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Now showing items 1 - 16 of 63

  • Fundamental limits of jumping droplet heat transfer

    Foulkes, Thomas   Sett, Soumyadip   Sokalski, Peter   Oh, Junho   Miljkovic, Nenad  

    Liquid-vapor phase-change cooling has a significant potential to facilitate the development of highly dense electronics by leveraging latent heat during the phase transition to remove heat from hotspots. A promising form of liquid-vapor phase-change cooling is coalescence-induced jumping droplet condensation, where droplet growth results in coalescence and gravity-independent jumping from the cold surface due to capillary-inertial energy conversion. Once the departed droplets reach the hotspot, heat is extracted via evaporation and through vapor return, subsequently spreading to the cold surface via condensation. Realizing the full potential of jumping droplet cooling requires a detailed understanding of the physics governing the process. Here, we examine the fundamental thermal and hydrodynamic limits of jumping droplet condensation. We demonstrate that jumping is mainly governed by the rate of droplet growth and fluid thermophysical properties. Timescale analysis demonstrates that the upper bound of water vapor jumping droplet condensation critical heat flux is similar to 20kW/cm(2), significantly higher than that experimentally observed thus far due to surface structure limitations. Analysis of a wide range of available working fluids shows that liquid metals such as Li, Na, and Hg can obtain superior performance when compared to water.
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  • High-Throughput Stamping of Hybrid Functional Surfaces

    Hoque, Muhammad Jahidul   Yan, Xiao   Keum, Hohyun   Li, Longnan   Cha, Hyeongyun   Park, Jun Kyu   Kim, Seok   Miljkovic, Nenad  

    Hydrophobic-hydrophilic hybrid surfaces, sometimes termed biphilic surfaces, have shown potential to enhance condensation and boiling heat transfer, anti-icing, and fog harvesting performance. However, state of art techniques to develop these surfaces have limited substrate selection, poor scalability, and lengthy and costly fabrication methods. Here, we develop a simple, scalable, and rapid stamping technique for hybrid surfaces with spatially controlled wettability. To enable stamping, rationally designed and prefabricated polydimethylsiloxane (PDMS) stamps, which are reusable and independent of the substrate and functional coating, were used. To demonstrate the stamping technique, we used silicon wafer, copper, and aluminum substrates functionalized with a variety of hydrophobic chemistries including heptadecafluorodecyltrimethoxy-silane, octafluorocyclobutane, and slippery omniphobic covalently attached liquids. Condensation experiments and microgoniometric characterization demonstrated that the stamped surfaces have global hydrophobicity or superhydrophobicity with localized hydrophilicity (spots) enabled by local removal of the functional coating during stamping. Stamped surfaces with superhydrophobic backgrounds and hydrophilic spots demonstrated stable coalescence induced droplet jumping. Compared to conventional techniques, our stamping method has comparable prototyping cost with reduced manufacturing time scale and cost. Our work not only presents design guidelines for the development of scalable hybrid surfaces for the study of phase change phenomena, it develops a scalable and rapid stamping protocol for the cost-effective manufacture of next-generation hybrid wettability surfaces.
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  • Environment-Friendly Antibiofouling Superhydrophobic Coatings

    Razavi, Seyed Mohammad Reza   Oh, Junho   Haasch, Richard T.   Kim, Kyungsub   Masoomi, Mahmood   Bagheri, Rouhollah   Slauch, James M.   Miljkovic, Nenad  

