1,720,980 research outputs found
Capillary-Limited Evaporation From Well-Defined Microstructured Surfaces
Get PDFThermal management is increasingly becoming a bottleneck for a variety of high power density applications such as integrated circuits, solar cells, microprocessors, and energy conversion devices. The performance and reliability of these devices are usually limited by the rate at which heat can be removed from the device footprint, which averages well above 100 W/cm[superscript 2] (locally this heat flux can exceed 1000 W/cm[superscript 2]). State-of-the-art air cooling strategies which utilize the sensible heat are insufficient at these large heat fluxes. As a result, novel thermal management solutions such as via thin-film evaporation that utilize the latent heat of vaporization of a fluid are needed. The high latent heat of vaporization associated with typical liquidvapor phase change phenomena allows significant heat transfer with small temperature rise. In this work, we demonstrate a promising thermal management approach where square arrays of cylindrical micropillar arrays are used for thin-film evaporation. The microstructures control the liquid film thickness and the associated thermal resistance in addition to maintaining a continuous liquid supply via the capillary pumping mechanism. When the capillary-induced liquid supply mechanism cannot deliver sufficient liquid for phase change heat transfer, the critical heat flux is reached and dryout occurs. This capillary limitation on thin-film evaporation was experimentally investigated by fabricating well-defined silicon micropillar arrays using standard contact photolithography and deep reactive ion etching. A thin film resistive heater and thermal sensors were integrated on the back side of the test sample using e-beam evaporation and acetone lift-off. The experiments were carried out in a controlled environmental chamber maintained at the water saturation pressure of ≈3.5 kPa and ≈25 °C. We demonstrated significantly higher heat dissipation capability in excess of 100 W/cm[superscript 2]. These preliminary results suggest the potential of thin-film evaporation from microstructured surfaces for advanced thermal management applications.United States. Office of Naval ResearchNational Science Foundation (U.S.). Graduate Research Fellowship ProgramBattell
Evaporation-Induced Cassie Droplets on Superhydrophilic Microstructured Surfaces
Get PDFA droplet deposited on a rough, lyophilic surface satisfying the imbibition condition, results in complete wetting. However, in this work, we demonstrate that this behavior can be altered by superheating the substrate such that droplets can reside in a non-wetting Cassie state due to evaporation. Photolithography and deep reactive ion etching were used to fabricate a well-defined silicon micropillar array with diameter, height, and center-to-center spacings of 5.3, 21.7 and 27.5 μm, respectively. Water droplets placed on this microstructured surface at room temperature demonstrated superhydrophilic behavior with liquid filling the voids between pillars resulting in a vanishing contact angle. However, when the microstructured surface was superheated above a critical value, the superhydrophilicity was lost and non-wetting Cassie droplets were formed. The superheat required to deposit a Cassie droplet (>75°C) was found to be significantly higher than that required to sustain an already deposited Cassie droplet (<35°C). Interestingly, the superheat required to sustain a Cassie droplet after the initial deposition was found to decrease with the square of the droplet radius. These observations where an inherently superhydrophilic structured surface turns into superhydrophobic at nominal superheats has implications for phase change based heat transfer applications where the loss of contact between the substrate and the heat transfer fluid can be detrimental to the device performance. Topics: Drops, EvaporationUnited States. Office of Naval Researc
Experimental Characterization and Modeling of Capillary-Pumped Thin-Film Evaporation From Micropillar Wicks
No full textGeneration of concentrated heat load in confined spaces in integrated circuits and advanced microprocessors has presented a thermal management challenge for the semiconductor industry. Compared to state-of-the-art single phase cooling strategies, phase-change based approaches are promising for cooling the next generation microelectronic devices. In particular, thin-film evaporation from engineered surfaces has received significant attention in the last few decades as a potential candidate since it enables passive transport of the working fluid via capillarity in addition to increasing the evaporation area via extending the liquid meniscus and three-phase contact line. Thin-film evaporation, however, is coupled with nucleate boiling making experimental characterization as well as modeling of the fluidic and thermal transport a challenging task for thermal engineers. Furthermore, quantifying the relative contributions of nucleate boiling and thin-film evaporation from