Electrokinetic Phenomena

1
Electrokinetic Phenomena

• Electrokinetic phenomena result from the
  differential movement of two phases where the
  interface is an electrical double layer. The
  region containing the double layered is sheared
  at some distance from the solid surface creating
  a thin film associated with the solid. The
  electrical potential at the shearing plane is the
  zeta potential.

2
Electrokinetic Phenomena

• Electrophoresis a suspended, charged particle
  moves as a result of an applied electrical field
• Sedimentation potential an electrical potential
  created by the movement of charged particles
  through a liquid by gravity
• Electrosmosis a liquid flows along a charged
  surface when an electric field is applied
  parallel to the surface
• Streaming potential an electric potential
  created when a liquid is forced to move along a
  charged surface

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Importance of Electrokinetic Phenomena in Porous
Media

• Streaming potential used to map subsurface flow
  variations faults, oil reservoirs
• Monitoring and prediction of earth quakes
• Water leakage from reservoirs
• Monitoring of volcanoes and earthquakes

4
Electrophoresis

• Electric field applied to a suspension of
  particles carrying a DDL
• Particles move toward electrode of opposite
  charge and counter-ions move the opposite
  direction (except those inside the plane of
  shear)
• In a short time particles move with constant
  velocity total force acting on a particle is
  zero Felectrical Fhydrodynamic friction
• Two other forces oppose the electrical force, a
  force associated with the friction of water
  moving in the direction opposite the particles
  with the counterions and a retarding force caused
  by distortion of the DDL

5
Retarding Forces

• The DDL in front of the moving particle must
  constantly be restored as it is broken off behind
  the particle
• Formation of the DDL at the front of the particle
  takes a finite amount of time
• The DDL is asymmetric resulting in a force
  retarding movement of the particle
• The frictional force associated with movement of
  waters of hydration on counterions is the
  electrophoretic retardation force
• The force resulting from an asymmetric DDL is the
  relaxation force

6
Zeta Potential

• The classical equation relating the
  electrophoretic velocity,v, and the zeta
  potential, ?, ignores the retardation and
  relaxation forces and thus, is only approximate
  in nature

where e is the relative permittivity, E is the
strength of the electrical field and ? is the
viscosity of the fluid
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Streaming Potential
Flow of counterions with liquid
Conduction current in opposite direction of the
flow
Convection current
The flow of liquid through a capillary tube
induces a convection current in the direction of
flow. A conduction current is induced by the
potential difference at the ends of the
capillary. At the stationary state, the
conduction current is equal to the convection.
Evaluation of the currents gives
where Kw is the specific conductance of the
liquid and P is the applied pressure drop in the
capillary per unit length
8
Difficulties Associated with Determining the Zeta
Potential

• Influenced by surface topology
• Surface potential
• Liquid conductivity
• Counterion
• Practically the zeta potential potential at the
  Stern layer
• Slipping plane 3 to 5 A (hydrated counterion
  radius