    Hydrophobic surfaces have the potential to enhance the efficiency of a plethora of applications, from heat exchangers, to underwater structures, to food industry and oil-water filtration. A large fraction of currently available hydrophobic coatings consist of perfluorinated compounds or organosilane-based chemistries, both of which can be toxic and bioaccumulate in nature. Here, we develop environmentally friendly and economical superhydrophobic coatings using naturally abundant sepiolite nanoparticles functionalized with naturally extracted fatty acids from cinnamon and myristica. We demonstrate our coating on a variety of metallic and nonmetallic surfaces with dip-coating of aluminum, absorbent fabrics, glass, and even paper. Contact angle measurements revealed the ability to scalably produce high apparent advancing contact angles (>160 degrees) with low contact angle hysteresis (<5 degrees). We characterized our coated surfaces for their antibiofouling characteristics using Gram negative and Gram positive bacteria. The results showed that the bacterial attachment considerably decreased (<5%) compared to the untreated surfaces (similar to 30%), resulting in lower biofouling. The chemical, mechanical, and thermal durabilities of the coating were studied, with results showing that immersing the samples in different pH aqueous solutions (4 <=3D pH <=3D 10) and exposing the samples to different temperatures (T < 200 degrees C) for various times does not have a significant effect on the superhydrophobicity of the samples. Our work not only presents the development of naturally-derived and environment-friendly superhydrophobic antibiofouling coatings, it demonstrates a pathway for future research on the development of sustainable and ecological functional coatings.
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  • Self-assembled liquid bridge confined boiling on nanoengineered surfaces

    Foulkes, Thomas   Oh, Junho   Pilawa-Podgurski, Robert   Miljkovic, Nenad  

    Increasing electrification of mechanically controlled or driven systems has created a demand for the development of compact, lightweight electronics. Removing waste heat from these high volumetric and gravimetric power dense assemblies, especially in mobile applications, requires non-traditional thermal management strategies with high heat flux potential and low integration penalty. Here, we develop and study confined subcooled pool boiling on nanoengineered surfaces which enables self-assembly of liquid bridges capable of high heat flux dissipation without external pumping. Using high-speed optical imaging coupled with high-fidelity heat transfer experiments in pure vapor environments, we study the physics of liquid bridge formation, bridge lifetime, and heat transfer. We demonstrate heat flux dissipations >100 W/cm(2) from a gallium nitride (GaN) power transistor residing above a horizontally parallel superhydrophobic nanostructured aluminum cold plate. To understand the confined bridge dynamics, we develop a hydrodynamic droplet bridging model and design rules capable of predicting the effects of gravity, intrinsic contact angle, contact angle hysteresis, and device heat flux. Our work not only demonstrates an ultra-efficient mechanism of heat dissipation and spreading using nanoengineered surfaces coupled to fluid confinement, but also enables the development of fully three-dimensional integrated electronics. (C) 2018 Elsevier Ltd. All rights reserved.
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  • Thin Film Condensation on Nanostructured Surfaces

    Oh, Junho   Zhang, Runyu   Shetty, Pralav P.   Krogstad, Jessica A.   Braun, Paul V.   Miljkovic, Nenad  

    Water vapor condensation is a ubiquitous process in nature and industry. Over the past century, methods achieving dropwise condensation using a thin (<1 mu m) hydrophobic "promoter" layer have been developed, which increases the condensation heat transfer by ten times compared to filmwise condensation. Unfortunately, implementations of dropwise condensation have been limited due to poor durability of the promoter coatings. Here, thin-film condensation which utilizes a promoter layer not as a condensation surface, but rather to confine the condensate within a porous biphilic nanostructure, nickel inverse opals (NIO) with a thin (<20 nm) hydrophobic top layer of decomposed polyimide is developed. Filmwise condensation confined to thicknesses <10 mu m is demonstrated. To test the stability of thin-film condensation, condensation experiments are performed to show that at higher supersaturations droplets coalescing on top of the hydrophobic layer are absorbed into the superhydrophilic layer through coalescence-induced transitions. Through detailed thermal-hydrodynamic modeling, it is shown that thin-film condensation has the potential to achieve heat transfer coefficients approaching approximate to 100 kW m(-2) while avoiding durability issues by significantly reducing nucleation on the hydrophobic surface. The work presented here develops an approach to potentially ensure durable and high-performance condensation comparable to dropwise condensation.
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  • Results of EAHP's 2018 Survey on Medicines Shortages

