<P> Particles will, in effect, "stick" to the surface of an object moving through it . This layer of particles is called the boundary layer . At the surface of the object, it is essentially static due to the friction of the surface . The object, with its boundary layer is effectively the new shape of the object that the rest of the molecules "see" as the object approaches . This boundary layer can separate from the surface, essentially creating a new surface and completely changing the flow path . The classical example of this is a stalling airfoil . The delta wing image clearly shows the boundary layer thickening as the gas flows from right to left along the leading edge . </P> <P> As the total number of degrees of freedom approaches infinity, the system will be found in the macrostate that corresponds to the highest multiplicity . In order to illustrate this principle, observe the skin temperature of a frozen metal bar . Using a thermal image of the skin temperature, note the temperature distribution on the surface . This initial observation of temperature represents a "microstate". At some future time, a second observation of the skin temperature produces a second microstate . By continuing this observation process, it is possible to produce a series of microstates that illustrate the thermal history of the bar's surface . Characterization of this historical series of microstates is possible by choosing the macrostate that successfully classifies them all into a single grouping . </P> <P> When energy transfer ceases from a system, this condition is referred to as thermodynamic equilibrium . Usually this condition implies the system and surroundings are at the same temperature so that heat no longer transfers between them . It also implies that external forces are balanced (volume does not change), and all chemical reactions within the system are complete . The timeline varies for these events depending on the system in question . A container of ice allowed to melt at room temperature takes hours, while in semiconductors the heat transfer that occurs in the device transition from an on to off state could be on the order of a few nanoseconds . </P>

When would a gas be described as a vapor