Droplet transportation using a pre-charging method for digital microfluidics

Abstract

A new droplet-driving scheme for digital microfluidics termed the ''pre-charging of a droplet'' is demonstrated. In this method, a droplet is initially charged by applying ''pre-charging'' voltage between the droplet and an electrode buried under dielectric layers. The droplet is then driven to the next electrode by applying ''driving'' voltage between two adjacent buried electrodes. The concept of pre-charging was proved by the polarity of the charge stored in the droplet. When the droplet is precharged with positive voltage, it is driven with negative voltage and vice versa. Therefore, the magnitudes of the pre-charging and driving voltages are identical, but only with the opposite polarity. A 2.5-lL deionized water droplet is pre-charged and driven at a minimal voltage of 12 V. The charge stored in the droplet by this pre-charging method remained for more than 2 min, and the driving actuation could be repeated more than 150 times while the droplet remained its charged state. This method suggests a new means of driving a droplet for digital microfluidics at a relatively low voltage by utilizing both the electrostatic and dielectrophoretic force in the droplet transport process with a simpler structure compared to other single-plate structured devices.

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

“…The Faradaic charging of water droplets was also employed in digital microfluidic devices to manipulate water droplets above a system of microelectrodes. 13 Instead of the dielectric oil phase, the actuation electrodes were covered by a thin film of an insulator. More information about droplet handling in microfluidic devices can be found e.g.…”

Next generation digital microfluidic technology: Electrophoresis of charged droplets

“…Modeling of drop/particle motion experiencing the contact charging phenomenon is one of the key subjects in this field [4][5][6]66,[74][75][76][77][78][79][80][81][82]. In microfluidics field, the direct contact charging is used to manipulate droplet [38,[83][84][85][86] or particle motions [20,75] in microchannels in addition to digital microfluidic approaches [6,23,70,77,[87][88][89]. The details of each category will be covered in the following fundamentals and applications section.…”

“…They showed some potentials for the ECD as a digital microfluidic actuation method; however, in their work, the system size was too large and the actuation voltage was too high (few kilovolts), which limited the applicability of the technology. For that reason, there is a preconception that a high actuation voltage on the order of kilovolts is required for ECD actuation [87], which is not the case for smaller systems [6]. Later, Im et al resolved this issue by decreasing system size and demonstrated various biochemical applications such as hydrogel and crystal formation as shown in Fig.…”

“…Fortunately, the viability issue was confirmed for human cells [77] as well as for mammalian cells [89]. The ECD method was also adopted in other digital microfluidic field such as pre-charging method in electrowetting on dielectrics (EWOD) applications [87].…”

Electrophoretic manipulation of multiple-emulsion droplets

“…Choi et al recently presented a droplet manipulation technique, whereby a water droplet is given a charge from an electrode and is then driven by electric fields (Choi et al, 2012). Apart from carrying a native charge, water-in-oil droplets can also be deliberately charged through direct contact with a voltagebiased electrode (Khayari et al, 2003).…”