Electrophoretic deposition under modulated electric fields: a review

Abstract

Classical electrophoretic deposition (EPD) relies on continuous direct current (CDC) to deposit charged particles on electrodes. In recent decades, modulated electric fields such as pulsed direct current (PDC) and alternating current (AC) have been investigated. This paper reviews EPD under these modulated electric fields and major applications of the deposited microstructures. The paper starts with a short overview of EPD principals such as the electrical double layer of the charged particle, electrophoretic mobility and main suspension parameters including zeta potential, particle size, conductivity, viscosity and stability of the suspension. The EPD mechanisms from the earliest model reported by Hamaker and Verwey to latest models including Sarkar and Nicholson model and influence of the electrohydrodynamics and electroosmosis as well as electrode surface and its electrochemical double layer on the deposition process have been briefly discussed. Two categories of modulated electric fields, PDC and AC fields have been addressed with their advantages and disadvantages. It is found that compared to CDC, PDC offers the advantage of: i) reducing the coalescence between gas bubbles induced by water electrolysis from aqueous suspensions, hence yielding deposition of smooth and uniform coatings, ii) reducing aggregation and disaggregation of nanometer sized particles, leading to formation of uniform and homogenous deposits and, iii) PDC generates low change in pH near the electrode, thus it is convenient for deposition of biochemical and biological species in their highly active states. The main disadvantage of PDC over CDC lies in the decrease of the deposition yield. The latter can be more pronounced if low time-pulses are used. Various categories of AC signals including symmetrical fields with no net DC component and asymmetrical AC signals without and with net DC component have been discussed. Overall, the deposition rate under AC fields increases with polarization time and amplitude. With respect to frequency, the deposition rate increases with frequency up to certain value then drops at elevated frequencies. It is noted that deposition under AC signals offers the possibility to produce superior quality coatings from aqueous suspensions because electrolysis of water as well as particle orientation during the deposition could be controlled. From the application standpoint, PDC and AC, offers new application perspectives such as in biotechnology. Because under modulated electric fields, EPD can now be accomplished from aqueous suspensions with low water electrolysis rates, a variety of biochemical and biological species can be deposited to yield highly active layers suitable for a wide range of applications including biosensors, biofuel cells and bioreactors.