Droplet transport through dielectrophoretic actuation using line electrode

Abstract

We explore a novel transverse line electrode configuration for droplet transport through dielectrophoretic actuation with potential lab-on-chip applications. Using a lumped electromechanical model, we show a weak dependence of DEP actuation force on electrode spacing in this configuration. The configuration successfully triggers translational drop motion with minimal changes in contact angle at considerably low voltages. Two sessile, deionized water drops placed horizontally apart on a indium-tin–oxide-coated glass with additional coatings of polydimethylsiloxane, and a thin layer of Teflon is merged by applying an AC field (88 V_rms at 150 kHz) through a common horizontal wire electrode. A lateral motion of two drops is induced along the horizontal electrode, eventually leading to coalescence. The drop motion is unique compared to electrowetting in its near-constant dynamic contact angle, and irreversibility on withdrawal of electric field. The effect of frequency on the drop behavior is examined through a parametric study on single drops within the range of 2–200 kHz. It is interesting to observe a switch-over from DEP behavior at high frequency to EWOD behavior at low frequency around a critical frequency (Jones in Langmuir 18:4437–4443, 2002).

Appendix

ς can be related to the viscosity of the liquid η and reversible work of adhesion $\gamma \,\left( {1 + \cos \theta_{0} } \right)$ as (Blake and De Coninck 2002)

$$\varsigma = \frac{{\eta V_{L} }}{{\lambda^{3} }}{ \exp }\left[ {\frac{{\gamma \lambda^{2} \left( {1 + \cos \theta_{0} } \right)}}{{K_{B} T}}} \right]$$

where V _L is the molecular volume of the unit of flow and λ is the jump length. The ratio of ς at different equilibrium contact angles (between the surface used herein, 98^o and that reported in Wang et al. 2007; 115°) can be found as

$$\frac{{\varsigma_{1} }}{{\varsigma_{2} }} = { \exp }\left[ {\frac{{\gamma \lambda^{2} }}{{K_{B} T}}\left( {\cos (\theta_{1} ) - \cos (\theta_{2} )} \right)} \right]$$

For θ₁ = 98° and θ₁ = 115°, $\varsigma_{98} /\varsigma_{110}$ is about 30.