Electrical Switching of Droplets and Fluid Segments

Switching operations in transport of microfluidic compartments are of high interest in miniaturized biotechnology, cell cultivation and screening programs as well as for future applications in miniaturized and automated diagnostics and in particular for automated experiments in ultraminiaturized combinatorial chemistry and combinatorial screenings in multidimensional parameter spaces. In addition to switching by laser actuation, surface forces, by centrifugal forces and electrowetting, electrical switching using electrostatic or dielectric manipulation represents an important class of microfluidic actuation principles. Electrical operations are of particular interest for fast switching and for addressing single selected fluid segments. Thus, they can be used for defining distances and orders of fluid segments and for sorting of droplets and segments in dependence on individual properties. Principles of electrostatic manipulation and the specific conditions for multi phase systems with strong differences in the electrical conductivity and electrochemical behaviour of the involved liquids are described in this chapter. The manipulation by DC fields is compared with the manipulation using AC fields by positive and negative dielectrophoresis. The manipulation by potential switching in Y-shaped micro channels is an example for efficient electrical manipulation of segments without galvanic contact. It can be shown that simple segment manipulation without any electrochemical changing of liquid composition is possible by switching under non-galvanic conditions. It is demonstrated that electronic data sets can be converted into a fluid pattern proving the high reliability of segment operations by potential switching. The robustness related to chemical composition and the applicability to cell suspensions will be shown and the potential for cell cultivation and miniaturized screenings will be discussed. For further development, the possibilities and requirements related to the successive reduction of volumes of fluid segments, downscaling of functional elements and enhancement of switching frequencies are treated. Finally, future tasks for switching coming both from applications with different segment composition, involving cell cultures, multicellular systems, single cell operations, biomolecular diagnostics and from organic and inorganic synthesis, generation and application of nanomaterials, catalysis, single particle processes and supermolecular chemistry are discussed.