Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session E36: Drops: Condensation and Freezing |
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Chair: Alexandre Ponomarenko, Harvard University Room: Ballroom C |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E36.00001: Instant freezing of impacting wax drops Alexandre Ponomarenko, Emmanuel Virot, Shmuel Rubinstein We present the impact of hot liquid drops of wax on surfaces whose temperature is below the solidifying temperature of the drops. During the fall the drops remain mostly liquid, but upon impact, their temperature quickly decreases resulting in the solidification of the drop. Depending on the impact energy, drops size and the temperature difference between the drop and the surface this results in plethora of solid shapes: simple lenses, triangular drops, spherical caps and popped popcorn shapes. [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E36.00002: Spatial Control of Condensation using Chemical Micropatterns Kevin Murphy, Ryan Hansen, Saurabh Nath, Scott Retterer, Patrick Collier, Jonathan Boreyko Surfaces exhibiting wettability patterns can spatially control the nucleation of condensation to enable enhanced fog harvesting and phase-change heat transfer. To date, studies of patterned condensation have utilized a combination of chemical and topographical features, making it difficult to isolate the effects of intrinsic wettability versus surface roughness on spatially controlling the condensate. Here, we fabricate chemical micropatterns consisting of hydrophilic silicon oxide and a smooth hydrophobic silane monolayer to isolate the effects of changes in intrinsic wettability on the spatial control of condensation. Complete spatial control, defined as every nucleation and growth event occurring exclusively on the hydrophilic features, was observed even for supercooled droplets at high water vapor supersaturation. However, this complete spatial control was found to break down beyond a critical spacing that depended upon the extent of supersaturation. The average diameter of condensate was found to be smaller for the chemically micropatterned surfaces compared to a uniformly hydrophobic surface. Control of inter-droplet spacing between supercooled condensate through chemical patterning can be employed to minimize the growth of inter-droplet frost on cold surfaces. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E36.00003: Can Ice Prevent Frost Growth? Saurabh Nath, Ryan Hansen, Kevin R. Murphy, Scott Retterer, Patrick Collier, Jonathan Boreyko So-called icephobic surfaces that exhibit special wettability characteristics can delay the onset of ice nucleation in supercooled water. However, to date no icephobic surface has been able to passively prevent frost growth once ice nucleates. Here, we demonstrate that the growth rate of frost can be tuned and even halted with a chemically patterned surface that controls the spatial distribution of supercooled condensation. The success and speed of inter-droplet frost growth is found to depend upon two primary factors: the extent of spacing between hydrophilic regions where liquid nucleation occurs and the time allowed for condensation growth prior to the initial freezing event. Instead of delaying the onset of freezing, we initiate freezing as early as possible. This creates a ``dry zone'' where no frost and condensation can occur. The underlying mechanism behind the ``dry zone'' involves the saturation vapor pressure over ice that is lower than that over water at the same temperature, causing ice to behave like a passive humidity sink. Thus, quite remarkably it appears that ice itself may be the solution to the frosting problem. [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E36.00004: Freezing Behavior of a Supercooled Water Droplet Impacting on Surface Using Dual-Luminescent Imaging Technique Mio Tanaka, Katsuaki Morita, Makoto Yamamoto, Hirotaka Sakaue A collision of a supercooled-water droplet on an object creates ice accretion on its surface. These icing problems can be seen in any cold environments and may lead to severe damages on aircrafts, ships, power cables, trees, road signs, and architectures. To solve these problems, various studies on ice-prevention and ice-prediction techniques have been conducted. It is very important to know the detail freezing mechanism of supercooled water droplets to propose or improve those techniques. The icing mechanism of a single supercooled-water droplet impacting on object surface would give us great insights for constructing those techniques. In the present study, we use a dual-luminescent imaging technique to measure the time-resolved temperatures of a supercooled water droplet impacting with different speed. The technique we applied consists of high-speed color camera and two luminescent probes. We will report the current status of this experiment in the presentation. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E36.00005: Dry Zones Around Frozen Droplets Caitlin Bisbano, Saurabh Nath, Jonathan Boreyko The saturation pressure of water vapor above supercooled water exceeds that above ice at the same temperature. A frozen droplet will therefore grow by harvesting water vapor from neighboring supercooled condensate, which has recently been demonstrated to be a primary mechanism of in-plane frost growth on hydrophobic surfaces. The underlying physics of this source-sink interaction is still poorly understood. In this work, a deposited water droplet is frozen on a dry hydrophobic surface initially held above the dew point. We demonstrate that when the surface is then cooled beneath the dew point, the frozen droplet harvests nearby water vapor in the air. This results in an annular dry zone that forms between the frozen droplet and the forming supercooled condensation. For a given ambient temperature and humidity, the length of the dry zone varied strongly with surface temperature and weakly with droplet volume. The dependence of the dry zone on surface temperature is due to the fact that the vapor pressure gradients between the ambient and the surface and between the liquid and frozen water are both functions of temperature. [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E36.00006: Ice Formation Delay on Penguin Feathers Elaheh Alizadehbirjandi, Faryar Tavakoli-Dastjerdi, Judy St. Leger, Stephen H. Davis, Jonathan P. Rothstein, H. Pirouz Kavehpour Antarctic penguins reside in a harsh environment where air temperature may reach -40 $^{\circ}$C with wind speed of 40 m/s and water temperature remains around -2.2 $^{\circ}$C. Penguins are constantly in and out of the water and splashed by waves, yet even in sub-freezing conditions, the formation of macroscopic ice is not observed on their feathers. Bird feathers are naturally hydrophobic; however, penguins have an additional hydrophobic coating on their feathers to reinforce their non-wetting properties. This coating consists of preen oil which is applied to the feathers from the gland near the base of the tail. The combination of the feather's hydrophobicity and surface texture is known to increase the contact angle of water drops on penguin feathers to over 140 $^{\circ}$ and classify them as superhydrophobic. We here develop an in-depth analysis of ice formation mechanism on superhydrophobic surfaces through careful experimentations and development of a theory to address how ice formation is delayed on these surfaces. Furthermore, we investigate the anti-icing properties of warm and cold weather penguins with and without preen oil to further design a surface minimizing the frost formation which is of practical interest especially in aircraft industry. [Preview Abstract] |
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