Evaporation

Evaporation is one of the two forms of vaporization. It is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state. It's the opposite process of condensation.

The thermal motion of a molecule of liquid must be sufficient to overcome the surface tension and evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface. Evaporation therefore, proceeds more quickly at higher temperature, at higher flow rates between the gaseous and liquid phase and in liquids with lower surface tension (i.e. higher vapor pressure). Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant, the rate of evaporation is limited. Also, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases. This phenomenon is also called evaporative cooling. This is the reason that evaporating sweat cools the human body.

Gas has less order than liquid or solid matter, and thus the entropy of the system is increased, which always requires energy input. This means that the enthalpy change for evaporation (ΔH_evaporation) is always positive. Evaporation is a cooling process.

Evaporation is a critical component of the water cycle, which is responsible for clouds and rain. Solar energy drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. In hydrology, evaporation and transpiration (which involves evaporation within plant stomata) are collectively termed evapotranspiration.

Evaporative equilibrium

If the evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapour above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapour increases. When the process of escape and return reaches an equilibrium, the vapour is said to be "saturated," and no further change in either vapor pressure and density or liquid temperature will occur.

Evaporation of water

Evaporation is the process where a liquid, in this case water changes from a liquid to a gaseous state. It is a misconception that at 1 atm, water vapor only exists at 100 °C. Water molecules are in a constant state of evaporation and condensation flux near the surface of liquid water. If a surface molecule receives enough energy, it will leave the liquid and turn into vapour pending an allowable vapor pressure. Under a pressure of 1 atm, water will boil at 100 °C.

The ratio of the heat loss from a pond by evaporation to the heat loss due to convection, independent of windspeed, is given by:

{Q_{c,pa} \over Q_e} = {0.46(T_p -T_a) \over P_{wp} - P_a}{p \over 760}

where $Q_{c,pa}$ is the heat loss from the pond by convection, in W/(m²·K), $Q_e$ is the heat loss from the pond by evaporation, in W/(m²·K), $T_p$ and $T_a$ are the Kelvin (or Celsius) temperatures of the water and air, and $P_{wp}$ and $P_a$ are the vapor pressures of the pond surface and air, and $p$ is the barometric pressure, with all pressures in mm Hg. (Bowen, 1926)

Factors influencing rate of evaporation

Concentration of the substance evaporating in the air. If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
Concentration of other substances in the air. If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
Flow rate of air. This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. In addition, molecules in motion have more energy than those at rest, and so the stronger the flow of air, the greater the evaporating power of the air molecules.
Concentration of other substances in the liquid. If the liquid contains other substances (such as salts), it will have a lower capacity for evaporation. This is due to Raoult's law.
Temperature of the substance. If the substance is hotter, then evaporation will be faster.
Inter-molecular forces. The stronger the forces keeping the molecules together in the liquid or solid state the more energy that must be input in order to evaporate them.

Applied evaporation

Forced evaporation

Forced evaporation or distillation is a process used in the separation of mixtures, in which a mixture is heated to drive off the more volatile component with a higher vapor pressure.

Combustion vaporisation

The fuel droplets vaporize as they receive heat by mixing with the hot gases in the combustion chamber. Heat(energy) can also be received by radiation from any hot refractory wall of the combustion chamber.

Film deposition

Evaporation is a common method of thin film deposition used in industry. Sputtering tends to be slower compared to evaporative deposition. However plastic substrates typically cannot tolerate the bombardment with energetic neutral atoms that unavoidably occurs in a sputter chamber. An important example of an evaporative process is the production of aluminized PET film packaging film in a roll-to-roll web system. Often, the aluminum layer in this material is not thick enough to be entirely opaque since a thinner layer can be deposited more cheaply than a thick one. The main purpose of the aluminum is to isolate the product from the external environment by creating a barrier to the passage of light, oxygen, or water vapor.

For some applications, the fact that evaporative deposition proceeds in a line-of-sight fashion is an important advantage. The energy distribution of evaporated material tends to be Maxwellian, with a temperature derived from the evaporation source. In contrast, the plasmas encountered in sputtering systems are far from thermal equilibrium and may have high-energy tails that contain particles with large random velocities. Evaporation is therefore a gentler process with a better defined beam of source material that can be used to coat just one side of a substrate or even the side of etched surface features, as in MEMS processing.

The three main kinds of evaporation are thermal, electron-beam and resistive. In the thermal method, the crucible that holds the source material is radiatively heated by a filament that winds around it. In the electron-beam method, the current that heats the crucible is boiled off a filament and is attracted to the crucible by a high voltage. Electron-beam evaporation is used with the highest melting elements. Resistive evaporation is accomplished by passing a large current through a wire or foil of the material that is to be deposited. Molecular beam epitaxy is a particularly sophisticated kind of thermal evaporation.

External links

References

Semiconductor Devices: Physics and Technology, by S.M. Sze, ISBN 0471333727, has an especially detailed discussion of film deposition by evaporation.

Physical chemistry | Thermodynamics | atmospheric thermodynamics | Materials science | Thin film deposition

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