Interdisciplinary Applied Mathematics

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7.1 Electrokinetic Effects


The electrokinetic phenomena can be divided into the following four categories (Probstein, 1994):


• Electroosmosis is the motion of ionized liquid relative to the stationary charged surface by an applied electric field.


• Electrophoresis is the motion of the charged surfaces and macromolecules relative to the stationary liquid by an applied electric field.


• Streaming potential is the electric field created by the motion of ionized fluid along stationary charged surfaces (the opposite of electroosmosis).


• Sedimentation potential is the electric field created by the motion of charged particles relative to a stationary liquid (the opposite of electrophoresis).


Electrokinetic effects are important for micro- and nanoscale transport applications. For example, electroosmosis enables fluid pumping and flow control using electric fields, eliminating the need for mechanical pumps or valves with moving components. Furthermore, comparisons between the electroosmotically and pressure-driven flows reveal several important differences. First, the volumetric flowrate (per channel width) in electroosmotic flows varies linearly with the channel height (h), enabling useful flowrates in microchannels and nanochannels with low-driving electric fields. However, the flowrate (per unit width) in pressure-driven flows varies as the cube of the channel height (h3). This requires unrealistically large pressure drops, making pressure-driven flows impractical for nanochannel applications. Second, the velocity profiles for electroosmotic flows are (mostly) uniform (plug-like), while the pressure-driven flow velocity profiles are parabolic. This difference has significant effects in species transport and dispersion in microfluidic applications (see Section 7.5.3).


Typical physical and electrochemical conditions employed in electrokinetic flows    are presented    in    Table    7.1.    Since    we    are    mostly interested    in


bulk fluid and particle/species transport, we cover electroosmotic and electrophoretic transport in detail. We also present dielectrophoresis, which enables separation and detection of similar-size particles based on their polarizability.

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