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Evelyn N. Wang

• Title: Associate Professor
• Department: Department of Mechanical Engineering

Postdoctoral Associates

• Martin Cleary
• Ryan Enright
• Jiansheng Feng
• Xiansen Li
• Shankar Narayanan
• Teresa Peters
• Rishi Raj
• Sungwoo Yang

Collaborators

• Prof. John G. Brisson, M.I.T.
• Prof. Cullen Buie, M.I.T.
• Prof. Gang Chen, M.I.T.
• Prof. Carlos Hidrovo, UT Austin
• Prof. Jeffrey Lang, M.I.T.
• Prof. Tomas Palacios, M.I.T.
• Prof. Marin Soljacic, M.I.T.
• Prof. Carl V. Thompson, M.I.T.
• Prof. Omar Yaghi, UC Berkeley

Visiting Faculty

• Prof. TieJun Zhang, Masdar Institute

• Graduate Students

• Solomon Adera (ME)
• Kevin Bagnall (ME)
• David Bierman (ME)
• Siyu Chen (ME)
• Kuang-Han (Hank) Chu (ME)
• Daniel Hanks (ME)
• Tom Humplik (ME)
• Arthur Kariya (ME)
• Andrej Lenert (ME)
• Nenad Miljkovic (ME)
• Heena Mutha (ME)
• Dan Preston (ME)
• Rong Xiao (ME)
• Yangying Zhu (ME)

Undergraduate Students

• Kristyn Kadala (Aerospace)
• Jean Sack (ME)

Support Staff

• Theresa Werth, Administrative Assistant

Publications

(see drl.mit.edu for full listing)

K.-H. Chu, Y.S. Joung, R. Enright, C.R. Buie, E.N. Wang, “Hierarchically Structured Surfaces for Boiling Critical Heat Flux Enhancement,” Applied Physics Letters, 102, p. 151602-00, 2013.

R. Enright, N. Miljkovic, N. Dou, Y. Nam, E.N. Wang, “Condensation on Superhydrophobic Copper Oxide Nanostructures,” Journal of Heat Transfer, 2013, in press.

A. Lenert, Y. Nam, B.S. Yilbas, E.N. Wang, “Focusing of phase change microparticles for local heat transfer enhancement in laminar flows,” International Journal of Heat and Mass Transfer, 56(1), p. 380-389, 2013.

N. Miljkovic, R. Enright, Y. Nam, K. Lopez, N. Dou, J. Sack, E.N. Wang, “Jumping-Droplet-Enhanced Condensation on Scalable Superhydrophobic Nanostructured Surfaces,” Nano Letters, 13(1), p. 179-187, 2013.

N. Miljkovic, E.N. Wang, “Condensation heat transfer on superhydrophobic surfaces,” MRS Bulletin, 38(5), p. 397-406, 2013.

N. Miljkovic, R. Enright, E.N. Wang, “Modeling and Optimization of Superhydrophobic Condensation, Journal of Heat Transfer, 2013, in press.

R. Raj, S.C. Maroo, E.N. Wang, “Wettability of Graphene,” Nano Letters, 13, p. 1509-1514, 2013.

R. Xiao, N. Miljkovic, R. Enright, E.N. Wang, “Immersion Condensation on Oil-Infused Heterogeneous Surfaces for Enhanced Heat Transfer,” Scientific Reports, 3(1988), p. 1-6, 2013.

R. Xiao, S.C. Maroo, E.N. Wang, “Negative Pressures in Nanoporous Membranes for Thin Film Evaporation,” Applied Physics Letters, 102(00), p. 123103-00, 2013.

