Water-oil core-shell droplets for electrowetting-based digital microfluidic devices

Abstract

Digital microfluidics based on electrowetting-on-dielectric (EWOD) has recently emerged as one of the most promising technologies to realize integrated and highly flexible lab-on-a-chip systems. In such EWOD-based digital microfluidic devices, the aqueous droplets have traditionally been manipulated either directly in air or in an immiscible fluid such as silicone oil. However, both transporting mediums have important limitations and neither offers the flexibility required to fulfil the needs of several applications. In this paper, we report on an alternative mode of operation for EWOD-based devices in which droplets enclosed in a thin layer of oil are manipulated in air. We demonstrate the possibility to perform on-chip the fundamental fluidic operations by using such water-oil core-shell droplets and compare systematically the results with the traditional approach where the aqueous droplets are manipulated directly in air or oil. We show that the core-shell configuration combines several advantages of both the air and oil mediums. In particular, this configuration not only reduces the operation voltage of EWOD-based devices but also leads to higher transport velocities when compared with the manipulation of droplets directly in air or oil.

Numerical modeling of electrowetting transport processes for digital microfluidics

“…8, we thus made simulations of transport processes with these values of the contact angles. We found an average velocities of about 40 cm/s, which is at least 2-3 times larger than experimental velocities achieved in similar real EWOD devices (Brassard et al 2008). We believe this discrepancy between the LBM simulations and the velocities values obtained in real EWOD devices is due to CAH (that was not considered in the simulations).…”

“…6. Thus, we conclude that despite the low density ratios usually obtained in multiphase lattice Boltzmann simulations, the To compare our simulation results with the values obtained in real EWOD devices, we should first note that real devices typically operate with rather small differences between ON and OFF contact angles (Brassard et al 2008) compared to the simulations we showed previously. For example, a pair of contact angles often employed in real EWOD devices is 110°for the OFF state (inactive regions on the plates) and 80°for the active electrodes (ON states).…”

“…It is also possible to link the interaction strength parameter G LS to the electric potential used for the activation of the electrodes if the electric potential dependence of the contact angle at these electrodes is known. One example of such an experimental dependence can be found in (Brassard et al 2008). At relatively small values of the applied voltage (before the contact angle saturation) a simple quadratic dependence of the contact angle with the applied voltage cos (h) µ U 2 can be employed (Li and Fang 2009).…”

“…2) are designed as regions of tunable interaction potential (the size of each electrode being of about 110 lu, that is approximately 0.5 mm). We considered in our simulations this comb-shaped geometry of the electrodes since it is largely used in the design of EWOD devices (Brassard et al 2008) so comparisons to experimental data already available in the literature would be easier and more appropriate than having simple rectangular electrodes. These regions can be activated or deactivated by setting different potentials G LS at different times (iterations) of the simulation.…”

Digital Microfluidic System with Vertical Functionality

“…Immersing the water droplet in oil increases the contact angle [62] and reduces contact angle hysteresis, which makes it possible to insert droplets into channels with a smaller radius and makes vertical functionality easier to achieve (Figure 6b).…”