Nanoengineered Surfaces

Wettability and Microfluidics

Interactions between liquids and solids are ubiquitous in our physical environment and are typically characterized by the wetting angle that a liquid droplet makes on the solid surface. While wettability on flat and homogenous surfaces has been researched quite extensively, recent advances in micro-/nano-fabrication and coating technologies have enabled the development of smart engineered surfaces. Fundamental understanding of the wetting and liquid propagation behavior on these surfaces is important for a range of applications such as microfluidics, thermal management, lab-on-a-chip, water harvesting, optical, and biological systems.

At the Device Research Laboratory, we are working towards developing a better understanding of the change in wettability due to surface engineering [1-5], chemical heterogeneities [6-7], and in the presence of liquid-vapor phase change phenomena [8]. These studies are critical for elucidating the underlying physical mechanisms behind the other research topics being investigated in our lab. For example, hydrophilic surfaces made superhydrophilic due to structuring have resulted in enhancement of boiling and thin-film evaporation heat transfer coefficients. Conversely, hydrophobic structured surfaces, i.e. superhydrophobic surfaces have recently shown a promise to push the limits of condensation heat transfer. In addition to these fundamental studies, we are also investigating avenues to actively manipulate droplet morphology [9] and wetting states [10] to design microfluidics devices for practical applications. For example, we developed dynamically tunable micropillar arrays [11], where the tilt angle is controlled by an external magnetic field. The tunable surface design promises great potential for thermal management, microfluidics, biological and optical applications.

The wettability and microfluidics initiatives at the Device Research Laboratory are supported by a number of agencies including National Science Foundation, Office of Naval Research, Air Force Office of Scientific Research, and Battelle National Security Global Business.

Boiling

Boiling is a very effective mode of heat transfer and is used in a multitude of applications from day-to-day cooking to industrial scale thermal management and power generation. Despite the ubiquitous presence of boiling, the phenomenon is not completely understood mainly due to the complex phase change process, liquid-vapor wetting dynamics, convective flows, and temperature fluctuations involved. Furthermore, there exists a growing demand for the capability of transferring heat at higher rates as well as for improving device efficiencies.

At the Device Research Laboratory, we have experimentally demonstrated that, by introducing superhydrophilic surfaces with micro-and nanostructures into pool boiling systems, the maximum heat flux of boiling can be increased before catastrophic dryout occurs [12,13]. We have also investigated and explained the underlying mechanism of delayed dryout, which is attributed to the enhanced liquid transport on superhydrophilic structures aided by capillary pressure. In continuation of this effort, we have also implemented microstructured superhydrophilic surfaces in flow boiling systems where heat transfer performance are also influenced by many other factors such as flow rate, channel geometry, and subcooling. We are currently working to better understand the role of microstructures in flow boiling in order to develop better thermal management strategies [14]. In addition to structured surfaces, we are investigating how surfactant additives that adsorb to solid-liquid and liquid-vapor interfaces can enhance boiling performance by increasing bubble nucleation [15].

Boiling work at the Device Research Laboratory is funded by the Masdar Institute and the Battelle Memorial Institute.

Thin Film Evaporation

With the increase in processing speed in compact electronic devices, passive heat transfer cooling technologies with the ability cool heat fluxes of up to 100 W/cm2 are highly desired in the microelectronics industry. Conventional air cooling strategies are insufficient at these large heat fluxes. As a result, novel thermal management solutions such as thin-film evaporation that utilize the latent heat of vaporization of a fluid are needed. The high latent heat of vaporization associated with typical liquid-vapor phase change phenomena allows significant heat transfer with little temperature rise.

At the Device Research Laboratory, we use state-of-the-art silicon fabrication techniques to fabricate microstructured surfaces to implement thin-film evaporation. We started by characterizing and optimizing liquid propagation on these micropillar array structures [3,4]. The microstructures improve liquid spreading by generating capillary pressure in addition to increasing the thin-film region where the majority of the evaporation occurs. Moreover, the liquid-film thickness and the associated thermal resistance is minimum in the thin-film region making thin-film evaporation an attractive choice for dissipating high heat flux. However, one limitation in using microstructured surfaces for thin-film evaporation is drying out which occurs when the generated capillary pressure cannot transport enough liquid to sustain the evaporation. To overcome this limitation on the capillary pressure budget, we use biporous wicks (two level of porosity) to enhance liquid spreading as well as to achieve high heat flux and heat transfer coefficient. Closely spaced micropillar arrays (first level porosity) are used to generate higher capillary pressure that assists liquid spreading while larger microchannels (second level porosity) are used to reduce the overall viscous loss by providing a less-viscous bypass path for fluid flow. Preliminary experimental results indicate that thin-film evaporation is a promising strategy to dissipate higher heat fluxes in excess of 100 W/cm2 [16,17].