Non-Wetting Droplets on Superheated Textured Hydrophilic Surfaces

• Authors: S. Adera, R. Raj, R. Enright, and E. N. Wang

Engineered surfaces have received significant interest for a wide range of applications including microfluidics, drag reduction, self-cleaning, water harvesting, anti-corrosion, anti-icing, and thermal management.  Significant efforts have focused on using chemical functionalization to change the wettability of such surfaces from superhydrophobic to superhydrophilic.  In this work, we demonstrate that we can achieve non-wetting water droplets on superhydrophilic surfaces via evaporation. We demonstrate that when a wall superheat is below a critical superheat, the droplet spreads spontaneously following surface irregularities with near zero contact angle. However, when the superheat is above this critical superheat, the droplet rests on top of the structured surface without wetting with apparent contact angles as high as 160˚ due to evaporation. We studied this evaporation-induced non-wetting behavior of droplets by fabricating silicon micropillar arrays in a square pattern with pillar diameter (D), height (H), and pitch (L) ranging from 3-9 µm, 15-40 µm, and 10-30 µm, respectively. These samples were experimentally investigated at superheated conditions near the critical superheat to identify the effect of pillar array geometry (D, H, and L) and droplet size on the wetting behavior of droplets. A one-dimensional force balance model is developed that agrees reasonably well with experimental data. This phenomenon is distinct from the widely researched Leidenfrost and offers an expanded parametric space for fabricating surfaces with desired temperature-dependent wettability. Moreover, the framework can be extended to investigate the effect of pillar array geometry and material properties for the design of superhydrophobic surfaces at elevated temperatures.



Figure 1: Effect of wall temperature on the wetting state of a water droplet. (a) Due to roughness-enhanced wettability, a water droplet deposited on a superhydrophilic surface at 120 °C spontaneously spreads into a thin film and wets the surface following surface irregularities with a near zero contact angle. The inset shows the scanning electron micrograph (SEM) image of the microstructured surface (D = 6.6 μm, H = 18.3 μm, and L = 20.0 μm) acquired at 10˚ inclination. (b) A similar size droplet at a higher wall temperature of 160 °C did not wet the same superhydrophilic surface; instead it rests on top of the structured surface forming a Cassie-like droplet. The inset shows a magnified view of the boxed section near the droplet base of a similar experiment indicating that the droplet remained in contact with the pillar tops. The scale bar in the inset represents 100 μm.

Improving Water Desalination Capabilities through the Use of Engineered Nanostructured Materials

• Authors: T. Humplik, H. Mutha, T. Laoui, E. N. Wang
• Sponsorship: Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Saudi Arabia

Manipulation and control of transport at nanometer length scales offers new opportunities to enhance the performance of water desalination systems^[1]. We report on two projects that utilize nanometer and sub-nanometer sized pores to better separate salt from water.

Capacitive deionization (CDI), a method that captures ions in the electric double layer (EDL), can be used to desalinate brackish waters^[2]. Similar to supercapacitors, ideal CDI electrodes should have a large electrolyte-accessible specific surface area available for ion adsorption. To study the role of porous geometries in ion adsorption, we developed carbon nanotube (CNT) carpet electrodes, using an Au-Au self-diffusion bond to transfer vertically aligned CNT carpets onto an electrochemically-compatible substrate^[3]. We are investigating the role of ion solution concentration on the capacity of the electrodes. Preliminary results show a three-fold increase of capacitance from 5mM solution (brackish water limit) to 100mM solution (Figure 1). At present, we are investigating the effect of pore lengths on capacitance and ion adsorption.

