Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session M11: Drops: Freezing and Condensation |
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Chair: Christophe Josserand, Ecole Polytechnique Room: Georgia World Congress Center B216 |
Tuesday, November 20, 2018 8:00AM - 8:13AM |
M11.00001: Frozen rivulet Axel Huerre, Donatien Mottin, Christophe Josserand, Thomas Seon We investigate experimentally the freezing of a rivulet, obtained by flowing water on a cold solid substrate (0°C to –35°C). |
Tuesday, November 20, 2018 8:13AM - 8:26AM |
M11.00002: Drop Impact on Supercooled Surfaces Soumyadip Sett, Zona Hrnjak, Peter Sokalski, Junho Oh, Nenad Miljkovic Frost formation and accretion present major economic and performance challenges for several industries, including aerospace, power transmission, and building environmental control. In the recent years, due to significant progress in nanoengineering and chemical functionalization, much attention has been given to the rational development of anti-icing surfaces via reduction of ice adhesion and delayed frost formation. Yet, comprehensive understanding of the impact and freezing dynamics of ambient-temperature water droplets on highly-supercooled (>100K) surfaces is limited. Here, we experimentally study ambient (≈ 22°C) water droplet impact on supercooled nanoengineered metallic surfaces (°C) having a range of wettabilities (). To characterize the freezing processes, we measure the dependence of freezing time and contact area of water droplets during impact. By varying the incoming droplet Bond number (0.5 < Bo < 5), Weber number (10 < We < 1000) and Ohnesorge number (0.001 < Oh < 0.01), we establish the underlying physics, characterized by the delicate interplay of not only liquid inertia, viscosity, surface tension, and gravitational body forces, but also on the substrate wettability and temperature, on freezing dynamics. |
Tuesday, November 20, 2018 8:26AM - 8:39AM |
M11.00003: Competition after liquid droplet contact on super-cooled surfaces: spreading vs solidification Robin Koldeweij, Kirsten Harth, Michiel Limbeek, Pallav Kant, Jacco Snoeijer, Detlef Lohse Additive manufacturing processes are based on liquid droplet solidification after their deposition on super-cooled surfaces. However, the exact processes leading to certain splat properties are yet unknown. Observing spreading hexadecane droplets on super-cooled plates by means of total internal reflection (TIR) imaging allows completely new, temporally and spatially resolved, insights into the solidification patterns and dynamics. Various regimes of solidification are observed, depending on the surface temperature. The total rate of solidification can be theoretically described, based by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. We also quantify the nucleation rate and dendritic growth during the solidification, to investigate the arrest behavior of spreading droplets. |
Tuesday, November 20, 2018 8:39AM - 8:52AM |
M11.00004: Freezing of Impacted Drops: Contact Line Dynamics Virgile Thiévenaz, Thomas Séon, Christophe Josserand We experimentally explore the mechanisms of a water drop freezing while impacting a cold surface. The liquid drop spreads and partially freezes during the impact and then retracts upon ice. In the meantime the contact line is pinned thoughout a certain time of residence, unlike the case without solidification. Experimental measurements reveal non-trivial variations of this duration with the substrate thermal properties and drop impact velocity. A scaling law model enables us to interpret this phenomenon as the dynamic of the contact angle, which is affected by the solidification. |
Tuesday, November 20, 2018 8:52AM - 9:05AM |
M11.00005: Freezing drop impact Thomas Seon, Virgile Thievenaz, Christophe Josserand We investigate experimentally the solidification of a water drop during its impact on a sub-zero cooled metallic plate. As the drop impacts the substrate, a first thin layer of ice builds-up in the briefest moment. Once this frozen disc is formed, the remaining liquid retracts on the top of it. The competition between this capillary retraction and the liquid solidification leads to a variety of frozen drop patterns. Typically, with a change of temperature the drop can freeze into a fried egg or a doughnut shape. Experimental measurements of the frozen drop profile, a 1D solidification model, and relevant scaling laws allow us to characterize the frozen disc thickness, and the different patterns, as a function of the control parameters. |
Tuesday, November 20, 2018 9:05AM - 9:18AM |
M11.00006: Solidification during impact: freezing the shape of a splash Michela Geri, Gareth H. McKinley The impact of a drop on an immiscible bath can generate different patterns depending on several parameters including the initial velocity of the drop and the density difference with the liquid bath. If the impacting drop can readily solidify upon cooling, the shape of the splash can be frozen in time. In this work, we study the morphologies obtained upon impacting a hot solution of paraffin wax in mineral oil on a cold immiscible bath. As it cools down, the impacting drop solidifies during the complex deformation processes associated with splashing, generating different morphologies that vary from cup-like to floral shapes with increasing number of petals. The anatomy of the final solidified wax is the result of a competition between different physical processes and instabilities that all occur over the time-scales of impact, including droplet breakup, wrinkling and crack formation /propagation in a thin stretched elastic sheet. Using high-speed imaging we show how different control parameters affect these fascinating morphologies that span the range from transient pattern formation in deforming fluids to the permanent indelible shape of thin elastic solid shells. |