The thin-film evaporation division of the Device Research Laboratory is funded by the Office of Naval Research (ONR) with Mark Spector as program manager and the SMART program.

Condensation

Condensation is a phase change phenomenon often encountered in nature but also harnessed for industrial applications including power generation, thermal management, desalination, and environmental control. For the past eight decades, researchers have focused on creating surfaces which allow condensed droplets to be easily removed by gravity for enhanced heat transfer performance [18]. Recent advancements in nanofabrication have enabled increased control of surface structuring for the development of superhydrophobic surfaces with even higher droplet mobility and, in some cases, coalescence-induced droplet jumping [19].

At the Device Research Laboratory, we theoretically [20] and experimentally [21] study superhydrophobic [22] and oleophobic [23] surfaces to enhance condensation heat transfer for water and refrigerant based condensation systems. We work on identifying challenges and new opportunities to advance these surfaces for broad implementation into thermo-fluidic systems [24]. For example, the recent discovery of jumping droplet electrostatic charging [25] has led to applications in condensation heat transfer enhancement [26] and energy harvesting [27].

In addition, we study the fabrication, characterization, wettability, and interfacial dynamics of superhydrophobic materials during condensation to examine the role of surface structure on emerging droplet morphology [28], nucleation density [29], droplet growth rate [30], and departure characteristics [31]. Furthermore, we seek to develop scalable [32] fabrication techniques for creating superhydrophobic surfaces with experimentally demonstrated heat transfer enhancement, and we investigate the robustness of these surfaces under industrial conditions [33] as well as the effects on contamination on wetting properties [34].

The condensation work in the Device Research Laboratory is supported by the Office of Naval Research (ONR) as well as the Electric Power Research Institute and the Abu Dhabi National Oil Company.

