Electroosmotic flow (or electro-osmotic flow, often abbreviated EOF;
synonymous with electroosmosis or electroendosmosis) is the motion of
liquid induced by an applied potential across a porous material,
capillary tube, membrane, microchannel, or any other fluid conduit.
Because electroosmotic velocities are independent of conduit size, as
long as the electrical double layer is much smaller than the
characteristic length scale of the channel, electroosmotic flow will
have little effect. Electroosmotic flow is most significant when in
small channels. Electroosmotic flow is an essential component in
chemical separation techniques, notably capillary electrophoresis.
Electroosmotic flow can occur in natural unfiltered water, as well as
Electroosmotic flow schematic
4.2 Vascular plant biology
6 See also
8 Further reading
Electroosmotic flow was first reported in 1809 by F. F. Reuss in the
Proceedings of the Imperial Society of Naturalists of Moscow. He
showed that water could be made to flow through a plug of clay by
applying an electric voltage.
Clay is composed of closely packed
particles of silica and other minerals, and water flows through the
narrow spaces between these particles just as it would through a
narrow glass tube. Any combination of an electrolyte (a fluid
containing dissolved ions) and an insulating solid would generate
electro-osmotic flow, though for water/silica the effect is
particularly large. Even so, flow speeds are typically only a few
millimeters per second.
Electroosmotic flow is caused by the
Coulomb force induced by an
electric field on net mobile electric charge in a solution. Because
the chemical equilibrium between a solid surface and an electrolyte
solution typically leads to the interface acquiring a net fixed
electrical charge, a layer of mobile ions, known as an electrical
double layer or Debye layer, forms in the region near the interface.
When an electric field is applied to the fluid (usually via electrodes
placed at inlets and outlets), the net charge in the electrical double
layer is induced to move by the resulting Coulomb force. The resulting
flow is termed electroosmotic flow.
The resulting flow from applying a voltage is a plug flow. Unlike a
parabolic profile flow generated from a pressure differential, a plug
flow’s velocity profile is approximately planar, with slight
variation near the electric double layer. This offers significantly
less deleterious dispersive effects and can be controlled without
valves, offering a high performance method for fluid separation,
although many complex factors prove this control to be difficult.
Because of difficulties measuring and monitoring flow in micro fluidic
channels, primarily disrupting the flow pattern, most analysis is done
through numerical methods and simulation. Electroosmotic flow
through micro channels can be modeled after the Navier-Stokes equation
with the driving force deriving from the electric field and not the
pressure differential. Thus it is governed by the continuity equation
displaystyle nabla cdot mathbf U =0
displaystyle rho frac Dmathbf U Dt =-nabla p+mu nabla ^ 2
mathbf U +rho _ e nabla left(psi +phi right),
where U is the velocity vector, ρ is the density of the fluid,
is the material derivative, μ is the viscosity of the fluid, ρe is
the electric charge density, Φ is the applied electric field, and ψ
is the electric field due to the zeta potential at the walls.
Laplace’s equation can describe the external electric field
displaystyle nabla ^ 2 phi =0,
while the potential within the electric double layer is governed by
displaystyle nabla ^ 2 psi = frac -rho _ e epsilon epsilon _ 0
where ε is the dielectric constant of the electrolyte solution and
ε0 is the vacuum permittivity. This equation can be further
simplified using the Debye-Hückel approximation
displaystyle nabla ^ 2 psi =k^ 2 psi ,
where 1 / k is the Debye Length, used to describe the characteristic
thickness of the electric double layer. The equations for potential
field within the double layer can be combined as
displaystyle rho _ e =-epsilon epsilon _ 0 k^ 2 psi .
Electro-osmotic flow is commonly used in microfluidic devices,
soil analysis and processing, and chemical analysis, all of
which routinely involve systems with highly charged surfaces, often of
oxides. One example is capillary electrophoresis, in which
electric fields are used to separate chemicals according to their
electrophoretic mobility by applying an electric field to a narrow
capillary, usually made of silica. In electrophoretic separations, the
electroosmotic flow affects the elution time of the analytes.
Electro-osmotic flow is actuated in a
FlowFET to electronically
control fluid flow through a junction.
It is projected that micro fluidic devices utilizing electroosmotic
flow will have applications in medical research. Once controlling this
flow is better understood and implemented, the ability to separate
fluids on the atomic level will be a vital component for drug
dischargers. Mixing fluids at the micro scale is currently
troublesome. It is believed that electrically controlling fluids will
be the method in which small fluids are mixed.
A controversial use of electro-osmotic systems is being done to
control rising damp in structure of a building. While there is
little evidence to suggest that these systems can be useful in moving
salts in walls, such systems are claimed to be especially effective in
structures with very thick walls. However some claim that there is no
scientific base for those systems, and cite several examples for their
In fuel cells, electro-osmosis causes protons moving through a proton
exchange membrane (PEM) to drag water molecules from one side (anode)
to the other (cathode).
Vascular plant biology
In vascular plant biology, electro-osmosis is also used as an
alternative or supplemental explanation for the movement of polar
liquids via the phloem that differs from the cohesion-tension theory
supplied in the mass flow hypothesis and others, such as cytoplasmic
streaming. Companion cells are involved in the "cyclic" withdrawal
of ions (K+) from sieve tubes, and their secretion parallel to their
position of withdrawal between sieve plates, resulting in polarisation
of sieve plate elements alongside potential difference in pressure,
and results in polar water molecules and other solutes present moved
upward through the phloem.
St Petersburg University
St Petersburg University graduates applied direct electric
current to 10 mm segments of mesocotyls of maize seedlings
alongside one-year linden shoots; electrolyte solutions present in the
tissues moved toward the cathode that was in place, suggesting that
electro-osmosis might play a role in solution transport through
conductive plant tissues.
Maintaining an electric field in an electrolyte requires Faradaic
reactions to occur at the anode and cathode. This is typically
electrolysis of water, which generates hydrogen peroxide, hydrogen
ions (acid) and hydroxide (base) as well as oxygen and hydrogen gas
bubbles. The hydrogen peroxide and/or pH changes generated can
adversely affect biological cells and biomolecules such as proteins,
while gas bubbles tend to "clog" microfluidic systems. These problems
can be alleviated by using alternative electrode materials such as
conjugated polymers which can undergo the Faradaic reactions
themselves, dramatically reducing electrolysis.
Wikimedia Commons has media related to Electro-osmosis.
Electrical double layer
electrical double layer
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