Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session M13: Drops: Heat Transfer and Evaporation II |
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Chair: David B. Thiessen, Washington State University Room: 3020 |
Tuesday, November 25, 2014 8:00AM - 8:13AM |
M13.00001: Marangoni Effect on the Shape of Freely Receding Evaporating Sessile Droplets of Perfectly Wetting Liquids Yannis Tsoumpas, Sam Dehaeck, Alexey Rednikov, Pierre Colinet Freely receding evaporating sessile droplets of perfectly wetting liquids (HFE-7100, 7200 and 7500), with small finite contact angles induced by evaporation, are studied with a Mach-Zehnder interferometer. Surprisingly, the experimentally obtained profiles turn out to deviate from the classical macroscopic static shape of a sessile droplet (as determined by gravity and capillarity), often used when modeling evaporating droplets. These deviations can be seen in two ways. Namely, either the droplet appears to be inflated as compared to the classical static shape assuming the same contact angle and contact radius, or the apparent contact angle appears lower than the classical static one assuming the same volume and contact radius. In reality, the experimental profiles exhibit a local decrease of the slope near the contact line, which we attribute to the Marangoni effect in an evaporating sessile droplet. In this case, the radially inward (along the liquid-air interface) direction of the flow delivers more liquid to the center of the droplet making it appear inflated. When the Marangoni effect is weak, as in the case of the poorly volatile HFE-7500, no significant influence is noticed on the drop shape. The experimental results are compared with the predictions of a lubrication-type theoretical model that incorporates the evaporation-induced Marangoni flow. [Preview Abstract] |
Tuesday, November 25, 2014 8:13AM - 8:26AM |
M13.00002: How long does it take for sessile droplets to evaporate? Stephen Wilson, Jutta Stauber, Brian Duffy, Khellil Sefiane The evaporation of sessile droplets plays a crucial part in many practical applications, and in many of these applications it is important to be able to understand and/or control the lifetimes of droplets. The lifetime of an evaporating droplet depends on the manner in which it evaporates. There are various qualitatively different modes of droplet evaporation, of which the most extreme are the constant radius mode (in which the contact line is always pinned) and the constant angle mode (in which the contact angle $\theta$ always takes its initial value $\theta=\theta_0$), and probably the most commonly occurring is the stick-slide mode (in which the drop initially evaporates in a constant radius phase until $\theta$ reaches a critical transition angle $\theta^*$, and thereafter evaporates in a constant angle phase with $\theta=\theta^*$). In this talk we describe a theoretical model for the stick-slide mode and discuss the relationship between $\theta_0$ and $\theta^*$ and its implications. Theoretical predictions for the lifetimes of droplets are compared with previously published experimental results. Further details of the theoretical model are given in the recent paper by Stauber, Wilson, Duffy and Sefiane [{\it J.\ Fluid Mech.} {\bf 744}, R2 (2014)]. [Preview Abstract] |
Tuesday, November 25, 2014 8:26AM - 8:39AM |
M13.00003: Wettability Patterning for Enhanced Dropwise Condensation Aritra Ghosh, Ranjan Ganguly, Constantine Megaridis Dropwise condensation (DwC), in order to be sustainable, requires removal of the condensate droplets. This removal is frequently facilitated by gravity. The rate of DwC heat transfer depends strongly on the maximum departing droplet diameter. Based on wettability patterning, we present a facile technique designed to control the maximum droplet size in DwC within vapor/air atmospheres, and demonstrate how this approach can be used to enhance the corresponding heat transfer rate. We examine various hydrophilic-superhydrophilic patterns, which, respectively sustain DwC and filmwise (FwC) condensation on the substrate. The fabrication method does \textit{not }employ any hydrophobizing agent. By juxtaposing parallel lines of hydrophilic (CA $\sim$ 78$^{\circ})$ and superhydrophilic (CA $\sim$ 0$^{\circ})$ regions on the condensing surface, we create alternating domains of DwC and FwC. The average droplet size on the DwC domain is reduced by $\sim$ 60{\%} compared to the theoretical maximum, which corresponds to the line width. We compare heat transfer rate between unpatternend DwC surfaces and patterned DwC surfaces. Even after sacrificing 40{\%} of condensing area, we achieve up to 20{\%} improvement in condensate collection rate using an interdigitated superhydrophilic pattern, inspired by the vein network of plant leaves. The bioinspired interdigitated pattern is found to outperform the straight hydrophilic-superhydrophilic pattern, particularly under higher vapor loadings in an air/vapor ambient atmosphere. [Preview Abstract] |
