Electric field assisted vertical alignment of micro particles in a microfluidic system

The effect of a uniform AC electric field on the vertical alignment of polystyrene micro particles and Escherichia coli bacterial cells has been studied. A microfluidic system consisting of parallel plate electrodes was designed and fabricated for the study. The polystyrene micro particles were found to form single stranded pearl chains in the direction perpendicular to the plate electrodes with uniform separation. The separation was further found to increase in size with increasing electric field frequency and voltage. It is speculated that the formation of pearl chains is due to dipole-dipole interactions, and the variation in chain separation is caused by changes in the electrostatic force between the particles. The electric field was also shown to be effective in aligning E. coli cells in the microfluidic system. Unlike the polystyrene particles, the E. coli cells are non-spherical and the alignment takes place along the long axis of the cylindrical-shaped cell bodies. Temporary pearl chains of E. coli were observed during the alignment process. However, the pearl chains were short lived and break up easily. This is speculated to be due to the fact that the E. coli cells have rotating flagella which allow them to resist the influence of the electric field.