Lab on a Chip PAPER Cite this: Lab Chip, 2014, 14, 494 Received 15th August 2013, Accepted 7th October 2013 DOI: 10.1039/c3lc50949d www.rsc.org/loc Flow of suspensions of carbon nanotubes carrying phase change materials through microchannels and heat transfer enhancement Sumit Sinha-Ray, a Suman Sinha-Ray, a Hari Sriram ab and Alexander L. Yarin * ac This work explores the potential of nano-encapsulated phase change materials (PCMs) in applications related to microelectronics cooling. PCMs (wax or meso-erythritol) were encapsulated in carbon nanotubes (CNTs) by a method of self-sustained diffusion at room temperature and pressure. These nano-encapsulated wax nanoparticles alone allowed heat removal over a relatively wide range of temperatures (different waxes have melting temperatures in the range 40–80 °C). On the other hand, nano-encapsulated meso-erythritol nanoparticles allowed heat removal in the range 118–120 °C. The combination of these two PCMs (wax and meso-erythritol) could extend the temperature range to 40–120 °C, when both types of nanoparticles (wax and meso-erythritol intercalated) would be suspended in the same carrier fluid (an oil). The nanoparticles possess a short response time of the order of 10 −7 s. Such nano-encapsulation can also prevent the PCM from sticking to the wall. In this work, experiments with wax-intercalated CNTs, stable aqueous suspensions of CNTs with concentrations up to 3 wt% with and without nano-encapsulated wax were prepared using a surfactant sodium dodecyl benzene sulfonate (NaDDBS). These suspensions were pumped through two channels of 603 μm or 1803 μm in diameter subjected to a constant heat flux at the wall. It was found that the presence of the surfactant in CNT suspensions results in a pseudo-slip at the channel wall which enhances the flow rate at a fixed pressure drop. When aqueous solutions of the surfactant were employed (with no CNTs added), the enhanced convection alone was responsible for a ~2 °C reduction in temperature in comparison with pure water flows. When CNTs with nano-encapsulated wax were added, an additional ~1.90 °C reduction in temperature due to the PCM fusion was observed when using 3 wt% CNT suspensions. In addition, suspensions of meso-erythritol-intercalated CNTs in alpha-olefin oil were used as coolants in flows through the 1803 μm-diameter microchannel. These suspensions (1.5 wt% CNT) revealed a temperature reduction due to the PCM fusion of up to 3.2 °C, and a fusion temperature in the range 118–120 °C. Introduction The continuing miniaturization of microelectronic, optoelec- tronic and radiological devices and powerful computers is accompanied by further increases in their functionality, complexity and integration. According to Moore's law, the number of chips per circuit is expected to increase exponen- tially every year, resulting in a dramatic increase in the amount of heat released per unit volume. Thermal reasons have been reported to be responsible for more than half of the failure incidents of microelectronic devices. 1 Therefore effective cooling and temperature reduction become critical issues. Bulky fans and other traditional means have their own severe limitations (e.g. bulky space, thermal inertia, etc.) and can hardly resolve these emerging cooling problems. The use of liquid coolants for cooling microelectronics attracts wider attention. Liquid coolants such as liquid metals 2 and dielectric coolants 3 have been explored in the context of microelectronics cooling. The use of phase change materials (PCMs) holds great promise for the enhancement of cooling. PCMs absorb heat as latent heat of fusion, and release this heat when they solidify elsewhere. 4 Several different types of PCMs are available, such as fatty acids, 5 hydrated salts, paraffin waxes, and eutectic compositions, 6–8 and numerical investiga- tions of potential single-phase or multi-phase PCM effects on microelectronics cooling reveal their benefits. 9–12 Paraffin waxes are attractive PCMs due to their relatively high latent heat of fusion (~200 J g −1 ), chemical inertness, and minor phase segregation, although their drawbacks are low thermal conductivity and diffusivity. 13 The low thermal conductivity of wax extends its melting time to a level inappropriate for flow-through systems such as microchannels. Efforts to a Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 W. Taylor St., Chicago, IL 60607-7022, USA. E-mail: ayarin@uic.edu b Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, USA c College of Engineering, Korea University, Seoul, South Korea 494 | Lab Chip, 2014, 14, 494–508 This journal is © The Royal Society of Chemistry 2014 Published on 09 October 2013. Downloaded by University of Illinois at Chicago on 06/01/2014 05:11:32. View Article Online View Journal | View Issue