This paper presents a study of electrowetting of ionic liquids (ILs) under AC voltages, where nine different ILs (including mono-, di-, and tricationic varieties) with three different AC frequencies (60 Hz, 1 kHz, 10 kHz) were experimentally investigated. The main foci of this study are (i) an investigation of AC frequency dependence on the electrowetting of ILs; (ii) obtaining theoretical relationships between the relevant factors that explain the experimentally achieved frequency dependence; and (iii) a systematic comparison of electrowetting of ILs using AC vs DC voltage fields. The frequency of the AC voltage was found to be directly related to the apparent contact angle change (Deltatheta) of the ILs. This relationship was further analyzed and explained theoretically. The electrowetting properties of ILs under AC voltages were compared to that under DC voltages. All tested ILs showed greater apparent contact angle changes with AC voltage conditions than with DC voltage conditions. The effect of structure and charge density also was examined. Electrowetting reversibility under AC voltage conditions was studied for few ILs. Finally, the physical properties and AC electrowetting properties of ILs were measured and tabulated.
Thermal management in electronics become more challenging as the size of electronics decreases, yet, the heat generated from electronics still increases. To enhance cooling efficiency of conventional cooling schemes such as heat pipes, we experimentally present a use of electrowetting on dielectric (EWOD) digital microfluidic technique to force the cooling liquid medium to move to hot spot area. In this paper, firstly, two different EWOD devices were compared in their cooling performance. One is a system using one plane device and sessile droplet of cooling medium and the other is a system using two parallel planes and liquid is sandwiched in between. Secondly, two types of liquids were used and compared as the cooling medium. De-ionized (DI) water and room temperature ionic liquid (RTIL) have been investigated. RTILs are thermally stable thanks to their low vapor pressure. In addition to thermal stability, RTIL can be tailored task specifically by altering cations and anions. Different experiments were conducted to study the capacity of IL’s to change the surface temperature of the hotspot generated and this was compared with that of DI water. The latter showed higher capacity to remove heat, while evaporation problem was predominant in the sandwiched setup. Three different ionic liquids, 1-butyl-3-methylimidazolium chloride or [BMIM]Cl, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide or [BMIM]Ntf2, and [CMIM]FeCl4 showed less effect on changing the surface temperature compared to water. It is due to generally lower heat conductivity and higher viscosity of ILs than water. However, RTILs showed high thermal stability by resulting in no evaporation during cooling process while water had vigorous evaporation. Nanofluid of RTIL and multiwall carbon nanotubes (MWCNT) mixture has been tested as the first step toward enhancing thermal conductivity of RTIL.
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