Background

Electrophoresis is the process whereby suspended particles travel through a fluid in response to an applied electric field. The particles themselves carry an electric charge.

The direction that the particle travel is dictated by their charge. Positively charged particles will migrate towards the negatively charged electrode or cathode by a process called catophoresis. Similarly, negatively charged particles will move towards the anode via anophoresis.
Electrophoresis versus Electroplating

Electrophoresis and electroplating are similar processes. The differences are outlined in table 1.

Deposition Technique


Electrophoresis


Electroplating

Moving Species


Solid Particles


Ions

Charge Transfer on Deposition


None


Ion Reduction

Required Conductance of Liquid Medium


Low


High

Preferred Liquid


Organic


Water

Typical Deposition Rate (µm/min)


~1000


~0.1
Charging of Particles

Particles can become charged by the addition of the liquid medium. Mechanisms leading to charged particles may include:

· Selective adsorption of ions onto the particles’ surface

· Dissociation of ions from the solid phase into the liquid

The charge on the particle depends not only which ions are present, but which mechanism dominates. E.g. if beta alumina particles are suspended in alcohol, with only a small amount of water present, the particles acquire a net positive charge, thus:

Milling of beta alumina results in a polarity reversal:

However, at a particular water content, the effects are cancel each other out and no deposition will take place.
Particle Size

Particles less than 1µm in diameter behave as colloids, i.e. they tend to be largely unaffected by gravity and do not settle, as do larger particles. Brownian motion also assists them in being able to stay in suspension. This makes them extremely stable and often the repulsive forces between them are not overcome by the electric field, so deposition cannot occur.

One of the conditions for electrophoresis to take place is that particles remain suspended in the liquid. Therefore, particles to be deposited by electrophoresis are typically 1-20µm in diameter. However, for larger particles to be deposited, either a very strong surface layer must be obtained or the double layer region must be increased in size. These conditions are satisfied when the electrolyte is low in concentration, typically when liquids are of a low dielectric constant.
Factors Affecting Deposition Rates

The main factors affecting the amount of material deposited on an electrode via electrophoresis are:

· Surface properties of the colloidal particles or zeta potential

· Electrical properties of the suspending media such as dielectric constant

· Properties of the suspension such as solids content and viscosity

· Particle concentration

· Applied voltage and time

· Surface area of the electrode

· Distance between the electrodes

Other factors such as pH of the liquid, temperature, particle surface topography and chemical environment may also play a role in determining the deposition rate for a given system.

The rate of deposition can be approximated using Hamakers equation:

where:

M=Mass deposited in time t

T = Deposition time

a = co-efficient representing the fraction of particles near the electrode that are being deposited

A = Surface area of the electrode

C = particle concentration in the suspension (kg/m3)

µ = Electrophoretic mobility (m2/Vs)

E = Electric field (V/m)
Double Layers

At the interface between two planes, an electric double layer forms, where each of the opposing faces carries a different charge similar to a parallel plate capacitor. Such a layer exists at all boundaries between different phases, such as between solids and liquids. This implies that such a layer exists on the surface of all particles in suspension.
Applications

The main application of electrophoresis is for biomolecular separation and detection, although it can also be used for depositing coatings