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the fusion of drops or bubbles upon contact with each other in a mobile medium (liquid or gas) or on the surface of a body. It is accompanied by enlargement of drops (bubbles) and is brought about by the effect of forces of inter-molecular attraction. It is a spontaneous process accompanied by a decrease in the free energy of the system. As a result of coalescence, emulsions and foams may cease to exist as disperse systems and undergo complete separation into two macrophases, liquid-liquid or liquid-gas. In a liquid dispersion medium, coagulation frequently precedes coalescence. A special case of coalescence is known as self-adhesion, during which the interface between the agglutinating particles or fused lumps of the plastic polymer disappears as a result of slow diffusion of macromolecules.
Along with isothermal distillation, coalescence of water droplets causes the precipitation of atmospheric residue (rain or dew) from clouds and fog. The coalescence of droplets of paint or lacquer sprayed onto a surface to be painted leads to the formation of a continuous film. Coalescence is the basis of many other production processes and natural phenomena.