Abstract
Even though droplet microfluidics has been developed since the early 1980s, the number of applications that have resulted in commercial products is still relatively small. This is partly due to an ongoing maturation and integration of existing methods, but possibly also because of the emergence of new techniques, whose potential has not been fully realized. This review summarizes the currently existing techniques for manipulating droplets in two-phase flow microfluidics. Specifically, very recent developments like the use of acoustic waves, magnetic fields, surface energy wells, and electrostatic traps and rails are discussed. The physical principles are explained, and (potential) advantages and drawbacks of different methods in the sense of versatility, flexibility, tunability and durability are discussed, where possible, per technique and per droplet operation: generation, transport, sorting, coalescence and splitting.
Highlights
Droplet microfluidics has been around since the early 1980s, but the number of examples that have made it towards commercial applications are limited
The first tool required for droplet microfluidics is, a system to generate droplets in the order of picoliter or even femtoliter [2] volumes
A third electrostatic manipulation technique that needs to be mentioned is the pre-charging of droplets
Summary
Droplet microfluidics has been around since the early 1980s, but the number of examples that have made it towards commercial applications are limited. Great advances have been made since leading to commercial devices for genome sequencing, Polymerase Chain Reaction (PCR) and flow cytometry Each of these techniques uses some form of microfluidics and droplet control for the automation of complex laboratory protocols. One needs a flow geometry (sometimes supplemented with an external field) that causes the continuous (mostly aqueous) phase to form a neck, followed by a break-up This should preferably work in a highly controlled manner leading to a large continuous production of monodisperse droplets. This paper is further organized as follows: we first discuss the existing techniques from the physics point of view, and separately how they have been translated to passive and active manipulations for which are able to execute laboratory protocols for biophysical and biochemical purposes
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