Additionally, we are fabricating size-exclusion based reverse osmosis (RO) membranes, which allow for water permeation while perfectly rejecting solvated salt ions. To achieve this, we are utilizing the ~5.5 Å pore size of MFI-type zeolites, which falls between the size of a water molecule (~3 Å) and a solvate salt ion (~7-8 Å)^[4]. We investigated the permeability of water across the pores of these zeolites using water adsorption analysis. By varying the compositional silicon to aluminum (Si/Al) ratio of the zeolite, we can control the affinity of water to the zeolite. Decreasing the Si/Al ratio increases the amount of water within the zeolite pores at reverse osmosis operating pressures. However, decreasing this ratio has a significant detrimental effect on the permeability. Figure 2, which shows the water permeability as a function of the inverse Si/Al ratio, reveals that the higher Si/Al ratio zeolites exhibit enhanced water permeability, which is within an order of magnitude of current state-of-the-art RO membranes. We are in the process of fabricating defect-free zeolite-based membranes to further investigate the water and salt permeability. The authors would like to thank the King Fahd University of Petroleum and Minerals in Dhahran, Saudi Arabia, for funding the research reported in this paper through the Center for Clean Water and Clean Energy at MIT and KFUPM.



Figure 1: Cyclic voltammetry scan at 500mV/s of CNT electrode in NaCl solution. Increasing NaCl concentration, increases electrical capacitance, and decreases ion adsorption and removal from water



Figure 2: Permeability of water within zeolites as a function of the inverse Si/Al ratio. Increasing the Si/Al ratio increases the permeability of water through the zeolite pores. The polymeric RO permeability values can be found in^[5].




1. T. Humplik, J. Lee, S. C. O’Hern, B. A. Fellman, M. A. Baig, S. F. Hassan, M. A. Atieh, F. Rahman, T. Laoui, R. Karnik, and E. N. Wang, “Nanostructured materials for water desalination,” Nanotechnology, vol. 22, no. 29, p. 292001, Jun. 2011. [↩]
2. Y. Oren, “Capacitive deionization (CDI) for desalination and water treatment – past, present and future (a review),” Desalination, vol. 228, no. 1, pp. 10–29, 2008. [↩]
3. R. Enright, R. Mitchell, H. Mutha, C. Lv, M. Christiansen, C. V. Thompson and E. N. Wang, “Diffusion-bonded CNT carpets for fundamental CDI studies,” MRS Symposium Proceedings, vol. 1407, Boston, MA, Dec. 2011. [↩]
4. S. C. Maroo, T. Humplik, T. Laoui, and E. N. Wang, “Water Transport in Sub-nanometer MFI Zeolites for Efficient Water Desalination,” in Proc. ASME 2012 3rd Micro/Nanoscale Heat & Mass Transfer International Conference (2012). [↩]
5. T. Humplik, J. Lee, S. C. O’Hern, B. A. Fellman, M. A. Baig, S. F. Hassan, M. A. Atieh, F. Rahman, T. Laoui, R. Karnik, and E. N. Wang, “Nanostructured materials for water desalination,” Nanotechnology, vol. 22, no. 29, p. 292001, Jun. 2011. [↩]

Design and Fabrication of Magnetically Tunable Microstructured Surfaces

• Authors: Y. Zhu, R. Xiao, E. N. Wang
• Sponsorship: Air Force Office of Scientific Research

Micro- and nanostructured surfaces have broad applications ranging from liquid transport in microfluidics and cell manipulation in biological systems to light tuning in optical applications^[1][2]. While significant efforts have focused on fabricating static micro/nanostructured arrays^[3], uniform arrays that can be dynamically tuned have not yet been demonstrated. We present a novel fabrication process for magnetically tunable microstructured surfaces, where the tilting angle can be controlled upon application of an external magnetic field. We also demonstrate this platform for droplet and bubble manipulation in heat transfer applications.

The tunable surfaces consist of ferromagnetic (Ni) pillars on a soft PDMS substrate. The pillars have a diameter of 23-35 µm, pitch of 60-70 µm, and height of 70-80 µm. We used vibrating sample magnetometry to obtain hysteresis loops of the Ni pillar arrays, which match well the properties of bulk Ni. With a field strength of 0.5±0.1 Tesla and a field angle of 60±15°, a uniform 10.5±0.5° tilting angle of the pillar arrays was observed. Meanwhile, simulations using Abaqus to determine the equilibrium positions of the pillars under different applied fields show good agreement with the experiments. We also investigated how these tunable pillars changed the contact angle of water droplets on the surface.  An external magnetic field of 0.3 Tesla changed the water droplet contact angle by ~15°. Future work will focus on using these surfaces to actively transport water droplets and spread the liquid film via pillar movement. These tunable surfaces promise new fluid manipulation capability for applications in condensation, evaporation, and boiling.



