1,721,000 research outputs found
Liquid Freezing Dynamics on Hydrophobic and Superhydrophobic Surfaces
No full textFalse color environmental scanning electron microscope (ESEM) images of water freezing on smooth (θe ≈ 120°) and nanostructured (l ~ 50 nm, θe ≈ 170 - 180°) hydrophobic surfaces are presented. To obtain the freezing dynamics of water droplets, the vapor pressure in the ESEM chamber was set to 800 Pa. The images were obtained with a beam potential of 20 kV and variable probe current. The surface temperature was initially set to 5 ± 1.5 °C using a cold stage, resulting in water droplet condensation. After a period of time, the surface temperature was decreased to -20 ± 1.5 °C, rapidly freezing the condensed droplets. The three images capture the freezing dynamics on hydrophobic and superhydrophobic surfaces. Figure 1a shows a snapshot of droplet coalescence between a large droplet (left) and a smaller droplet (right) on a nanostructured surface. Due to rapid freezing (Figure 1b), ice droplets maintained an ‘amorphous’ spherical structure (blue). Subsequent deposition of ice preferentially initiated on the droplet interface and formed crystallographic ice (red). Figure 2 shows a larger scale droplet (R ~ 100 μm) undergoing rapid freezing with notable protrusions. The heat flux (q” ~ 1 GW/m2) due to the latent heat of freezing could not be removed rapidly enough to match the velocity of the freezing front (vf ≈ 1500 m/s), resulting in evaporation and ejection of the liquid (protrusions) from the droplet interface. These distinct freezing droplet behaviors can significantly alter dynamics of phasechange phenomena on these surfaces. The visualizations provide insight into these complex droplet-surface interactions, which are important for the development of de-icing surfaces
Modeling and Optimization of Superhydrophobic Condensation
No full textSuperhydrophobic micro/nanostructured surfaces for dropwise condensation have recently received significant attention due to their potential to enhance heat transfer performance by shedding water droplets via coalescence-induced droplet jumping at length scales below the capillary length. However, achieving optimal surface designs for such behavior requires capturing the details of transport processes that is currently lacking. While comprehensive models have been developed for flat hydrophobic surfaces, they cannot be directly applied for condensation on micro/nanostructured surfaces due to the dynamic droplet-structure interactions. In this work, we developed a unified model for dropwise condensation on superhydrophobic structured surfaces by incorporating individual droplet heat transfer, size distribution, and wetting morphology. Two droplet size distributions were developed, which are valid for droplets undergoing coalescence-induced droplet jumping, and exhibiting either a constant or variable contact angle droplet growth. Distinct emergent droplet wetting morphologies, Cassie jumping, Cassie nonjumping, or Wenzel, were determined by coupling of the structure geometry with the nucleation density and considering local energy barriers to wetting. The model results suggest a specific range of geometries (0.5–2 μm) allowing for the formation of coalescence-induced jumping droplets with a 190% overall surface heat flux enhancement over conventional flat dropwise condensing surfaces. Subsequently, the effects of four typical self-assembled monolayer promoter coatings on overall heat flux were investigated. Surfaces exhibiting coalescence-induced droplet jumping were not sensitive (2 μm). This work provides a unified model for dropwise condensation on micro/nanostructured superhydrophobic surfaces and offers guidelines for the design of structured surfaces to maximize heat transfer. Keywords: superhydrophobic condensation, jumping droplets, droplet coalescence, condensation optimization, environmental scanning electron microscopy; micro/nanoscale water condensation, condensation heat transfer.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-FG02-09ER46577)National Science Foundation (U.S.) (Award ECS-0335765
Growth Dynamics During Dropwise Condensation on Nanostructured Superhydrophobic Surfaces
Get PDFCondensation on superhydrophobic nanostructured surfaces offers new opportunities for enhanced energy conversion, efficient water harvesting, and high performance thermal management. Such surfaces are designed to be Cassie stable, which minimize contact line pinning and allow for passive shedding of condensed water droplets at sizes smaller than the capillary length. In this work, we investigated in situ water condensation on superhydrophobic nanostructured surfaces using environmental scanning electron microscopy (ESEM). The "Cassie stable" surfaces consisted of silane coated silicon nanopillars with diameters of 300 nm, heights of 6.1 μm, and spacings of 2 μm, but allowed droplets of distinct suspended (S) and partially wetting (PW) morphologies to coexist. With these experiments combined with thermal modeling of droplet behavior, the importance of initial growth rates and droplet morphology on heat transfer is elucidated. The effect of wetting morphology on heat transfer enhancement is highlighted with observed 6× higher initial growth rate of PW droplets compared to S droplets. Consequently, the heat transfer of the PW droplet is 4-6× higher than that of the S droplet. To compare the heat transfer enhancement, PW and S droplet heat transfer rates are compared to that of a flat superhydrophobic silane coated surface, showing a 56% enhancement for the PW morphology, and 71% degradation for the S morphology. This study provides insight into importance of local wetting morphology on droplet growth rate during superhydrophobic condensation, as well as the importance of designing CB stable surfaces with PW droplet morphologies to achieve enhanced heat transfer during dropwise condensation. Topics: Dynamics (Mechanics), CondensationUnited States. Department of Energy. Office of Science. Solid-State Solar Thermal Energy Conversion Cente
