Introduction

It was shown in Chapter 2 that as the length scale of microchannels approaches the nanoscale, pressure-driven flow becomes increasingly difficult to achieve. The precise cutoff depends on the desired flow rate, but for a volume flow rate of Q = 1 µL/min, electro-osmotic flow (EOF) becomes significantly more efficient around an individual channel height of h = 40 nm. Using the material studied in Chapters 4–8, we can now investigate flows of ionic and biomolecular species in micro- and nanochannels.

Electrokinetic phenomena are generally grouped into four classes:

1. 1. Electro-osmosis (EOF): the bulk motion of a fluid caused by an electric field

2. 2. Electrophoresis: the motion of a charged particle in an otherwise motionless fluid or the motion of a charged particle relative to a bulk motion

3. 3. Streaming potential or streaming current: the potential induced by a pressure gradient at zero current flow of an electrolyte mixture

4. 4. Sedimentation potential: the electric field induced when charged particles move relative to a liquid under a gravitational or centrifugal or other force field

Note that electrophoresis and sedimentation potential refer to particles and electro-osmosis and streaming potential refer to electrolyte solutions. The first three of these are by far the most important for our applications, and in this chapter, we discuss the phenomenon of electro-osmosis first, followed by a discussion of electrophoresis and streaming potential.