Figure 1: Fabrication process and images of fabricated tunable micropillars with diameters of 24 27 µm, heights of 60 80 µm, and spacings of 60 µm. a) Fabrication process flow. b) Optical microscope image of a sample after step 2. c) Optical microscope image of a sample after step 4. d) SEM of a sample after step 6.



Figure 2: Simulation and experimental demonstration of tilting pillars. a) Finite element (Abaqus) simulation of tilt angle when h=80μm d=26μm, field is 0.7 Tesla at 60°. Legend represents the strain. b) Simulation result of tilt angle as a function of field. Square symbol shows experimentally demonstrated result. c) Time-lapse side view image of a sample with field of ~0.5 Tesla at ~60°. d) Time-lapse top view image of a sample with field of~0.5 Tesla at ~60°.




1. R. Xiao and E. N. Wang, “Microscale Liquid Dynamics and the Effect on Macroscale Propagation in Pillar Arrays,” Langmuir, vol. 27, pp. 10360-10364, 2011. [↩]
2. J. Fu, Y. Wang, M. T. Yang, R. A. Desai, X. Yu, Z. Liu and C. S. Chen, “Mechanical regulation of cell function with geometrically modulated elastomeric substrates,” Nature Methods, vol. 7, pp. 733-736, 2010. [↩]
3. K. Chu, R. Xiao and E. N. Wang, “Uni-directional Liquid Spreading on Asymmetric Nanostructured Surfaces,” Nature Materials, vol. 9, pp. 413-417, 2010. [↩]

Optimizing Emitter-to-Absorber Area Ratio for Improved Efficiency in Planar Solar Thermophotovoltaics

• Authors: A. Lenert, D. M. Bierman, E. N. Wang
• Sponsorship: MIT S3TEC Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under DE-FG02-09ER46577

Solar thermophotovoltaic (STPV) devices have the potential to overcome the Shockley-Queisser limit by converting solar radiation to a narrow-band thermal emission matched to the spectral response of a photovoltaic (PV) cell^[1],^[2]. However, limiting the influence of non-idealities of the individual STPV components (absorber/emitter/PV) through an understanding of the highly coupled energy conversion process is needed to approach high efficiencies. Although reported TPV efficiencies (thermal-to-electric) have exceeded 10%^[3], measured overall STPV conversion efficiencies are below 1%^[4],^[5]. One of the most prohibitive aspects of this system can be the thermal transfer efficiency due to the high temperature operation of the device.

In this work, we designed an experimental system for characterizing high-temperature planar STPVs, schematically shown in Figure 1, aimed to bridge the gap between potential and measured STPV performance. We show how increasing the emitter-to-absorber area ratio (AR) can compensate for non-ideal spectral selectivity on the absorber side. This concept was implemented in planar devices through a process of seeded growth of vertically aligned multi-walled carbon nanotube forests on smooth tungsten surfaces (shown in Figure 2). The TPV-side of the system is composed of a one-dimensional Si/SiO₂ photonic crystal emitter (1D PhC) paired with a low band-gap PV cell (InGaAsSb). With a 4:1 emitter-to-absorber area ratio (AR 4), we experimentally demonstrate a two-fold increase in thermal transfer efficiency (relative to AR 1) and a significant boost in STPV performance leading to efficiencies exceeding 2%.



Figure 1: Schematic of the planar STPV experimental layout. Incoming solar radiation is converted to heat at the absorber (MWCNT/W); heat is selectively radiated by the emitter (1D PhC) and converted to electrical power at the PV (InGaAsSb – 0.55 eV).



