A microemulsion is an emulsion which forms spontaneously, without mechanical action, from a mixture of two liquid phases under certain conditions (surfactant concentration, salt content, temperature).

The droplet size is less than the wavelength of visible light, as a result of which microemulsions are transparent, unlike conventional emulsions. A further characteristic is the phase inversion at micro level; i.e. droplets of the emulsified phase can in turn contain droplets of the bulk phase.

The formation of microemulsions is used, for example, in enhanced oil recovery EOR (surfactant flooding), in solvent-free degreasing and in the administration of hydrophobic active ingredients in the pharmaceutical industry.

The interfacial tension of micro-emulsions is extremely low (down to 10-6 mN/m). Spinning drop tensiometers extend to this measuring range, enabling the conditions for the formation of a micro-emulsion to be investigated using spinning drop measurements.