Digital Microfluidic Device Using Ionic Liquids for Electronic Hotspot Cooling

Abstract

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.