A colloid is another name for a mixture whose particles are too small to be seen with the naked eye but too large to be molecules. In electrophoretic terms, a colloid is referred to as the matrix -- the solution that also contains the particles being separated. In the case of gel electrophoresis, the matrix is comprised of a mixture of agarose gel and the molecules being separated.
This classic electrophoretic technique utilizes an agarose gel to migrate charged DNA particles of different sizes. Because DNA is negatively charged, it moves through the gel matrix when an electric current is applied. Larger DNA particles move more slowly than smaller particles, so the end product is a distinct row of visible bands in the agarose that create a fingerprint for the particles being tested. Fluorescent labels are typically used to mark particles. This visual often represents DNA testing results in popular television shows.
This separation technique uses a capillary to migrate charged particles with unique radius ratios against an applied electric field. How fast or slow a particle moves with this technique depends on its ionic velocity, and the results reflect this much as gel electrophoresis results illustrate particle size. The CE matrix also consists of an electrolyte -- often an aqueous buffer solution -- and UV or fluorescent labels are used. The related technique, capillary zone electrophoresis (CZE), is strictly limited to ionic compound separation.
Whereas gel electrophoresis and CE use electrical current to move charged particles, dielectrophoresis (DEP) uses radio frequencies to separate cells with different polarizabilities. The cells respond according to their unique characteristics; particles of similar genetic constitution and/or geometric shape respond distinctly enough for researchers to tell them apart. The cells move across an electric field gradient when a dielectrophoretic force is applied. This technique is innovative because it doesn't require particles to be labeled and cells can be used for further study, following isolation and recovery.