Tuesday, November 20, 2018 9:18AM - 9:31AM |
M11.00007: Temperature measurements inside freezing drops in Poiseuille flow Mohammed Fazel, Victor Voulgaropoulos, Stuart F Wright, Alexandros Charogiannis, Omar K Matar, Christos N Markides Water is cooled down close to its melting point and is introduced through a syringe in the form of drops in an oil-continuous pipe flow. The oil has a temperature below that of the melting point of water and flows at a Reynolds number in the order of 103. The heat transfer and solidification of the water drops is investigated downstream of the test section with planar laser-induced fluorescence (PLIF). A YLF laser sheet is emitted along the cross-stream direction of the flow and is used to harmonically excite a pair of fluorescent dyes with different emission spectra that are introduced in the water drop. Two cameras are connected to a beam-splitter and the fluorescence signal of each dye is captured in each camera individually. A ratiometric approach is used to accurately measure the temperature field inside the drop and account for reflection effects at the drop interface. The temperature gradients inside the drop and close to the thermal boundary layer are recorded for different Reynolds numbers and drop sizes, but also for different water and oil temperatures. |
Tuesday, November 20, 2018 9:31AM - 9:44AM |
M11.00008: The platonic pancake: phase change and the ultimate shape of spreading droplets Jerome Neufeld, Cayley Neufeld, Miles Neufeld, Wendell Neufeld, Mark Hallworth The spreading of viscous fluids, coupled with phase change, presents a compelling weekend experiment for many families, with important implications for our understanding of magmatic intrusions and many industrial processes. A key question is the extent to which the fluid spreads. Here we show that for many materials, even those with a temperature-dependent rheology, the ultimate shape may be set by the dynamics of phase change (e.g. cooking for pancakes or solidification for many fluid droplets) so that the final radius scales with the volume as $R_\infty \sim V^{5/12}$. Further, we show that with a single, non-dimensional parameter, $\Lambda$, two limiting behaviours in $R_\infty$ may be observed. For very inviscid droplets, $\Lambda << 1$, spreading is first halted by surface tension followed by phase change, while for very viscous droplets $\Lambda >> 1$, spreading is halted by the change of phase itself. We test our simple analytical theory against data from several hundred experiments, from the cooking of a large number of pancakes and crepes, showing good collapse with the variation in measured viscosity, to the solidification of a range of viscosity fluids. Finally, we use our results to speculate on the origin of pancakelike domes observed on the surface of Venus. |
Tuesday, November 20, 2018 9:44AM - 9:57AM |
M11.00009: Effect of condensation phenomena on the nanostructured hydrophobic polymer film during heat transfer Cheonji Lee, Muhammad Salman Abbasi, Seungchul Park, Hyuneui Lim, Jinkee Lee Polymer has drawn attention to use as heat exchanger due to their special characteristics such as flexibility, low weight, corrosion, and bio-fouling resistance, as well as their ease of manufacturing. However, since its thermal conductivity is low, it requires a way to increase its heat transfer rate. We investigated the effect of modifying the surface wettability of polymer films on the heat transfer rate during the condensation process, both theoretically and experimentally. Here, we used four different types of polymer films, which have different thermal conductivity and controlled wettability properties. We designed a thermal resistance model to compare the heat transfer rate of each sample and analyzed which resistance is dominant in the thermal resistance. We found that the convective thermal resistance dominantly affects the heat transfer rate and treated polymers showed over 200% higher total heat transfer than bare polymer. |
Tuesday, November 20, 2018 9:57AM - 10:10AM |
M11.00010: Dropwise Condensation on Hydrophobic Bumps and Dimples Yuehan Yao, Joanna Aizenberg, Kyoo-Chul Park The role of macroscopic surface topography in droplet growth by condensation has been overlooked compared to micro- or nano-texture. In this work, we studied the droplet growth on hydrophobic surfaces with millimeter scale radii of curvature with a positive or negative sign under a supersaturation condition created by controlling the surface temperature lower than the dew point. By analyzing the spatiotemporal droplet size distribution on convex surface structures such as bumps and concave surface structures such as dimples, we show detailed local droplet growth and its relation with the sign and magnitude of surface curvature. In particular, we report unexpected, more unfavorable condensation on dimples with a smaller radius of curvature, in contrast to preferential condensation in micro-cavities (i.e., capillary condensation). To explain these experimental results, we numerically calculated water vapor concentration gradient, showing that the diffusion flux is higher on bumps and lower on dimples at the center of them than a flat surface. We envision that our understanding of millimetric surface topography can be applied to improve the energy efficiency of condensation applications such as water harvesting devices, heat exchangers, and desalination plants. |
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