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
Electric-Field-Enhanced Condensation on Superhydrophobic Nanostructured Surfaces
No full textWhen condensed droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump due to the conversion of excess surface energy into kinetic energy. This phenomenon has been shown to enhance condensation heat transfer by up to 30% compared to state-of-the-art dropwise condensing surfaces. However, after the droplets jump away from the surface, the existence of the vapor flow toward the condensing surface increases the drag on the jumping droplets, which can lead to complete droplet reversal and return to the surface. This effect limits the possible heat transfer enhancement because larger droplets form upon droplet return to the surface, which impedes heat transfer until they can be either removed by jumping again or finally shedding via gravity. By characterizing individual droplet trajectories during condensation on superhydrophobic nanostructured copper oxide (CuO) surfaces, we show that this vapor flow entrainment dominates droplet motion for droplets smaller than R ≈ 30 μm at moderate heat fluxes (q″ > 2 W/cm2). Subsequently, we demonstrate electric-field-enhanced condensation, whereby an externally applied electric field prevents jumping droplet return. This concept leverages our recent insight that these droplets gain a net positive charge due to charge separation of the electric double layer at the hydrophobic coating. As a result, with scalable superhydrophobic CuO surfaces, we experimentally demonstrated a 50% higher overall condensation heat transfer coefficient compared to that on a jumping-droplet surface with no applied field for low supersaturations (<1.12). This work not only shows significant condensation heat transfer enhancement but also offers avenues for improving the performance of self-cleaning and anti-icing surfaces as well as thermal diodes.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-FG02-09ER46577)United States. Office of Naval ResearchNational Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374)Irish Research Council for Science, Engineering, and Technolog
Nanoengineered Surfaces for Efficient Energy Systems
Get PDFNanoengineered surfaces offer new possibilities to manipulate fluid transport and enhance heat dissipation characteristics for the development of efficient energy systems. In particular, nanostructures on these surfaces can be harnessed to achieve superhydrophilicity and superhydrophobicity, and to control liquid behavior and phase-change processes. In this work, we will describe recent developments focused on using superhydrophilic nanostructure design to manipulate liquid spreading behavior and directionalities. In the presence of asymmetric nanopillars, uni-directional spreading of water droplets can be achieved where the liquid spreads only in the direction of the pillar deflection and becomes pinned on the opposite interface. In the presence of fine features on the pillars, we observed a multi-layer spreading effect due to their associated energy barriers. For both cases, we have developed energy-based models to accurately predict the liquid behavior as functions of pertinent parameters. Furthermore, we developed a semi-analytical model to predict liquid propagation rates in pillar arrays driven by capillarity. The results offer design guidelines to optimize propagation rates for fluidic wicks. These investigations offer insights and significant potential for the development and integration of advanced nanostructures to achieve efficient energy conversion systems. Copyright © 2011 by ASME.National Science Foundation (U.S.) (Award EEC-0824328)United States. Defense Advanced Research Projects Agency (Young Faculty Award)United States. Office of Naval ResearchNorthrop Grumman Corporation (New Faculty Innovation Grant)Intel Corporation (Higher Education Grant
Electrostatic charging of jumping droplets
No full textWith the broad interest in and development of superhydrophobic surfaces for self-cleaning, condensation heat transfer enhancement and anti-icing applications, more detailed insights on droplet interactions on these surfaces have emerged. Specifically, when two droplets coalesce, they can spontaneously jump away from a superhydrophobic surface due to the release of excess surface energy. Here we show that jumping droplets gain a net positive charge that causes them to repel each other mid-flight. We used electric fields to quantify the charge on the droplets and identified the mechanism for the charge accumulation, which is associated with the formation of the electric double layer at the droplet–surface interface. The observation of droplet charge accumulation provides insight into jumping droplet physics as well as processes involving charged liquid droplets. Furthermore, this work is a starting point for more advanced approaches for enhancing jumping droplet surface performance by using external electric fields to control droplet jumping.United States. Dept. of Energy. Office of Basic Energy Sciences (Solid-State Solar-Thermal Energy Conversion Center Award DE-FG02-09ER46577)United States. Office of Naval ResearchNational Science Foundation (U.S.) (Major Research Instrumentation Grant for Rapid Response Research (MRI- RAPID))National Science Foundation (U.S.) (Award ECS-0335765)National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 1122374
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
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
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