    Miljkovic, Nenad   Gibbons, Nicholas   Batista, Aida   Fitzpatrick, Raymond William   Underhill, Jonathan   Horak, Petr  

    Aims and objectives The aim of the 2018 EAHP Survey on Medicines Shortages was to provide a clearer picture on the issue of medicines shortages, including the impact on hospital pharmacists' time, budgets and the effect on patient care. Methods A survey was conducted by the EAHP, collecting information from European hospital pharmacists on the shortage situation in their respective countries. The survey ran from 19 March 2018 to 11 June 2018. Keele University, UK analysed and compared the results to those of the 2014 survey. Results There were 1666 responses to the 2018 survey, which represented a threefold increase from the 2014 survey which received 607 responses. Ninety per cent of respondents answered ' Yes' when asked if shortages of medicines are a current problem in delivering the best care to patients, while only 7% of respondents answered ' No', and 3% ' Unsure'. Problems with shortages of antimicrobials were most commonly reported (77% of respondents reporting this as an issue in 2018 vs 57% in 2014), followed by preventative medicines (43% in 2018 vs 20% in 2014) and anaesthetics (39% in 2018 vs 27% in 2014). Fiftynine per cent of respondents have seen care delayed as a consequence of medication shortages, with cancellations of care (31% of respondents), medication errors (25% of respondents) and suboptimal treatment for patients (25% of respondents) also being frequently reported. Sixty-three per cent of respondents reported having had to pay a higher price to procure from alternate sources most of the time or always when there was a shortage of a medicine. Conclusions Medicines shortages is an increasing problem across Europe and is having an adverse impact on patient care. Medicines shortages are adding to hospital pharmacists' time pressures and have an adverse budgetary impact. More timely information about impending shortages and how long they will last is seen as necessary to help manage the problem.
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  • Numerical Simulation of Jumping Droplet Condensation

    Birbarah, Patrick   Chavan, Shreyas   Miljkovic, Nenad  

    Jumping droplet condensation has been shown to enhance heat transfer performance (approximate to 100%) when compared to dropwise condensation by reducing the time-averaged droplet size (approximate to 10 mu m) on the condensing surface. Here, we develop a rigorous, three-dimensional numerical simulation of jumping droplet condensation to compute the steady-state time-averaged droplet size distribution. To characterize the criteria for achieving steady state, we use maximum radii (R-max) tracking on the surface, showing that R-max settles to an average in time once steady state is reached. The effects of the minimum jumping radius (0.1-10 mu m), maximum jumping radius, apparent advancing contact angle (150-175 degrees), and droplet growth rate were investigated. We provide a numerical fit for the droplet size distribution with an overall correlation coefficient greater than 0.995. The heat transfer performance was evaluated as a function of apparent contact angle and hydrophobic coating thickness, showing excellent agreement with prior experimentally measured values. Our simulations uncovered that droplet size mismatch during coalescence has the potential to impede the achievement of steady state and describe a new flooding mechanism for jumping droplet condensation. Our work not only develops a unified numerical model for jumping droplet condensation that is extendable to a plethora of other conditions but also demonstrates design criteria for nonwetting surface manufacture for enhanced jumping droplet condensation heat transfer.
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  • Self-assembled liquid bridge confined boiling on nanoengineered surfaces

    Foulkes, Thomas   Oh, Junho   Pilawa-Podgurski, Robert   Miljkovic, Nenad  

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  • Hierarchical Condensation

    Yan, Xiao   Chen, Feng   Sett, Soumyadip   Chavan, Shreyas   Li, Hang   Feng, Lezhou   Li, Longnan   Zhao, Fulong   Zhao, Chongyan   Huang, Zhiyong   Miljkovic, Nenad  

