Editorial Feature

Electrophoretic Deposition as Coating Technique

In the process of electrophoresis, suspended particles pass through a fluid upon applying an electric field. An electric charge is carried by the particles themselves.

The direction in which the particles travel is controlled by their charge. Particles that are positively charged will move toward the cathode (i.e. negatively charged electrode) through a process known as cataphoresis. In a similar way, particles that are negatively charged will migrate toward the anode through anaphoresis.

Electrophoresis vs. Electroplating

Both electroplating and electrophoresis are similar processes. Table 1 outlines the differences between the two.

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

The addition of a liquid medium can result in the formation of charged particles. Mechanisms resulting in charged particles may include:

  • Dissociation of ions from the solid phase into the liquid phase
  • Selective adsorption of ions onto the surface of the particles

Particle Size

Particles with a diameter of less than 1 µm act as colloids, that is, they are not much affected by gravity and do not settle, unlike larger particles. In addition, these particles remain in suspension form as a result of Brownian motion, making them extremely stable. Usually, the repulsive forces existing between these particles are not overcome by the electric field, and therefore deposition does not take place.

Various conditions can promote electrophoresis. One such condition is that particles should remain suspended in the liquid. Hence, particles that need to be deposited through electrophoresis usually measure 1–20 µm in diameter. But to deposit larger particles, an extremely strong surface layer needs to be obtained or the double layer region has to be increased in size. Such conditions are fulfilled when the electrolyte has low concentration, usually when liquids have a low dielectric constant.

Factors Affecting Deposition Rates

The major factors that have an impact on the quantity of material deposited on an electrode through electrophoresis are mentioned below:

  • Electrical characteristics of the suspending media like dielectric constant
  • Zeta potential or surface properties of the colloidal particles
  • Properties of the suspension, like content and viscosity of solids
  • Time and applied voltage
  • Concentration of particles
  • Distance existing between the electrodes
  • The electrode’s surface area

Other factors like particle surface topography, pH of the liquid, chemical environment, and temperature are also likely to play a role in governing the rate of deposition for a specified system.

Double Layers

An electric double layer forms at the interface between two planes, with each opposing face carrying a different charge similar to a parallel plate capacitor. A layer like that exists at all boundaries between different phases, for example, between liquids and solids. This means the surface of all particles in suspension will have this layer.

Applications

Electrophoresis is mainly used for separating and detecting biomolecules, but it can also be utilized for depositing coatings.

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