Biomedical Engineering Reference
In-Depth Information
Table 1
. Classification of AC and DC electrokinetic phenomena
Type of force AC electrokinetics DC electrokinetics
Body force on fluid
Electrothermal
Surface force on fluid
AC electroosmosis
Electroosmosis
Force on suspended
Dielectrophoresis
Electrophoresis
particles
reduce electrolysis. Further, the characteristic voltages necessary to accomplish
useful work are typically of the order of tens of volts—much smaller than what
is required for DC electrokinetics. AC electrokinetics can be classified into three
broad areas: dielectrophoresis (DEP), electrothermal forces, and AC electroos-
mosis (17).
2.
DC ELECTROKINETICS
2.1. Electroosmosis
Electroosmosis is a good place to begin discussing electrokinetic effects
because the geometries involved can be idealized to considering a liquid in con-
tact with a planar wall. When a polar liquid, such as water, and a solid surface
are brought into contact, the surface acquires an electric charge. The surface
charge attracts oppositely charged ionic species in the liquid that are strongly
drawn toward the surface, forming a very thin tightly bound layer of ions, called
the Stern layer, in which the ions in the liquid are paired one for one with the
charges on the surface. Thermal energy prevents the ions from completely neu-
tralizing the surface charge. The surface charge not neutralized by the Stern
layer then influences the charge distribution deeper in the fluid, creating a
thicker layer of excess charges of the same sign as those in the Stern layer,
called the diffuse or Gouy-Chapman layer. Together these two layers are called
the electric double layer, or EDL. Because of the proximity of charges, the Stern
layer is fixed in place while the diffuse layer can be moved. In particular, the
diffuse layer has a net charge and can be moved with an electric field. Conse-
quently, the boundary between the Stern layer and the diffuse layer is called the
shear surface because of the relative motion across it. The potential at the wall is
called the wall potential G
w
, and the potential at the shear plane is called the zeta
potential [. This situation is shown in Figure 1 and is typical of the charge dis-
tributions observed in many microfluidic devices. Both glass- (10) and polymer-
based (18) microfluidic devices tend to have negatively charged or deprotonated