Tuesday, November 25, 2014 8:39AM - 8:52AM |
M13.00004: Simulation Prediction of Transient Dropwise Condensation Ashley Macner, Susan Daniel, Paul Steen In order to design effective surfaces for large-scale dropwise condensation, an understanding of how surface functionalization affects drop growth and coalescence is needed. The long term technological goal is a set of design conditions to help NASA achieve maximum heat transfer rates of waste heat generated from electronics and habitable environments under microgravity conditions. Prediction of condenser surface heat transfer performance requires accurate simulation and modeling of the evolution of populations of drops in time. At shorter times, drops are primarily isolated and grow mainly by condensation onto the liquid-gas interface. At longer times, drops grow mainly by coalescence with neighbors. Simulation of dropwise condensation on a neutrally wetting surface and comparison with our previous experimental results is reported. A steady-state single drop conduction model is empirically fitted to determine a temperature profile that captures the drop size evolution. The simulation accurately predicts the continuous time evolution of number-density of drops, drop-size distributions, total condensate volume, fractional coverage, and median drop-size for both transient and steady states, all with no free parameters. [Preview Abstract] |
Tuesday, November 25, 2014 8:52AM - 9:05AM |
M13.00005: Universality of Tip Singularity Formation in Freezing Water Drops Oscar Enriquez, Alvaro Marin, Philippe Brunet, Pierre Colinet, Jacco Snoeijer A drop of water on a cold plate freezes from the bottom up and forms a pointy tip in the last moments of the process. Although this phenomenon is known to be caused by the expansion of water upon freezing, a quantitative description of the tip singularity has remained elusive. Our systematic measurements of the angles of the conical tip, for a wide range of temperatures and wetting angles, suggest a universal, self-similar mechanism that does not depend on the rate of solidification. Furthermore, using a Hele-Shaw geometry, we have observed the dynamics of the solidification front. Here we demonstrate how the geometry of the freezing front, determined by heat transfer considerations, is crucial for the tip formation. We propose a geometrical model for the tip formation and derive resulting tip angles analytically, in good agreement with the experiments. [Preview Abstract] |
Tuesday, November 25, 2014 9:05AM - 9:18AM |
M13.00006: Edge effects on water droplet condensation Laurent Royon, Anne Montgruel, Marie Gabrielle Medici, Daniel Beysens The effect of geometrical or thermal discontinuities on the growth of water droplets condensing on a cooled substrate is investigated. Edges, corners, cooled/non cooled boundaries can have a strong effect on the vapor concentration profile and mass diffusion around the drops. In comparison to growth in a pattern where droplets have to compete to catch vapor, which results in a linear water concentration profile directed perpendicular to the substrate, droplets near discontinuities can get more vapor (outer edges, corners), resulting in faster growth or less vapor (inner edges), giving lower growth. When the cooling heat flux limits growth instead of mass diffusion (substrate with low thermal conductivity, strong heat exchange with air), edges effects can be canceled. In certain cases, the growth enhancement can reach nearly 500{\%} on edges or corners which, on an inclined substrate, make droplets near the edges detach sooner than in the middle of the substrate. This effect is frequently observed with dew condensing on windows or car windshields. Such droplets, acting as wipers, can thus appreciably increase dew collection on a substrate. [Preview Abstract] |
Tuesday, November 25, 2014 9:18AM - 9:31AM |
