Principles, technologies and droplet-based microfluidic devices. Part 2 (review)

The active and passive methods and the devices for coalescence of the droplets are considered in the second part of the review. Particular attention is paid to the methods of processing and surface modification of polydimethylsiloxane (the most frequently used material in microfluidics) in order to create the conditions for the stable droplet generation. Depending on the type of the emulsion (water drops in oil transport flow or vice versa) the surface should be hydrophilic or hydrophobic. The surface with the different wetting properties on the various local sections is required at generation of double emulsions. A single part describes the methods of stabilizing the emulsions. Usually the surfactants used for the droplets stabilizing. The surfactant reduces the surface tension between the two fluids through the formation of the oriented film at their interface. However, the choice of an appropriate surfactant should be carefully carried out, since these substances can affect on the components of sample and biomolecules.