Electrospray cooling for microelectronics Weiwei Deng a , Alessandro Gomez b,⇑ a Department of Mechanical, Material and Aerospace Engineering, University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA b Department of Mechanical Engineering and Material Science, Yale University, 9 Hillhouse Ave., New Haven, CT 06520, USA article info Article history: Received 6 July 2010 Received in revised form 2 August 2010 Keywords: Spray cooling MEMS Electrospray abstract The challenge of effectively removing high heat flux from microelectronic chips may hinder future advancements in the semiconductor industry. Spray cooling is a promising solution to dissipate high heat flux, but traditional sprays suffer from low cooling efficiency partly because of droplet rebound. Here we show that electrosprays provide highly efficient cooling by completely avoiding the droplet rebound, when the electrically charged droplets are pinned on the heated conducting surface by the electric image force. We demonstrate a cooling system consisting of microfabricated multiplexed electrosprays in the cone-jet mode generating electrically charged microdroplets that remove a heat flux of 96 W/cm 2 with a cooling efficiency reaching 97%. Scale-up considerations suggest that the electrospray approach is well suited for practical applications by increasing the level of multiplexing and by preserving the system compactness using microfabrication. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Advancements of integrated circuits (IC) have recently been hampered by the severe challenge of the removal of high heat flux. Effective chip cooling may become the bottleneck of further pro- gress in the microelectronic industry. Compared to conventional fan cooling that often relies on a thermal spreader, cooling by direct liquid impingement on the chip back side is promising for high heat flux removal, because it eliminates contact thermal resis- tance, promotes high velocity gradients that favor heat dissipation, and exploits the liquid latent heat when phase change occurs [1]. The coolant can take the form of impinging jets [1–3] or sprays [4,5]. Microjet arrays generated by silicon microfabricated nozzles with open [2] or closed drainage [3,4] are examples of jet cooling. Spray cooling, currently used in some supercomputers such as the CRAY X1, in principle is more effective than jet impingement cool- ing [6], mainly because the liquid film formed by sprays is typically much thinner (by a factor of 10) than that of liquid jets [7]. The physical process of spray cooling results from the impact of droplets on a heated surface, which, in turn, may lead to splash, spread, or rebound [8]. Especially when the surface temperature is higher than the Leidenfrost point of the liquid, the droplet tends to rebound because the pressure of the vapor below the liquid par- tially lifts the droplet [9]. As a result, in conventional sprays only a fraction of the liquid cooling capacity is exploited because of this rebound loss. A possible approach to reduce or even entirely eliminate this loss is to electrically charge the droplets with respect to the hot conducting surface and rely on Coulombic attraction, if charge leakage on contact is sufficiently slow [10]. In this context, the electrospray (ES) is potentially well-suited for cooling purposes because of its unique properties. Although there are numerous functioning modes of this device [11,12], the most appealing one from the point of view of achieving fine liquid dispersion with ensuing enhanced evaporation in a relatively short time is the so-called cone-jet mode [13]. In that mode an electrohydrodynam- ic process is established in which a spray of monodisperse droplets is formed by passing a liquid with sufficient electrical conductivity through a capillary charged to a high potential with respect to a ground electrode a short distance away. Under the effect of a high electric field, the liquid meniscus takes the shape of a cone from the tip of which a thin liquid thread emerges, leading to the cone-jet mode [13]. This microjet breaks into a stream of charged droplets that eventually spread to form a spray. Among the key features distinguishing the electrospray from other atomization techniques are: the quasi-monodispersity of the droplets; the Cou- lombic repulsion of the charged droplets, which induces spray self- dispersion, prevents droplet coalescence and enhances mixing; and the capability of producing droplets of uniform size even at the nanoscale. In addition, the number density is reasonably uniform throughout the spray. The inner diameter of the ES nozzle is typi- cally 10–100 larger than the droplet, which reduces the risk of clogging and dramatically decreases the liquid pressure drop, from 10 5 Pa of a conventional atomizer [14] to 10 3 Pa of ES systems. ES has been widely used in ionization mass spectroscopy [15]. In virtually all other applications, it has been plagued by one 0017-9310/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2011.02.038 ⇑ Corresponding author. Tel.: +1 203 432 4384; fax: +1 203 432 7654. E-mail address: Alessandro.Gomez@yale.edu (A. Gomez). International Journal of Heat and Mass Transfer 54 (2011) 2270–2275 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt