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Pujahari A, DasGupta S, Bhattacharya A. Electro-osmosis Aided Thin-Film Evaporation from a Micropillar Wick Structure. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2022; 38:8442-8455. [PMID: 35771505 DOI: 10.1021/acs.langmuir.2c01048] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
The heat-dissipating capacity of a surface having micropillar wick structures, which resembles the evaporator section of a vapor chamber, is mainly limited by the liquid flow rate through the porous structure (permeability) and the capillary pressure gradient. The efficacy of a regular vapor chamber is determined from two parameters, namely, the dry-out heat flux and temperature of the evaporator surface. These two parameters possess a counter relation to each other. The work described herein introduces and evaluates the performance of a novel idea of electro-osmosis-aided thin-film evaporation from a micropillar array structure. This study is conducted using a discretized approach that is validated against the thin-film evaporation model and additionally the electro-osmotic flow model with pre-existing pressure gradient conditions. The unique feature of this approach is that it results in an increment in the magnitude of dry-out heat flux without significantly changing the surface temperature, wherein the increase in permeability is due to the addition of electro-osmotic flow. This comprehensive model considers various geometries, zeta potentials, and extremal electric fields and establishes the beneficial effects of the application of an external electric field. The results are used to predict the sensitivity and the dependence of the dry-out heat flux and the evaporator surface temperature on these parameters. For a host of electro-osmotic parameters considered herein, a maximum increment of up to 320% in the dry-out heat flux is observed for an external electric field of 105 V/m. The study, therefore, conclusively demonstrates the beneficial impact of electro-osmosis in enhancing the dry-out heat flux without any significant Joule heating.
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Affiliation(s)
- Ankita Pujahari
- Mechanical Engineering Department, IIT Kharagpur, Kharagpur, West Bengal Pin 721302, India
| | - Sunando DasGupta
- Chemical Engineering Department, IIT Kharagpur, Kharagpur, West Bengal Pin 721302, India
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Akbari R, Antonini C. Contact angle measurements: From existing methods to an open-source tool. Adv Colloid Interface Sci 2021; 294:102470. [PMID: 34186300 DOI: 10.1016/j.cis.2021.102470] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2021] [Revised: 06/18/2021] [Accepted: 06/18/2021] [Indexed: 12/19/2022]
Abstract
Contact angle measurement is an effective way to investigate solid surface properties. The introduction of low-cost digital cameras, as well as software and libraries for image analysis, has made contact angle measurement potentially accessible to every laboratory. In this review, we provide a comparison of the main methods developed to evaluate contact angle from digital images, including the so-called Young-Laplace method, the circle and polynomial fittings, as well as the mask method. All methods have been implemented and compared analyzing virtual and real drop images in an open-source software, Dropen, developed as an app in MATLAB environment. The code enables single image analysis evaluation, for the robust automatic identification of the contact points and contact angle evaluation, with the goal of minimizing user inputs, automatizing the process and facilitating measurements for all users, from less experienced to advanced wetting experts. Dropen and its code are made available at BOA, the Bicocca Open Access public repository, for use and further development.
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Pu JH, Wang SK, Sun J, Wang W, Wang HS. Stable and Efficient Nanofilm Pure Evaporation on Nanopillar Surfaces. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2021; 37:3731-3739. [PMID: 33730854 DOI: 10.1021/acs.langmuir.1c00236] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
Molecular dynamics simulations were conducted to systematically investigate how to maintain and enhance nanofilm pure evaporation on nanopillar surfaces. First, the dynamics of the evaporation meniscus and the onset and evolution of nanobubbles on nanopillar surfaces were characterized. The meniscus can be pinned at the top surface of the nanopillars during evaporation for perfectly wetting fluid. The curvature of the meniscus close to nanopillars varies dramatically. Nanobubbles do not originate from the solid surface, where there is an ultrathin nonevaporation film due to strong solid-fluid interaction, but originate and evolve from the corner of nanopillars, where there is a quick increase in potential energy of the fluid. Second, according to a parametric study, the smaller pitch between nanopillars (P) and larger diameter of nanopillars (D) are found to enhance evaporation but also raise the possibility of boiling, whereas the smaller height of nanopillars (H) is found to enhance evaporation and suppress boiling. Finally, it is revealed that the nanofilm thickness should be maintained beyond a threshold, which is 20 Å in this work, to avoid the suppression effect of disjoining pressure on evaporation. Moreover, it is revealed that whether the evaporative heat transfer is enhanced on the nanopillar surface compared with the smooth surface is also affected by the nanofilm thickness. The value of nanofilm thickness should be determined by the competition between the suppression effect on evaporation due to the decrease in the volume of supplied fluid and the existence of capillary pressure and the enhancement effect on evaporation due to the increase in the heating area. Our work serves as the guidelines to achieve stable and efficient nanofilm pure evaporative heat transfer on nanopillar surfaces.
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Affiliation(s)
- Jin Huan Pu
- School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, United Kingdom
| | - Si Kun Wang
- School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, United Kingdom
| | - Jie Sun
- School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China
| | - Wen Wang
- School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, United Kingdom
| | - Hua Sheng Wang
- School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, United Kingdom
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Wang R, Jakhar K, Antao DS. Unified Modeling Framework for Thin-Film Evaporation from Micropillar Arrays Capturing Local Interfacial Effects. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2019; 35:12927-12935. [PMID: 31525296 DOI: 10.1021/acs.langmuir.9b02048] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Thin-film evaporation from micropillar array porous media has gained attention in a number of fields including energy conversion and thermal management of electronics. Performance in these applications is enhanced by leveraging the geometries of the micropillar arrays to both optimize flow through these arrays via capillary pumping and increase the curved liquid-vapor interface (meniscus) area for active phase-change heat transfer. In this work, we present a unified semianalytical modeling framework to predict the dry-out heat flux accurately for thin-film evaporation from micropillar arrays with the precise prediction of (i) the pressure profile along the wick achieved by discretizing the porous media domain and (ii) the local permeability that depends on the local meniscus shape. We validate the permeability model with 3D numerical simulations and verify the accuracy of the thin-film evaporation modeling framework with available experimental data from the literature. We emphasize the importance of predicting an accurate liquid-vapor interface shape for the prediction accuracy of both the permeability and the associated governing equations for liquid propagation and phase-change heat transfer through porous materials. This modeling framework is an accurate non-CFD-based methodology for predicting the dry-out heat flux during thin-film evaporation from micropillar arrays and will serve as a general framework for modeling steady liquid-vapor phase-change processes (evaporation and condensation) in porous media.
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Affiliation(s)
- Ruisong Wang
- J. Mike Walker '66 Department of Mechanical Engineering , Texas A&M University , College Station , Texas 77843-3123 , United States
| | - Karan Jakhar
- J. Mike Walker '66 Department of Mechanical Engineering , Texas A&M University , College Station , Texas 77843-3123 , United States
| | - Dion S Antao
- J. Mike Walker '66 Department of Mechanical Engineering , Texas A&M University , College Station , Texas 77843-3123 , United States
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Wang R, Antao DS. Capillary-Enhanced Filmwise Condensation in Porous Media. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:13855-13863. [PMID: 30372087 DOI: 10.1021/acs.langmuir.8b02611] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Condensation is prevalent in various industrial and heat/mass transfer applications, and improving condensation heat transfer has a direct effect on process efficiency. Enhancing condensation performance has historically been achieved via the use of low surface energy coatings to promote the efficient dropwise mode over the typical filmwise mode of condensation. However, low surface tension fluids condense on these coatings in the filmwise mode, and low surface energy coatings are generally not robust at thicknesses required to enhance condensation heat transfer. We present a robust and scalable condensation enhancement method where a high heat transfer coefficient is achieved by leveraging capillary forces within a high thermal conductivity porous wick to promote condensate removal. The capillary pressure is supported by a pump to sustain steady condensate removal, and the high thermal conductivity of the wick decreases the overall thermal resistance. This technique has the potential to enhance condensation for a variety of fluids including low surface tension fluids and is capable of operating in both a gravity and a micro- (or zero-) gravity environment. We highlight key characteristics and enhancements achieved through this capillary-enhanced filmwise condensation technique using a porous media flow model. The model results indicate that increased wick thickness and permeability increase the operational envelope and delay the failure that occurs when the condensate floods the wick. However, increasing the permeability is more favorable as both the heat transfer coefficient and the flooding threshold are increased. The working fluid thermophysical properties determine both the degree of enhancement possible and the relative contributions from gravitational and capillary pressure forces when condensation occurs in the presence of gravity. This study provides fundamental insight into an enhanced filmwise condensation technique and an improved framework for modeling porous media flows with mass addition via condensation.
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Affiliation(s)
- Ruisong Wang
- J. Mike Walker '66 Department of Mechanical Engineering , Texas A&M University , College Station , Texas 77843 , United States
| | - Dion S Antao
- J. Mike Walker '66 Department of Mechanical Engineering , Texas A&M University , College Station , Texas 77843 , United States
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Ghosh UU, DasGupta S. Field-Assisted Contact Line Motion in Thin Films. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:12665-12679. [PMID: 29664644 DOI: 10.1021/acs.langmuir.7b04322] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
The balance of intermolecular and surface forces plays a critical role in the transport phenomena near the contact line region of an extended meniscus in several technologically important processes. Externally applied fields can alter the equilibrium and stability of the meniscus with concomitant effects on its shape and spreading characteristics and may even lead to an oscillation. This feature article provides a detailed account of the present and past efforts in exploring the behavior of curved thin liquid films subjected to mild thermal perturbations, heat input, and electrical and magnetic fields for pure as well as colloidal suspensions, including the effects of particle charge and polarity. The shape-dependent intermolecular force field has been evaluated in situ by a nonobtrusive optical technique utilizing the interference phenomena and subsequent image processing. The critical role of disjoining pressure is identified along with the determination of the Hamaker constant. The spatial and temporal variations of the capillary forces are evaluated for the advancing and receding menisci. The Maxwell-stress-induced enhanced spreading during electrowetting, at relatively low voltages, and that due to the application of a magnetic field are discussed with respect to their distinctly different characteristics and application potentials. The use of the augmented Young-Laplace equation elicited additional insights into the fundamental physics for flow in ultrathin liquid films.
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Affiliation(s)
- Udita Uday Ghosh
- Chemical Engineering Department , Indian Institute of Technology, Kharagpur , Kharagpur 721302 , India
| | - Sunando DasGupta
- Chemical Engineering Department , Indian Institute of Technology, Kharagpur , Kharagpur 721302 , India
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Mazloomi Moqaddam A, Derome D, Carmeliet J. Dynamics of Contact Line Pinning and Depinning of Droplets Evaporating on Microribs. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:5635-5645. [PMID: 29667830 DOI: 10.1021/acs.langmuir.8b00409] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
The contact line dynamics of evaporating droplets deposited on a set of parallel microribs is analyzed with the use of a recently developed entropic lattice Boltzmann model for two-phase flow. Upon deposition, part of the droplet penetrates into the space between ribs because of capillary action, whereas the remaining liquid of the droplet remains pinned on top of the microribs. In the first stage, evaporation continues until the droplet undergoes a series of pinning-depinning events, showing alternatively the constant contact radius and constant contact angle modes. While the droplet is pinned, evaporation results in a contact angle reduction, whereas the contact radius remains constant. At a critical contact angle, the contact line depins, the contact radius reduces, and the droplet rearranges to a larger apparent contact angle. This pinning-depinning behavior goes on until the liquid above the microribs is evaporated. By computing the Gibbs free energy taking into account the interfacial energy, pressure terms, and viscous dissipation due to drop internal flow, we found that the mechanism that causes the unpinning of the contact line results from an excess in Gibbs free energy. The spacing distance and the rib height play an important role in controlling the pinning-depinning cycling, the critical contact angle, and the excess Gibbs free energy. However, we found that neither the critical contact angle nor the maximum excess Gibbs free energy depends on the rib width. We show that the different terms, that is, pressure term, viscous dissipation, and interfacial energy, contributing to the excess Gibbs free energy, can be varied differently by varying different geometrical properties of the microribs. It is demonstrated that, by varying the spacing distance between the ribs, the energy barrier is controlled by the interfacial energy while the contribution of the viscous dissipation is dominant if either rib height or width is changed. Main finding of this is study is that, for microrib patterned surfaces, the energy barrier required for the contact line to depin can be enlarged by increasing the spacing or the rib height, which can be important for practical applications.
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Affiliation(s)
- Ali Mazloomi Moqaddam
- Chair of Building Physics, Department of Mechanical and Process Engineering , ETH Zurich , 8092 Zurich , Switzerland
- Laboratory for Multiscale Studies in Building Physics, Empa , Swiss Federal Laboratories for Materials Science and Technology , 8600 Dübendorf , Switzerland
| | - Dominique Derome
- Laboratory for Multiscale Studies in Building Physics, Empa , Swiss Federal Laboratories for Materials Science and Technology , 8600 Dübendorf , Switzerland
| | - Jan Carmeliet
- Chair of Building Physics, Department of Mechanical and Process Engineering , ETH Zurich , 8092 Zurich , Switzerland
- Laboratory for Multiscale Studies in Building Physics, Empa , Swiss Federal Laboratories for Materials Science and Technology , 8600 Dübendorf , Switzerland
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Preston DJ, Wilke KL, Lu Z, Cruz SS, Zhao Y, Becerra LL, Wang EN. Gravitationally Driven Wicking for Enhanced Condensation Heat Transfer. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:4658-4664. [PMID: 29578348 DOI: 10.1021/acs.langmuir.7b04203] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Vapor condensation is routinely used as an effective means of transferring heat or separating fluids. Filmwise condensation is prevalent in typical industrial-scale systems, where the condensed fluid forms a thin liquid film due to the high surface energy associated with many industrial materials. Conversely, dropwise condensation, where the condensate forms discrete liquid droplets which grow, coalesce, and shed, results in an improvement in heat transfer performance of an order of magnitude compared to filmwise condensation. However, current state-of-the-art dropwise technology relies on functional hydrophobic coatings, for example, long chain fatty acids or polymers, which are often not robust and therefore undesirable in industrial conditions. In addition, low surface tension fluid condensates, such as hydrocarbons, pose a unique challenge because common hydrophobic condenser coatings used to shed water (with a surface tension of 73 mN/m) often do not repel fluids with lower surface tensions (<25 mN/m). We demonstrate a method to enhance condensation heat transfer using gravitationally driven flow through a porous metal wick, which takes advantage of the condensate's affinity to wet the surface and also eliminates the need for condensate-phobic coatings. The condensate-filled wick has a lower thermal resistance than the fluid film observed during filmwise condensation, resulting in an improved heat transfer coefficient of up to an order of magnitude and comparable to that observed during dropwise condensation. The improved heat transfer realized by this design presents the opportunity for significant energy savings in natural gas processing, thermal management, heating and cooling, and power generation.
