Microfluidic Drops as Microreactors
Charles N. Baroud
from: Lab-on-a-Chip Technology (Vol. 2): Biomolecular Separation and Analysis (Edited by: Keith E. Herold and Avraham Rasooly). Caister Academic Press, U.K. (2009)
The use of individual droplets to transport reagents in microchannels solves three of the fundamental difficulties encountered in continuous flow microfluidic reactors: (i) The reagents are trapped inside the droplets, limiting their dispersion, (ii) mixing of species is performed by using the flow field inside the drop, and (iii) the ability to individually manipulate the drops allows for precise manipulation of small volumes of reagents. These advantages come at the price of increased complexity of the flows and of some new fluid mechanical issues. In this chapter, we begin by describing the advantages and by raising these fundamental issues, namely the determination of the shape, velocity, and transport properties within the drops. At the same time, ways of reducing the complexity associated with the fluid mechanics and the physical-chemistry are discussed. Later, we consider methods for producing and manipulating drops in microchannels with the aim of providing ways to perform reactions inside the drops. Two approaches to manipulation are discussed: passive manipulation, which relies on the channel geometry, and active manipulation which involves external forcing through electrical, mechanical, or optical methods. Drop manipulation is based on certain fundamental steps, namely the production, division, merging, or storing of drops, as well as the mixing of their contents. The implementation of these steps by passive methods is shown, which offers robust and simple control over many operations by taking advantage of careful design of the the microchannel geometry. This is followed by a discussion of active control methods which allow the dynamic tuning of the operations performed by passive means, as well as providing more complex operations. For instance, operations such as drop sorting are shown, as well as the synchronization of two drop formations or switching the order of two drops. These operations are only possible by introducing active control into the channels. Finally, example implementations on cellular and biochemical systems are discussed, as well as a discussion of future trends read more ...