This paper examines the electrostatic force on a microdroplet transported via electrowetting on dielectric (EWOD). In contrast with previous publications, this article details the force distribution on the advancing and receding fluid faces, in addition to presenting simple algebraic formulae for the net force in terms of system parameters. Dependence of the force distribution and its integral on system geometry, droplet location, and material properties is described. The consequences of these theoretically and numerically obtained results for design and fabrication of EWOD devices are considered.
This paper proposes the use of electrowetting on dielectric (EWOD) as the driving force for digitized heat transfer (DHT), a novel approach to microscale thermal management in which system cooling is actively achieved via the manipulation of an array of discrete microdroplets. Galinstan, a nontoxic, readily available, inexpensive liquid alloy with 65 times less thermal resistance than water, is proposed as a viable DHT coolant. The nature of the EWOD driving force and the velocity of EWOD-actuated droplets are presented, along with an analysis demonstrating the advantages of DHT over some other methods of microscale heat control.
Active thermal management of compact microsystems by a periodic array of discrete liquid metal droplets is proposed and referred to as “digitized heat transfer.” This is in contrast to convective heat transfer by a continuous liquid flow. Two methods of droplet actuation, electrowetting on dielectric and continuous electrowetting, are described. Liquid metals or alloys support significantly higher heat transfer rates than other fluids, such as water or air. In addition, electrowetting is an efficient method of microscale fluid control, requiring low actuation voltages and very little power consumption. These concepts are used in this investigation to design an active management technique for high-power-density electronic and integrated micro systems. Preliminary calculations indicate that this technique could potentially offer a viable cooling strategy for achieving some of the most important objectives of electronic cooling, i.e., minimization of the maximum substrate temperature, reduction of the substrate temperature gradient and removing substrate hot spots. Numerical simulation of a droplet in a microchannel is also investigated. We propose a technique for dynamically calculating the slip velocity at the wall boundary including both the advancing and receding contact lines. The technique is based on the observed non-Newtonian behavior of a continuous liquid flow at high shear rates and its associated slip velocity (Thompson and Trioan 1997). While most of the wall boundary has negligible slip, significant slip at the advancing and receding contact lines are calculated from the data itself.
This article presents a unified model for the velocity of discrete microdroplets. Simple algebraic expressions for steady-state droplet velocities are presented, as well as exact and approximate transient solutions. Specific results in terms of known experimental parameters are derived for the cases of electrowetting on dielectric (EWOD), dielectrophoresis (DEP), continuous electrowetting (CEW), and thermocapillary pumping (TCP). Model predictions are shown to agree with previously published theoretical and experimental results, giving fluid velocities for a broad range of applications in digitized microfluidics. A relative comparison of the model's predictions for EWOD, CEW, DEP and TCP is also presented.
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