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Affiliation(s)
- Daniel J Preston
- Department of Mechanical Engineering , Massachusetts Institute of Technology , 77 Massachusetts Avenue , Cambridge , Massachusetts 02139 , United States
| | - Kyle L Wilke
- Department of Mechanical Engineering , Massachusetts Institute of Technology , 77 Massachusetts Avenue , Cambridge , Massachusetts 02139 , United States
| | - Zhengmao Lu
- Department of Mechanical Engineering , Massachusetts Institute of Technology , 77 Massachusetts Avenue , Cambridge , Massachusetts 02139 , United States
| | - Samuel S Cruz
- Department of Mechanical Engineering , Massachusetts Institute of Technology , 77 Massachusetts Avenue , Cambridge , Massachusetts 02139 , United States
| | - Yajing Zhao
- Department of Mechanical Engineering , Massachusetts Institute of Technology , 77 Massachusetts Avenue , Cambridge , Massachusetts 02139 , United States
| | - Laura L Becerra
- Shiley-Marcos School of Engineering , University of San Diego , San Diego , California 92110 , United States
| | - Evelyn N Wang
- Department of Mechanical Engineering , Massachusetts Institute of Technology , 77 Massachusetts Avenue , Cambridge , Massachusetts 02139 , United States
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Heat Transfer Enhancement During Water and Hydrocarbon Condensation on Lubricant Infused Surfaces. Sci Rep 2018; 8:540. [PMID: 29323200 PMCID: PMC5765152 DOI: 10.1038/s41598-017-18955-x] [Citation(s) in RCA: 54] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2017] [Accepted: 12/14/2017] [Indexed: 11/08/2022] Open
Abstract
Vapor condensation is routinely used as an effective means of transferring heat or separating fluids. Dropwise condensation, where discrete droplets form on the condenser surface, offers a potential improvement in heat transfer of up to an order of magnitude compared to filmwise condensation, where a liquid film covers the surface. Low surface tension fluid condensates such as hydrocarbons pose a unique challenge since typical hydrophobic condenser coatings used to promote dropwise condensation of water often do not repel fluids with lower surface tensions. Recent work has shown that lubricant infused surfaces (LIS) can promote droplet formation of hydrocarbons. In this work, we confirm the effectiveness of LIS in promoting dropwise condensation by providing experimental measurements of heat transfer performance during hydrocarbon condensation on a LIS, which enhances heat transfer by ≈450% compared to an uncoated surface. We also explored improvement through removal of noncondensable gases and highlighted a failure mechanism whereby shedding droplets depleted the lubricant over time. Enhanced condensation heat transfer for low surface tension fluids on LIS presents the opportunity for significant energy savings in natural gas processing as well as improvements in thermal management, heating and cooling, and power generation.
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Lu Z, Wilke KL, Preston DJ, Kinefuchi I, Chang-Davidson E, Wang EN. An Ultrathin Nanoporous Membrane Evaporator. NANO LETTERS 2017; 17:6217-6220. [PMID: 28926270 DOI: 10.1021/acs.nanolett.7b02889] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/19/2023]
Abstract
Evaporation is a ubiquitous phenomenon found in nature and widely used in industry. Yet a fundamental understanding of interfacial transport during evaporation remains limited to date owing to the difficulty of characterizing the heat and mass transfer at the interface, especially at high heat fluxes (>100 W/cm2). In this work, we elucidated evaporation into an air ambient with an ultrathin (≈200 nm thick) nanoporous (≈130 nm pore diameter) membrane. With our evaporator design, we accurately monitored the temperature of the liquid-vapor interface, reduced the thermal-fluidic transport resistance, and mitigated the clogging risk associated with contamination. At a steady state, we demonstrated heat fluxes of ≈500 W/cm2 across the interface over a total evaporation area of 0.20 mm2. In the high flux regime, we showed the importance of convective transport caused by evaporation itself and that Fick's first law of diffusion no longer applies. This work improves our fundamental understanding of evaporation and paves the way for high flux phase-change devices.
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Affiliation(s)
- Zhengmao Lu
- Department of Mechanical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States
| | - Kyle L Wilke
- Department of Mechanical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States
| | - Daniel J Preston
- Department of Mechanical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States
| | - Ikuya Kinefuchi
- Department of Mechanical Engineering, University of Tokyo , Bunkyo, Tokyo 113-8656, Japan
| | - Elizabeth Chang-Davidson
- Department of Mechanical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States
| | - Evelyn N Wang
- Department of Mechanical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States
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