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Electro Osmosis |
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In 1809, F.F. Reuss originally demonstrated electro-osmosis in an experiment that showed that water could be
forced to flow through a clay-water system when an external electric field was applied to the soil. Research has since shown
that flow is initiated by the movement of cations, (positively charged ions), present in the pore fluid of clay, or similar
porous medium such as concrete; and the water surrounding the cations moves with them.
Electro-osmosis can be used to arrest or cause flow of water as well as the ions in it. Electro-osmosis
has been used for many years in civil engineering to dewater dredgings and other high-water content waste solids, consolidate
clays, strengthen soft sensitive clays, and increase the capacity of pile foundations. It has also received significant attention
in the past 5 years as a method to remove hazardous contaminants from groundwater or to arrest water flow. The basic physics
and chemistry of electro osmosis can be found in several textbooks and treatises (e.g. Glasstone, 1946 and Tikhomolova, 1993).
Electro osmotic systems for waterproofing
masonry walls were introduced in the 1960’s by the Europeans, (Adams 1978, Smith, 1984). The first applications were to prevent
rising damp, (the wicking up of soil moisture by masonry due to capillary action). To prevent the upward movement
of water, these systems established an electric field at the point at which a damp course is installed. “Active” systems that supplied
a direct current, and “passive” systems that used the natural electrical potential between the saline saturated wall and the earth
were installed. The natural electrical potential difference was claimed to halt the migration of moisture above a copper strand
installed in the mortar joint between two courses of brick. In practice, however, there was little proof that the passive system
worked and ionisation of the copper anode resulted in physical depletion. Early active systems did work, but were also subject to
rapid copper anode corrosion. The introduction of newer conductive materials such as platinised titanium overcame these problems
and, provided that a calculated controlled D.C. current was used, longevity was assured.
Electrokinesis
Electrokinesis is the particle or fluid transport produced by an electric field acting on a fluid having a net mobile charge.
Electrokinesis was first observed by Reuss in 1809 and has been studied extensively since the 19th century. Such study is known as
electro-hydrodynamics or electro-kinetics, and was documented by Thomas Townsend Brown in 1921.
It was later refined in scientific terms during the 1930s in conjunction with Dr. Paul Alfred Biefeld. The flow rate in such a
mechanism is linear in the electric field. Electrokinesis is of considerable practical importance in micro-fluidics,
since it offers a way to manipulate and convey fluids in microsystems using only electric fields, with no moving parts.
If the electrodes are free to move within the fluid, while keeping their distance fixed from each other, then such a force will
actually propel the electrodes with respect to the fluid.
Electrophoresis
The migration of electrically charged particles in solution or suspension in the presence of an applied electric field. Each particle
moves toward the electrode of opposite electrical polarity. For a given set of solution conditions, the velocity with which a particle
moves divided by the magnitude of the electric field is a characteristic number called the electrophoretic mobility. The electrophoretic
mobility is directly proportional to the magnitude of the charge on the particle, and is inversely proportional to the size of the
particle. An electrophoresis experiment may be either analytical, in which case the objective is to measure the magnitude of the
electrophoretic mobility, or preparative, in which case the objective is to separate various species which differ in their electrophoretic
mobilities under the experimental solution conditions.
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