends or scatters light in a detectable way. At the interface, the molecules feel different forces than in the bulk phases and thus have special properties. Energy is always required to create interface from bulk, the amount per unit area being called the interfacial tension. Water is a fluid with an extremely high vapor-liquid (or surface) tension; this surface tension allows insects to crawl on ponds and causes sprinkler streams to break up into sprays of droplets.
In mixtures, the molecules respond differently to the interfacial forces, so the interfacial composition is generally different from that of the bulk. This has also been the basis of a separation process. If the difference of composition is great enough and it varies with time and position because of evaporation of one or more of the components, the interfacial forces can push the fluid into motion, as can be observed on the walls of a glass of brandy (the Marangoni effect). Some substances strongly adsorb at the interface because their chemical structure has one part that prefers to be in one phase, such as water, and another part that prefers the other phase, such as oil or air. Such surfactants or detergents help solubilize dirt into wash water, keep cosmetics and other immiscible mixtures together, and form foams when air and soapy water are whipped together.
Besides the relations among pressure, density, temperature, and composition of static or equilibrium fluids, there are also characteristics associated with fluid flow, heat transfer, and material transport. For example, when a liquid or gas flows through a tube, energy must be supplied by a pump, and there is a drop in pressure from the beginning to the end of the tube that matches the rise in pressure in the pump. The pump work and pressure drop depend on the flow rate, the tube size and shape, the density, and a property of the molecules called the viscosity. The effect arises because the fluid molecules at the solid tube wall do not move and there are velocity gradients and shear in the flow. The molecules that collide with one another transfer momentum to the wall and work against one another, in a sort of friction which dissipates mechanical energy into internal energy or heat. The greater the viscosity, the greater the amount of energy dissipated by the collisions and the greater the pressure drop. If only chemical constitution and physical state are needed to characterize the viscosity, and if shear stress is directly proportional to velocity gradient, the fluid is called newtonian and the relation for pressure drop is relatively simple. If the molecules are large or the attractive forces are very strong over long ranges, as in polymers, gels, and foods such as bread dough and cornstarch, the resistance to flow can also depend on the rate of flow and even the recent deformations of the substance. These fluids are called non-newtonian, and the relationship of flow resistance to the applied forces can be very complex. See Fluid flow, Newtonian fluid, Non-newtonian fluid, Viscosity
Another fluid-transport property, thermal conductivity, indicates the ability of a static fluid to pass heat from higher to lower temperature. This characteristic is a function of chemical constitution and physical state, as is the viscosity. In mixtures, these properties may involve simple or complex dependence on composition, the variation becoming extreme if the unlike species strongly attract each other. The values of both properties increase rapidly near a critical point. See Conduction (heat), Heat transfer
Finally, the ability of molecules to change their relative position in a static fluid is called the diffusivity. This is a particularly important characteristic for separation processes whose efficiency depends on molecular motion from one phase to another through a relatively static interface, or on the ability of some molecules to move faster than others in a static fluid under an applied force. See Diffusion