    With the recent advances in surface fabrication technologies, condensation heat transfer has seen a renaissance. Hydrophobic and superhydrophobic surfaces have all been employed as a means to enhance condensate shedding, enabling micrometric droplet departure length scales. One of the main bottlenecks for achieving higher condensation efficiencies is the difficulty of shedding sub-10 mu m droplets due to the increasing role played by surface adhesion and viscous limitations at nanometric length scales. To enable ultraefficient droplet shedding, we demonstrate hierarchical condensation on rationally designed copper oxide microhill structures covered with nanoscale features that enable large (similar to 100 mu m) condensate droplets on top of the microstructures to coexist with smaller (<1 mu m) droplets beneath. We use high-speed optical microscopy and focal plane shift imaging to show that hierarchical condensation is capable of efficiently removing sub-10-mu m condensate droplets via both coalescence and divergent-track-assisted droplet self-transport toward the large suspended Cassie-Baxter (CB) state droplets, which eventually shed via classical gravitational shedding and thereby avoid vapor side limitations encountered with droplet jumping. Interestingly, experimental growth rate analysis showed that the presence of large CB droplets accelerates individual underlying droplet growth by similar to 21% when compared to identically sized droplets not residing beneath CB droplets. Furthermore, the steady droplet shedding mechanism shifted the droplet size distribution toward smaller sizes, with similar to 70% of observable underlying droplets having radii of <=3D 5 mu m compared to similar to 30% for droplets growing without shading. To elucidate the overall heat transfer performance, an analytical model was developed to show hierarchical condensation has the potential to break the limits of minimum droplet departure size governed by finite surface adhesion and viscous effects through the tailoring of structure length scale, coalescence, and self-transport. More importantly, abrasive wear tests showed that hierarchical condensation has good durability against mechanical damage to the surface. Our study not only demonstrates the potential of hierarchical condensation as a means to break the limitations of droplet jumping, it develops rational design guidelines for micro/nanostructured surfaces to enable excellent heat transfer performance as well as extended durability.
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    Choi, Chang K.   Miljkovic, Nenad  

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  • Millimeter-scale liquid metal droplet thermal switch

    Yang, Tianyu   Kwon, Beomjin   Weisensee, Patricia B.   Kang, Jin Gu   Li, Xuejiao   Braun, Paul   Miljkovic, Nenad   King, William P.  

    Devices capable of actively controlling heat flow have been desired by the thermal management community for decades. The need for thermal control has become particularly urgent with power densification resulting in devices with localized heat fluxes as high as 1 kW/cm(2). Thermal switches, capable of modulating between high and low thermal conductances, enable the partitioning and active control of heat flow pathways. This paper reports a millimeter-scale thermal switch with a switching ratio >70, at heat fluxes near 10 W/cm(2). The device consists of a silicone channel filled with a reducing liquid or vapor and an immersed liquid metal Galinstan slug. Galinstan has a relatively high thermal conductivity (approximate to 16.5W/mK at room temperature), and its position can be manipulated within the fluid channel, using either hydrostatic pressure or electric fields. When Galinstan bridges the hot and cold reservoirs (the "ON" state), heat flows across the channel. When the hot and cold reservoirs are instead filled with the encapsulating liquid or vapor (the "OFF" state), the cross-channel heat flow significantly reduces due to the lower thermal conductivity of the solution (approximate to 0.03-0.6 W/mK). We demonstrate switching ratios as high as 15.6 for liquid filled channels and 71.3 for vapor filled channels. This work provides a framework for the development of millimeter-scale thermal switches and diodes capable of spatial and temporal control of heat flows. Published by AIP Publishing.
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  • Exploring the Role of Habitat on the Wettability of Cicada Wings

    Oh, Junho   Dana, Catherine E.   Hong, Sungmin   Roman, Jessica K.   Jo, Kyoo Dong   Hong, Je Won   Nguyen, Jonah   Cropek, Donald M.   Alleyne, Marianne   Miljkovic, Nenad  