M13.00007: Star-shaped oscillations of Leidenfrost droplets on a curved surface Xiaolei Ma, Juan-Jos\'{e} Li\'{e}tor-Santos, Justin Burton We investigate the spontaneous oscillations of a Leidenfrost droplet, which is levitated by a cushion of evaporated vapor on a hot surface. The oscillations exhibit a star-shaped pattern determined by a standing wave along the droplet periphery, and obey a quasi-2D dispersion relation. The bowl-shaped curvature of the surface suppresses the buoyancy-driven Rayleigh-Taylor instability in the vapor layer, allowing for very large droplets with up to 13 lobes. Although droplets of a given size can theoretically contain various oscillatory modes, we observe only one mode of oscillation, so that all star-shaped droplets have nearly the same frequency regardless of size. We suspect that the origin of this mode selection is due to a parametric coupling between vertical and azimuthal oscillations of the droplet, similar to experiments of droplets on hydrophobic, vibrated surfaces [1]. In order to investigate the phenomenon further, we also measure the pressure variations beneath the droplet during quiescent and oscillatory phases. \\[4pt] [1] P. Brunet and J. H. Snoeijer, Eur. Phys. J. Spec. Top. 192, 207 (2011). [Preview Abstract] |
Tuesday, November 25, 2014 9:31AM - 9:44AM |
M13.00008: Leidenfrost effect: accurate drop shape modeling and new scaling laws Benjamin Sobac, Alexey Rednikov, St\'ephane Dorbolo, Pierre Colinet In this study, we theoretically investigate the shape of a drop in a Leidenfrost state, focusing on the geometry of the vapor layer. The drop geometry is modeled by numerically matching the solution of the hydrostatic shape of a superhydrophobic drop (for the upper part) with the solution of the lubrication equation of the vapor flow underlying the drop (for the bottom part). The results highlight that the vapor layer, fed by evaporation, forms a concave depression in the drop interface that becomes increasingly marked with the drop size. The vapor layer then consists of a gas pocket in the center and a thin annular neck surrounding it. The film thickness increases with the size of the drop, and the thickness at the neck appears to be of the order of 10-100 $\mu $m in the case of water. The model is compared to recent experimental results [Burton et al., Phys. Rev. Lett., 074301 (2012)] and shows an excellent agreement, without any fitting parameter. New scaling laws also emerge from this model. The geometry of the vapor pocket is only weakly dependent on the superheat (and thus on the evaporation rate), this weak dependence being more pronounced in the neck region. In turn, the vapor layer characteristics strongly depend on the drop size. [Preview Abstract] |
Tuesday, November 25, 2014 9:44AM - 9:57AM |
M13.00009: Tracking liquid in drying colloidal fluids with polarized light microscopy Kun Cho, Jung Soo Park, Joon Heon Kim, Byung Mook Weon When colloidal fluids dry, tracking liquid surfaces around colloids is difficult with conventional imaging techniques. Here we show that polarized light microscopy (PM) is very useful in tracking liquid surfaces during drying processes of colloidal fluids. In particular, the PM mode is not a new or difficult way but is able to visualize liquid films above colloids in real time. We demonstrate that when liquid films above colloidal particles are broken, the PM patterns appear clearly: this feature is useful to identify the moment of liquid film rupture above colloids in drying colloidal fluids. This result is helpful to improve relevant processes such as inkjet printing, painting, and nanoparticle patterning (K.C. and J.S.P. equally contributed). [Preview Abstract] |
Tuesday, November 25, 2014 9:57AM - 10:10AM |
M13.00010: Secondary atomization pathways in burning functional droplets subjected to travelling pressure wave Ankur Miglani, Saptarshi Basu Self-induced internal boiling in burning multicomponent droplets and the resulting pressure upsurge is observed to initiate characteristic bubble ejection/droplet disruption events. These bubble ejections (also termed as secondary atomization events) corrugate the droplet surface and induce bulk shape deformation in the droplet. In this study, first, we identify the entire spectrum of secondary break-up modes that occur at distinct stages of droplet lifetime and at different temporal scales. Based on the increasing magnitude of their droplet-shape deformation inducing potential they range from high aspect ratio, high momentum needle type ligament break-up to low momentum, thick ligament break-up. Needle-type ejections are dominant at initial stages of droplet lifecycle and are primarily responsible for triggering only small-scale, localized surface wrinkling. In contrast, latter modes of atomization occur at later stages and initiate large-length scale droplet deformation. Second, we show that by exciting the droplet flame in its critical responsive frequency range (80 Hz $\le $ \textbf{\textit{f}}$_{\mathbf{P}} \le $ 120 Hz) the latter high intensity modes can be suppressed. [Preview Abstract] |
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