  1. K.-H. Chu, R. Xiao, E.N. Wang, "Uni-directional liquid spreading on asymmetric nanostructured surfaces," Nature Materials, 9(5), p. 413-417, 2010.
  2. R. Raj, S. Adera, R. Enright, E.N. Wang, "High Resolution Liquid Patterns via 3D Droplet Shape Control," Nat Comm, 5, 2014.
  3. M. McCarthy, K. Gerasopoulos, R. Enright, J.N. Culver, R. Ghodssi, E.N. Wang, "Biotemplated hierarchical surfaces and the role of dual length scales on the repellency of impacting droplets," Applied Physics Letters, 100(263701), p. 1-5, 2012.
  4. R. Xiao, E.N. Wang, "Microscale Liquid Dynamics and the Effect on Macroscale Propagation in Pillar Arrays," Langmuir, 27(17), p. 10360-10364, 2011.
  5. R. Xiao, R. Enright, E.N. Wang, "Prediction and Optimization of Liquid Propagation in Micropillar Arrays," Langmuir, 26(19), p. 15070-15075, 2010.
  6. R. Raj, R. Enright, Y. Zhu, S. Adera, E.N. Wang, "Unified Model for Contact Angle Hysteresis on Heterogeneous and Superhydrophobic Surfaces," Langmuir, 28(45), p. 15777-15788, 2012.
  7. R. Raj, S.C. Maroo, E.N. Wang, "Wettability of Graphene," Nano Letters, 13, p. 1509-1514, 2013.
  8. S. Adera, R. Raj, R. Enright, E.N. Wang, "Non-Wetting Droplets on Hot Superhydrophilic Surfaces," Nature Communications, 4, 2013.
  9. E.N. Wang, M. Bucaro, J.A. Taylor, P. Kolodner, J. Aizenberg, T.N. Krupenkin, "Droplet mixing using electrically tunable superhydrophobic nanostructured surfaces," Microfluidics and Nanofluidics, 7(1), p. 137-140, 2009.
  10. T.N. Krupenkin, J.A. Taylor, E.N. Wang, P. Kolodner, M. Hodes, T.R. Salamon, "Reversible Wetting-Dewetting Transitions on Electrically Tunable Superhydrophobic Nanostructured Surfaces," Langmuir, 23(18), p. 9128-9133, 2007.
  11. Y. Zhu, R. Xiao, E.N. Wang, "Design and Fabrication of Magnetically Tunable Microstructured Surfaces," Transducers, Barcelona, Spain, June 16-20, 2013.
  12. K.-H. Chu, R. Enright, E.N. Wang, "Structured surfaces for enhanced pool boiling heat transfer," Applied Physics Letters, 100(24), p. 241603-00, 2012.
  13. 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(00), p. 151602-00, 2013.
  14. Y. Zhu, D.S. Antao, K.-H. Chu, T.J. Hendricks, E.N. Wang, "Enhanced Flow Boiling Heat Transfer in Microchannels with Structured Surfaces," International Heat Transfer Conference, Kyoto, Japan, August 10-15, 2014.
  15. H.J. Cho, V. Sresht, D. Blankschtein, E.N. Wang, "Understanding Enhanced Boiling with Triton X Surfactants," Proceedings of the ASME 2013 Summer Heat Transfer Conference, Minneapolis, MN, July 14-19, 2013.
  16. S. Adera, R. Raj, E.N. Wang, "Capillary-limited evaporation from well-defined microstructured surfaces," Proceedings of the 4th International Conference on Micro/Nano Scale Heat and Mass Transfer, Hong Kong, December 11-14, 2013.
  17. R. Raj, S. Adera, Q. Liang, R. Xiao, C.S. Tan, E.N. Wang, "Experiments, Modeling and Optimization of Thin Film Evaporation in Microstructured Capillary Wicks," Proceedings of the ASME 2013 Summer Heat Transfer Conference, Minneapolis, MN, July 14-19, 2013.
  18. J. W. Rose, "Dropwise condensation theory and experiment: a review," Proceedings of the Institution of Mechanical Engineers Part a-Journal of Power and Energy, vol. 216, pp. 115-128, 2002.
  19. J. B. Boreyko and C. H. Chen, "Self-Propelled Dropwise Condensate on Superhydrophobic Surfaces," Physical Review Letters, vol. 103, pp. 184501-1 - 184501-4, Oct 30 2009.
  20. N. Miljkovic, et al., "Modeling and Optimization of Superhydrophobic Condensation," Journal of Heat Transfer, vol. 135, pp. 111004-111004, 2013.
  21. N. Miljkovic, et al., "Jumping-Droplet-Enhanced Condensation on Scalable Superhydrophobic Nanostructured Surfaces," Nano Letters, vol. 13, pp. 179-187, Jan 2013.
  22. A. Lafuma and D. Quere, "Superhydrophobic States," Nature Materials, vol. 2, pp. 457-460, Jul 2003.
  23. R. Xiao, et al., "Immersion Condensation on Oil-Infused Heterogeneous Surfaces for Enhanced Heat Transfer," Scientific Reports, vol. 3, Jun 13 2013.
  24. N. Miljkovic and E. N. Wang, "Condensation heat transfer on superhydrophobic surfaces," Mrs Bulletin, vol. 38, pp. 397-406, May 2013.
  25. N. Miljkovic, et al., "Electrostatic charging of jumping droplets," Nature Communications, vol. 4, Sep 2013.
  26. N. Miljkovic, et al., "Electric-Field-Enhanced Condensation on Superhydrophobic Nanostructured Surfaces," Acs Nano, vol. 7, pp. 11043-11054, Dec 2013.
  27. D. J. Preston, et al., "Jumping Droplet Electrostatic Charging and Effect on Vapor Drag," Journal of Heat Transfer, vol. accepted, 2014.
  28. N. Miljkovic, et al., "Jumping-droplet electrostatic energy harvesting," Applied Physics Letters, vol. 105, Jul 7 2014.
  29. R. Enright, et al., "Condensation on Superhydrophobic Surfaces: The Role of Local Energy Barriers and Structure Length Scale," Langmuir, vol. 28, pp. 14424-14432, Oct 9 2012.
  30. R. Enright, et al., "Condensation on Superhydrophobic Copper Oxide Nanostructures," Proceedings of the Asme Micro/Nanoscale Heat and Mass Transfer International Conference, 2012, pp. 419-425, 2012.).
  31. N. Miljkovic, et al., "Effect of Droplet Morphology on Growth Dynamics and Heat Transfer during Condensation on Superhydrophobic Nanostructured Surfaces," Acs Nano, vol. 6, pp. 1776-1785, Feb 2012.
  32. A. Cavalli, et al., "Electrically induced drop detachment and ejection," Physics of Fluids, vol. 28, Feb 2016.
  33. D. J. Preston, et al., "Scalable Graphene Coatings for Enhanced Condensation Heat Transfer," Nano Letters, vol. 15, pp. 2902-2909, May 2015.
  34. D. J. Preston, et al., "Effect of hydrocarbon adsorption on the wettability of rare earth oxide ceramics," Applied Physics Letters, vol. 105, Jul 7 2014.