Water evaporation phenomena on micro and nanostructured surfaces H. Azarkish a , A. Behzadmehr a, * , T. Fanaei Sheikholeslami b , S.M.H. Sarvari c , L.G. Fr  echette d a Mechanical Engineering Department, University of Sistan and Baluchestan, Zahedan, Iran b Electrical Engineering Department, University of Sistan and Baluchestan, Zahedan, Iran c Mechanical Engineering Department, Shahid Bahonar University, Kerman, Iran d Mechanical Engineering Department, Universit e de Sherbrooke, Sherbrooke, Qu ebec, Canada article info Article history: Received 29 July 2014 Received in revised form 12 December 2014 Accepted 12 December 2014 Available online Keywords: Surface modification Evaporation and boiling No flooded regime Maximum evaporation rate abstract Augmentation of a microevaporator performance has been investigated experimentally to provide high quality vapor flow. Silicon, Silicon dioxide (SiO 2 ), Silicon nanowires (SiNW), silicon pillars (PeSi), silicon pillars covered by silicon dioxide (PeSiO 2 ) and silicon pillars with nanowires etched on the top (PeSiNW) are considered as evaporation surfaces. These surfaces are fabricated based on deep reactive ion etching (DRIE) as well as electrochemically etched nanowires. Two regimes (no flooded evaporation regime and flooded evaporation regime) are called for evaporation based on different applications. Experiments are repeated three times to ensure repeatability of the observations. Results show that in the case of no flooded regime, evaporation rate are significantly affected by three mechanisms; water spreading be- tween micro and nanostructures, shape and thickness of water droplets on the surface and dynamic behavior of evaporation. In this regime, the PeSiO 2 surface has the highest performance among the other surfaces. However, in the case of the flooded regime, the nucleation sites of boiling are very important to achieve maximum rate of evaporation. In this regime the PeSiNW surface is the most efficient surface. © 2014 Elsevier Masson SAS. All rights reserved. 1. Introduction Demand for compact thermal management solutions for high heat flux applications such as cooling of micro-electronic devices and micro power generation systems have been increased. It is well known that boiling provides much higher heat transfer than other mechanisms, so it is commonly considered for thermal devices with high heat fluxes. At small scale, however, boiling flow leads to add the complexity compared to single-phase liquid or gas convective cooling, such as capillary forces from the formation of bubbles near the size of the microchannels, unsteady flow and pressure variations, and alternating liquid and vapor slug flow at the exit of the microchannel [1,2]. In some applications, such as the micro-Rankine power generation cycle on a chip, the quality of the vapor as it enters the turbine is critical, and the presence of liquid droplets in the vapor flow is unacceptable. By implementing this cycle, using micro-electromechanical systems (MEMS) technolo- gies, it could provide low-cost devices to generate electrical power from environmental and waste heat [3]. Although many of the required components to implement a micro-Rankine power plant- on-a-chip have been demonstrated [4,5], micro-evaporators remain a challenge to be overcome. The latter must not only accommodate heat rates up to 100 W/cm 2 , but also provide a constant high quality vapor (liquid-free steam). Arslan et al. [6] experimentally showed that a constant flow of high quality vapor could be achieved by using a thermal gradient along shaped channels with multiple expansions and contractions. Since this kind of micro evaporator has a limited range of evaporation rate, Azarkish et al. [7] introduced a novel silicon bi-textured micropillar array to provide fully evaporated steam for micro Rankine cycle application. Hsieh and Yao [8] used silicon microstructures to enhance the rate of evaporation. They used deep reactive ion etching (DRIE) to fabricate micro-pillars on an oxidized surface. Their results show that microstructures enhance the spreading rate of water droplets on the silicon surface. Yao et al. [9] used silicon substrate and copper nanowires with different heights to intensify pool boiling. They showed that the tallest nanowire structure results the highest boiling heat fluxes; it is almost 3 times higher than that for one on a plain Si surface. Cai and Bhunia [10] showed that using carbon * Corresponding author. E-mail address: amin.behzadmehr@eng.usb.ac.ir (A. Behzadmehr). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts http://dx.doi.org/10.1016/j.ijthermalsci.2014.12.005 1290-0729/© 2014 Elsevier Masson SAS. All rights reserved. International Journal of Thermal Sciences 90 (2015) 112e121