Electrophoresis of a Charged Droplet in a Dielectric Liquid for Droplet Actuation

Abstract

Electrophoretic motion of a charged droplet in a dielectric fluid under an electric field has been investigated experimentally for use as a microdroplet actuation method. The effects of the droplet size, electric field strength, and electrolyte concentration and ion species on the charging of an aqueous droplet have been examined. The amount of electrical charging has been measured by two different methods: indirect measurement using the image analysis of droplet motion and direct measurement using the electrometer. Quantitative comparison of the droplet charge measured experimentally and the theoretical value of a perfectly conductive sphere shows that an aqueous droplet is less charged than the corresponding perfectly conductive sphere. The limiting effect on electrical charging is more significant for an electrolyte droplet, and the effect is positively correlated to the electrolyte concentration rather than the ion species. This implies that the low electrical conductivity of water is not a major cause of the limiting effect. The scaling law of the charging amount for a deionized water droplet nearly follows that of the perfect conductor, whereas for an electrolyte droplet, the scaling law exponent is slightly higher. Some advantages and potentials of the current droplet actuation method are also discussed in comparison with the conventional ones.

Oscillatory motion of water droplets in kerosene above co-planar electrodes in microfluidic chips

“…The most probable explanation of this phenomenon relies on the droplet deformation that has been observed also in Ref. 20, 24, and 25. As a tiny tip is formed on the droplet, most of free electric charge accumulates in it and the tip is strongly attracted to the electrode edges.…”

“…The deceleration at the electrode surface has been shown but not discussed in Ref. 20. The most probable explanation of this phenomenon relies on the droplet deformation that has been observed also in Ref.…”

“…The effects of droplet size on the droplet velocity have been studied in systems with parallel electrodes without significant dielectrophoretic effects. 20,24 The droplet velocity always monotonously increased with growing droplet radius in the interval from 300 μm to 700 μm. As it was shown, the amount of electric charge is approximately proportional to R 2 (the power varied from 1.59 to 2.3 in different situations).…”

“…As it was shown, the amount of electric charge is approximately proportional to R 2 (the power varied from 1.59 to 2.3 in different situations). 20,24 …”

“…[20][21][22][23][24][25][26] Jung et al 24 and Im et al 20,22 studied the oscillatory migration of charged water droplets between two parallel electrodes. Oscillatory motion was also recorded for falling water droplets between parallelplated electrodes.…”