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Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. [1] A high concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidity affects rate of evaporation of water. [ 2 ]
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment.
Mass transfer is the net movement of mass from one location (usually meaning stream, phase, fraction, or component) to another.Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane filtration, and distillation.
The Hertz–Knudsen equation describes the non-dissociative adsorption of a gas molecule on a surface by expressing the variation of the number of molecules impacting on the surfaces per unit of time as a function of the pressure of the gas and other parameters which characterise both the gas phase molecule and the surface: [1] [2]
is the heat flux from the gas to the droplet surface (J.s −1) is the latent heat of evaporation of the species considered (J.kg −1) Analytical expressions for the droplet vaporization rate, ˙, and for the heat flux are now derived. A single, pure, component droplet is considered and the gas phase is assumed to behave as an ideal gas.
The total transport rate of the species is then given by a summation of the Stefan flow and diffusive contributions. An example of the Stefan flow occurs when a droplet of liquid evaporates in air. In this case, the vapor /air mixture surrounding the droplet is the flowing fluid, and liquid/vapor boundary of the droplet is the interface.
When a temperature is reached such that the sum of the equilibrium vapor pressures of the liquid components becomes equal to the total pressure of the system (it is otherwise smaller), then vapor bubbles generated from the liquid begin to displace the gas that was maintaining the overall pressure, and the mixture is said to boil.
The rate of evaporation heat loss is directly related to the vapor pressure at the skin surface and the amount of moisture present on the skin. [44] Therefore, the maximum of heat transfer will occur when the skin is completely wet.