    Evolutionary pressure has pushed many extant species to develop micro/nanostructures that can significantly affect wettability and enable functionalities such as droplet jumping, self-cleaning, antifogging, antimicrobial, and antire-flectivity. In particular, significant effort is underway to, understand the insect wing surface structure to establish rational design tools for the development of novel engineered materials. Most studies, however, have focused on super hydrophobic wings obtained from a single insect species, in particular, the Psaltoda claripennis cicada. Here, we investigate the relationship between the spatially dependent wing wettability, topology, and droplet jumping behavior of multiple cicada species and their habitat, lifecycle, and interspecies relatedness. We focus on cicada wings of four different species: Neotibicen pruinosus, N. tibicen, Megatibicen dorsatus, and Magicicada septendecim and take a comparative approach. Using spatially resolved microgoniometry, scanning electron microscopy, atomic force microscopy, and high speed optical microscopy, we show that within cicada species, the wettability of wings is spatially homogeneous across wing cells. All four species were shown to have truncated conical pillars with widely varying length scales ranging from 50 to 400 nm in height. Comparison of the wettability revealed three cicada species with wings that are superhydrophobic (>150 degrees) with low contact angle hysteresis (<5 degrees), resulting in stable droplet jumping behavior. The fourth, more distantly related species (Ma. septendecim) showed only moderate hydrophobic behavior, eliminating some of the beneficial surface functional aspects for this cicada. Correlation between cicada habitat and wing wettability yielded little connection as wetter, swampy environments do not necessarily equate to higher measured wing hydrophobicity. The results, however, do point to species relatedness and reproductive strategy as a closer proxy for predicting wettability and surface structure and resultant enhanced wing surface functionality. This work not only elucidates the differences between inter- and intraspecies cicada wing topology, wettability, and water shedding behavior but also enables the development of rational design tools for the manufacture of artificial surfaces for energy and water applications.
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  • Condensate droplet size distribution on lubricant-infused surfaces

    Weisensee, Patricia B.   Wang, Yunbo   Qian Hongliang   Schultz, Daniel   King, William P.   Miljkovic, Nenad  

    Condensation is a ubiquitous phenomenon in nature and industry. Heat transfer rates during dropwise condensation on non-wetting substrates can be 6-8X higher than heat transfer rates during traditional filmwise condensation on wetting substrates. Dropwise condensation on lubricant-infused surfaces (LIS, or SLIPS) is particularly interesting due to high droplet mobility on these surfaces. To accurately predict heat transfer rates during dropwise condensation, the distribution of droplet sizes must be known. Here we present condensation studies of water on aluminum-based lubricant-infused surfaces with a wide range of lubricant viscosities (12-2717 cSt) to determine droplet size distributions. Through optical imaging and microscopy, we show that the distribution of droplet sizes on LIS is independent of lubricant viscosity, and agrees well with the model developed by Rose for the distribution of droplet sizes on hydrophobic surfaces, especially in the range 10 < r < 100 mu m. Using artificial sweeping experiments and numerical modeling, we investigate the dependence of sweeping rates on the distribution of droplet sizes and on average heat transfer rates. The maximum size to which droplets grow before being swept decreases rapidly with only a modest decrease in sweeping period, from 750 to 62 mu m. Yet, the distribution of droplet sizes and heat transfer rates are nearly unaffected by the change in sweeping period, due to a relative insensitivity of heat transfer to droplets with radii r > 100 mu m due to a high conduction resistance within these droplets. Our work provides an experimental and analytical framework to predict heat transfer and sweeping rates for water condensation on a vertical plate coated with a LIS or SLIPS surface. (C) 2017 Elsevier Ltd. All rights reserved.
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  • Nanoscale-Agglomerate-Mediated Heterogeneous Nucleation

    Cha, Hyeongyun   Wu, Alex   Kim, Moon-Kyung   Saigusa, Kosuke   Liu, Aihua   Miljkovic, Nenad  