Figure 2: a) Absorber-side top view of a 4:1 emitter-to-absorber module showing spatially-defined MWCNT growth on a W-coated Si substrate. b) SEM side view of the MWCNT/W boundary, showing the vertically aligned nanotubes.




1. H. Nils-Peter and W. Peter, “Theoretical limits of thermophotovoltaic solar energy conversion,” Semicond. Sci. Technol. , vol. 18, no. 5, pp. 151-157, 2003. [↩]
2. W. Shockley and H.J. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells,” J. Appl. Phys., vol. 32, no. 3, pp. 510-519, 1961. [↩]
3. C.J. Crowley, N. A. Elkouh, S. Murray, and D. L. Chubb, “Thermophotovoltaic Converter Performance for Radioisotope Power Systems,” in AIP Conf. Proc., 2005, vol. 746, pp. 601-614. [↩]
4. A.S. Vlasov et al., “TPV Systems with Solar Powered Tungsten Emitters,” in AIP Conf. Proc., 2007, vol. 890, pp. 327-334. [↩]
5. A. Datas and C. Algora, “Development and experimental evaluation of a complete solar thermophotovoltaic system,” Prog. Photovolt: Res. Appl., Available: doi: 10.1002/pip.2201 [↩]

Scalable Superhydrophobic Copper Oxide Surfaces for Enhanced Heat Transfer

• Authors: N. Miljkovic, R. Enright, Y. Nam, K. Lopez, N. Dou, J. Sack, E. N. Wang
• Sponsorship: MIT S3TEC Center

Vapor condensation is observed in everyday life and routinely utilized in industry as an effective means of transferring heat. In industrial systems, the condensed vapor typically forms a thin liquid film due to the high surface energy associated with the majority of industrial heat exchanger materials (i.e., clean metals and metal oxides). This filmwise condensation mode is not desired due to the large thermal barrier for heat transfer.  If the condensing surface is coated with a functional hydrophobic coating, for example a long chain fatty acid or polymer coating, the vapor forms discrete liquid droplets. These droplets shed at sizes approaching the capillary length (≈2 mm for water) and refresh the surface for re-nucleation. Furthermore, when micro- or nanostructures are combined with the coating, a superhydrophobic surface can be created with smaller droplet shedding radii (≈10-100 µm) than dropwise condensation and potential droplet jumping during coalescence due to the conversion of surface energy to kinetic energy (Figure 1a). In this work^[1], we experimentally demonstrated that a 25% higher overall heat flux (Figure 2a) and 30% higher condensation heat transfer coefficient (Figure 2b) can be achieved using silanized CuO superhydrophobic surfaces (Figure 1b and c) compared to conventional dropwise condensing Cu surfaces. We show that these CuO surfaces offer ideal condensation behavior in terms of emergent droplet morphology and coalescence dynamics, and a significant enhancement in heat transfer performance. Furthermore, the chemical-oxidation-based CuO fabrication process provides a simple and readily scalable method to create superhydrophobic condensation surfaces that can sustain droplet jumping behavior. Accordingly, these surfaces are attractive for applications such as atmospheric water harvesting and dehumidification.



Figure 1: (a) Schematic of the droplet jumping mechanism showing two droplets with radii R, prior to (state 1) and immediately after (state 2) coalescence. (b) Side view field emission scanning electron microscopy FESEM image of a nanostructured CuO surface. The sharp, knife-like CuO structures have characteristic heights, h ≈ 1 μm, solid fraction, φ ≈ 0.023, and roughness factor, r ≈ 10. (c) Jumping-droplet superhydrophobic condensation on a nanostructured CuO tube (Inset: magnified view of the jumping phenomena). The tube sample has outer diameter DOD = 6.35 mm, inner diameter, DID = 3.56 mm, and length L = 131 mm.



