Liquid flow is an essential element of most
LOC systems. Liquid flow can be single-phase flow through microchannels or multi-phase flow of droplets on a surface. For single phase flow, the flow can be pressure driven, in which the pressure driving force is supplied by an external pump, or the fluid can be pumped by electroosmotic means where the motion of liquid is induced by an applied axial potential along a capillary tube or microchannel. Electroosmotic pumping depends on the electric double layer that forms in an electrolyte adjacent to a charged surface.
Alternately, individual droplets can be moved and manipulated on a surface (e.g. combined, separated or transported) by electric fields. Droplets form on a surface because the geometry of a droplet minimizes the energy of the system (including the energy of the liquid surface, exposed to the vapour phase, and the energy of the attraction between the liquid and the solid surface). Droplets can be manipulated on a surface in two different ways: 1) electrowetting, and 2) dielectrophoresis. Both of these techniques are actuated by manipulating electric fields around the droplets.
Electrowetting is based on the attractive forces between a solid surface and a liquid. When the attraction between the liquid and the solid surface is weak (hydrophobic for an aqueous system), then the droplet tends toward a spherical shape due to the dominance of surface tension energy. The surface force interaction with the fluid (i.e. the hydrophobicity) can be controlled by electric fields (electrowetting).
Dielectrophoresis requires alternating (AC) fields which induce a dipole in a discrete droplet. The droplet then experiences a net force due to the induced dipole when the frequency of the AC field is selected appropriately. Dielectrophoresis can be used to manipulate droplets on a surface using an array of surface electrodes, similar to that required for electrowetting.
from Herold and Rasooly (Eds)
in Lab-on-a-Chip Technology (Vol. 2)Further reading:
Labels: lab on a chip, Lab-on-a-Chip Technology, LOC