the experimentally reported heat fluxes has been difficult. Unlike previous studies, our work experimentally characterizes thin-film evaporation in the absence of nucleate boiling from arrays of silicon micropillars. In particular, we characterize the capillary-limit where the microstructured surface dries out due to liquid starvation when the capillary pressure that is generated due to the meniscus shape cannot overcome the viscous losses within the micropillar wick. We modeled the fluidic and thermal transport of the evaporating meniscus by solving the coupled heat and mass transfer equations. Compared to experiments, our model predicts the dryout heat flux with ±20% accuracy. The insights gained from this study provide a suitable platform to design and optimize micropillar wicks for phase-change based thermal management devices such as heat pipes and vapor chambers.National Science Foundation (U.S.). Graduate Research Fellowship (Grant No. 1122374)United States. Office of Naval ResearchSingapore-MIT Alliance. Low Energy Electronic Systems Progra
Unified Model for Contact Angle Hysteresis on Heterogeneous and Superhydrophobic Surfaces
No full textUnderstanding the complexities associated with contact line dynamics on chemically heterogeneous and superhydrophobic surfaces is important for a wide variety of engineering problems. Despite significant efforts to capture the behavior of a droplet on these surfaces over the past few decades, modeling of the complex dynamics at the three-phase contact line is needed. In this work, we demonstrate that contact line distortion on heterogeneous and superhydrophobic surfaces is the key aspect that needs to be accounted for in the dynamic droplet models. Contact line distortions were visualized and modeled using a thermodynamic approach to develop a unified model for contact angle hysteresis on chemically heterogeneous and superhydrophobic surfaces. On a surface comprised of discrete wetting defects on an interconnected less wetting area, the advancing contact angle was determined to be independent of the defects, while the relative fraction of the distorted contact line with respect to the baseline surface was shown to govern the receding contact angle. This behavior reversed when the relative wettability of the discrete defects and interconnected area was inverted. The developed model showed good agreement with the experimental advancing and receding contact angles, both at low and high solid fractions. The thermodynamic model was further extended to demonstrate its capability to capture droplet shape evolution during liquid addition and removal in our experiments and those in literature. This study offers new insight extending the fundamental understanding of solid–liquid interactions required for design of advanced functional coatings for microfluidics, biological, manufacturing, and heat transfer applications.National Science Foundation (U.S.) (Major Research Instrumentation Grant for Rapid Response Research (MRI- RAPID))Battelle Memorial Institut
Jumping Droplet Dynamics on Scalable Nanostructured Superhydrophobic Surfaces
No full textEnvironmental scanning electron microscope (ESEM) and high speed images of coalescence-induced droplet jumping on a nanostructured superhydrophobic copper oxide (CuO) surface are presented. Nanostructured CuO films were formed by immersing clean copper sheets into a hot (96 ± 3 °C) alkaline solution composed of NaClO2, NaOH, Na3PO4•12H2O, and DI water (3.75 : 5 : 10 : 100 wt.%). During the oxidation process, a thin (<200 nm) Cu2O layer was formed that then re-oxidized to form sharp, knife-like CuO oxide structures (Figure 1). Hydrophobic functionalization was obtained by depositing a fluorinated silane (trichloro(1H,1H,2H,2H-perfluorooctyl)silane) from a vapor phase. Individual droplet growth on the nanostructured CuO surfaces was characterized using an ESEM (Figure 2). The images were obtained with a beam potential of 20 kV and variable probe current. Droplets nucleated within the nanostructures and, while growing beyond the confines of the structures, their apparent contact angle increased as they developed a balloon-like shape with a liquid bridge at the base. Once droplets grew to diameters large enough to coalesce with neighboring droplets (R ≈ 7 μm), frequent out-of-plane jumping droplets were observed. To gain further understanding on jumping velocity, droplet jumping was studied in a pure saturated environment with a high speed camera. Recordings were taken at 2000 FPS. Figure 3 shows a time lapse of a coalescence event between two droplets. As the droplets coalesce, excess surface energy is converted into kinetic energy resulting in droplet jumping. The visualizations provide insight into these complex droplet-surface interactions, which are important for the development of enhanced phase change surfaces. In addition, these CuO surfaces offer ideal condensation behavior in terms of emergent droplet morphology and coalescence dynamics
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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