Figure 2: (a) Experimental steady state log mean water to vapor temperature difference (ΔTLMTD) as a function of overall surface heat flux (q”) for tube surfaces undergoing filmwise, dropwise, flooded, and jumping condensation with chamber vapor pressure Pv = 2700 ± 68 Pa. Rapid droplet removal due to coalescence induced droplet jumping results in the highest heat fluxes for the jumping sample (S < 1.12). (b) Experimental and theoretical steady state condensation coefficient (hc) as a function of saturated vapor pressure (Pv) for tube surfaces undergoing filmwise, dropwise, flooded, and jumping condensation. Jumping condensation shows the highest condensation heat transfer coefficient for low supersaturations (S < 1.12).

1. N. Miljkovic, R. Enright, Y. Nam, K. Lopez, N. Dou, J. Sack, and E. N. Wang, “Jumping-Droplet-Enhanced Condensation on Scalable Superhydrophobic Nanostructured Surfaces,” Nano Letters, vol. 13, no. 1, 2013, pp. 179-187. [↩]

Condensation on Micro/Nanostructured Superhydrophobic Surfaces

• Authors: N. Miljkovic, R. Enright, E. N Wang
• Sponsorship: MIT S3TEC Center

Water condensation on surfaces is a ubiquitous phase-change process that plays a crucial role in nature and across a range of industrial applications including energy production, desalination, and environmental control. Nanotechnology has created opportunities to manipulate this process through the precise control of surface structure and chemistry, thus enabling the biomimicry of natural surfaces, such as the leaves of certain plant species, to realize superhydrophobic condensation. However, this “bottom-up” wetting process is inadequately described using typical global thermodynamic analyses and remains poorly understood. In this work, we study condensation on well-defined structured surfaces spanning a wide range of length scales from 100 nm to 10 μm and functionalized using several hydrophobic thin films (Figure 1). We found that the emergent morphology of isolated droplets interacting with the surface structures during growth is primarily defined by the pinning behavior of the local contact line within the structures^[1]. Depending on the relationship between the structure length-scale and the droplet nucleation density, the dominant condensed droplet morphology can switch to one that is thermodynamically unfavorable in a global sense due to local contact line de-pinning. We show how these isolated condensed droplet morphologies arise by quantitatively describing growth in terms of characteristic local energy barriers and extend this view to explain the role of droplet-droplet interactions in determining emergent droplet morphology (Figure 2). This result contrasts the common macroscopic view of wetting behavior for individual droplets. The understanding of droplet growth behavior developed provides a rational basis to develop optimized condensing surfaces. This mechanistic framework also has implications for understanding condensation behavior on nature’s superhydrophobic surfaces and ice formation on structured surfaces that initiates with the condensation of sub-cooled water.



Figure 1: Scanning electron micrographs of fabricated pillar geometries. (a) Electrodeposited Au nanopillars defined by an anodic alumina template. (b) Si nanopillars fabricated using interference lithography and metal-assisted wet etching. (c) Si nanopillars fabricated using e-beam written mask and DRIE. (d) Si micropillars fabricated using optical lithography and DRIE.



Figure 2: Regime map characterizing the dominant wetting behavior observed during condensation with coordinates of 〈L〉/l and E*, where 〈L〉 is the average spacing between droplet nucleation sites, l is the pillar-to-pillar pacing, E^*=-1/(r∙cosθ_a), r=1+πdh/l^2, d is the pillar diameter, h is the pillar height, and θa is the smooth surface advancing contact angle. Cassie morphologies emerge at large 〈L〉/l and E^*≤1 (shaded region). Wenzel morphologies emerge at low 〈L〉/l and/or E^*≳1.

1. R. Enright, N. Miljkovic, A, Al-Obeidi, C. V. Thompson, and E.  N. Wang, “Superhydrophobic Condensation: The role of energy barriers and size scale,” Langmuir, 2012, vol. 28, no. 40, p. 14424–14432. [↩]