Enhanced Cooling By an Oscillating Droplet on DMF Platform

Significant change in the contact angle of a liquid droplet, placed on a dielectric solid brought about by externally applied electric field and subsequent wetting, is commonly known as Electrowetting on Dielectric (EWOD) and this has resulted into a new paradigm, Digital Microfluidics (DMF) as a faster and more efficient means of handling micro or nanoliters of liquid droplets [1]. Although many of the modern cooling techniques can produce significantly high heat transfer rates, they fail to meet the demand of dynamic cooling of hot-spots on a chip having non-uniform thermal profiles [2]. Digital Microfluidics (DMF) offers the benefit of controlling and manipulating individual droplets of volume less than a micro-liter, placed on an array of electrodes [3-4] and can also be effectively used in maximizing heat extraction from microchips with varying temperature distribution. DMF can use EWOD as a tool to drive discrete drops in the desired direction under externally controlled electrodes [5]. These drops transfer, store and remove heat from the integrated circuit surface and travel back to the coolant reservoir.

 The DMF methods discussed above employ static drops on the hot-spot and limit their heating to liquid phase only. The aim of the present work is to incorporate oscillation in the drop to create faster evaporation and thereby enhanced cooling. The ultimate strategy may involve a DMF platform wherein several drops would be moved to the different hot sites, oscillate simultaneously upon switching on the specific electrodes with a pulse input of DC, for enhanced heat extraction. The study of Chakraborty et al.[6] on the cooling enhancement by an oscillating droplet on EWOD platform is extended to an oscillating droplet on a DMF platform.

An array of electrode is designed (AutoCAD) for actuating and controlling the motion of droplets. The array comprises of eighteen electrodes, each consisting of two interconnected pads (Figure 1). The bigger pad (a square having dimension of 1.4x1.4 mm) acts as the electrode pad, while the smaller one (a square having dimension 1x1 mm) acts as the connection pad for the external circuit. A normal glass slide is coated with aluminum (195 nm thickness) using chemical vapor deposition technique.

Figure 1 Schematic of the Electrode

Next, electrodes are fabricated by photo-lithography techniques and are subsequently coated with PDMS (dielectric layer) using a spin coater (500 rpm for 30 seconds; 5000 rpm for 70 seconds) and then with a thin protective layer of Teflon (solution spin coated at 3000 rpm for 30 seconds;  cured at 112 °C for 10 minutes followed by at 175 °C for 25 minutes). This result in a 13µm of PDMS layer and 23.3 nm of Teflon layer thickness. Coating thicknesses have been measured using a profilometer. The coated substrate is attached to a printed circuit board (PCB) and connected to an external electronic circuit.

 On application of a pulsating electric field, a rapid, reversible and cyclic change in the shape of the drop, on the fabricated DMF, platform is noted which depended on the voltage and delay time of the pulse. The oscillation of the droplet is quantified by a frame by frame analysis of the   captured motion using a high speed camera attached with the goniometer.   It is observed that the frequency of drop oscillation increases with a decrease in the delay time of the pulse input but is independent of the applied voltage. The frequency of oscillation is higher in case of smaller microdrops. Maximum change of contact angle is found to take place at the beginning of a pulse input and it is a strong function of pulse voltage. The decrease in the droplet temperature is more for an oscillating drop compared to that with a static drop of same size. Results indicate that the oscillation of the drop enhances heat transfer with a marked decrease in the evaporation time of the droplet. The maximum percentage increase in evaporation rate is found to be 15.2 % at 250 V and 25 ms using a 3 μL droplet over that of an evaporating static drop of the same size. It is envisaged that the pulsating voltage induced internal flow within the drop would give rise to higher heat transfer rate.

References:

[1] G.M. Whitesides, "The origin and the future of microfluidics,"  Nature  vol. 442, pp.368-373, 2006

[2] P. Y. Paik, “Adaptive hot-spot cooling principles and design”, Artech house publ., Norwood, MA, USA, Chapter 3, 2007.

[3] K. Choi, A. H. C. Ng, R.  Fobel,  and A. R. Wheeler, “Digital microfluidics”, Annual Review of Analytical Chemistry vol 5, pp. 413-440, 2012.

[4] M. Abdelgawad,  A. R. Wheeler, “The digital revolution: A new paradigm for microfluidics”, Advanced Materials  vol 20, pp. 1-6, 2008.

[5]  R. B. Fair, “Digital microfluidics: is a true lab-on-a-chip possible?”, Microfluid Nanofluid  vol 3, pp. 245-281, 2007.

[6] M. Chakraborty, A. Ghosh, S. DasGupta,. " Enhanced microcooling by electrically induced droplet oscillation," RSC Advance vol 4, pp. 1074-1082, 2014.