    Water vapor condensation on hydrophobic surfaces has received much attention due to its ability to rapidly shed water droplets and enhance heat transfer, anti-icing, water harvesting, energy harvesting, and self-cleaning performance. However, the mechanism of heterogeneous nucleation on hydrophobic surfaces remains poorly understood and is attributed to defects in the hydrophobic coating exposing the high surface energy substrate. Here, we observe the formation of high surface energy nanoscale agglomerates on hydrophobic coatings after condensation/evaporation cycles in ambient conditions. To investigate the deposition dynamics, we studied the nanoscale agglomerates as a function of condensation/evaporation cycles via optical and field emission scanning electron microscopy (FESEM), microgoniometric contact angle measurements, nucleation statistics, and energy dispersive X-ray spectroscopy (EDS). The FESEM and EDS results indicated that the nanoscale agglomerates stem from absorption of sulfuric acid based aerosol particles inside the droplet and adsorption of volatile organic compounds such as methanethiol (CH3SH), dimethyl disulfide (CH3SSCH), and dimethyl trisulfide (CH3SSSCH3) on the liquidvapor interface during water vapor condensation, which act as preferential sites for heterogeneous nucleation after evaporation. The insights gained from this study elucidate fundamental aspects governing the behavior of both short- and long-term heterogeneous nucleation on hydrophobic surfaces, suggest previously unexplored microfabrication and air purification techniques, and present insights into the challenges facing the development of durable dropwise condensing surfaces.
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  • Electric Field-Based Control and Enhancement of Boiling and Condensation

    Shahriari, Arjang   Birbarah, Patrick   Oh, Junho   Miljkovic, Nenad   Bahadur, Vaibhav  

    This article reviews and analyzes recent advancements on boiling and condensation heat transfer enhancement via the use of electric fields. Historically, the majority of studies on phase change heat transfer enhancement have relied on passive approaches like surface engineering. Electric fields provide distinct options to enhance and control the nano/micro/mesoscale thermofluidic phenomena associated with boiling and condensation. This work focuses on the influence of electric fields on electrically conducting liquids like water and certain organic solvents. After a brief review of past work on electric field-based heat transfer enhancement using electrically insulating liquids, we summarize and discuss recent studies involving electrically conducting liquids. It is seen that electric fields can offer disruptive advancements and benefits in the control and enhancement of boiling and condensation. Perspectives, future research directions, and applications of these novel concepts are also discussed.
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  • Steady Method for the Analysis of Evaporation Dynamics

    Gunay, A. Alperen   Sett, Soumyadip   Oh, Junho   Miljkovic, Nenad  

    Droplet evaporation is an important phenomenon governing many man-made and natural processes. Characterizing the rate of evaporation with high accuracy has attracted the attention of numerous scientists over the past century. Traditionally, researchers have studied evaporation by observing the change in the droplet size in a given time interval. However, the transient nature coupled with the significant mass-transfer-governed gas dynamics occurring at the droplet three-phase contact line makes the classical method crude. Furthermore, the intricate balance played by the internal and external flows, evaporation kinetics, thermocapillarity, binary-mixture dynamics, curvature, and moving contact lines makes the decoupling of these processes impossible with classical transient methods. Here, we present a method to measure the rate of evaporation of spatially and temporally steady droplets. By utilizing a piezoelectric dispenser to feed microscale droplets (R approximate to 9 mu m) to a larger evaporating droplet at a prescribed frequency, we can both create variable-sized droplets on any surface and study their evaporation rate by modulating the piezoelectric droplet addition frequency. Using our steady technique, we studied water evaporation of droplets having base radii ranging from 20 to 250 mu m on surfaces of different functionalities (45 < theta(a,app) <=3D 162, where theta(a,app) is the apparent advancing contact angle). We benchmarked our technique with the classical unsteady method, showing an improvement of 140% in evaporation rate measurement accuracy. Our work not only characterizes the evaporation dynamics on functional surfaces but also provides an experimental platform to finally enable the decoupling of the complex physics governing the ubiquitous droplet evaporation process.
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