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 D26: Focus Session: Supericephobic Surfaces |
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Chair: Thomas Schutzius, ETH Zurich Room: Georgia World Congress Center B314 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D26.00001: Impact, spreading, and solidification of liquid droplets on supercooled surfaces Detlef Lohse, Robin Koldewij, Marise Gielen, Michiel van Limbeek, Kirsten Harth, Rielle de Ruiter, Pallav Kant, Jacco Snoeijer, Hanneke Gelderblom The problem of liquid droplet spreading on surfaces below the liquid's freezing temperature is relevant in nature and in (e.g. aircraft, rapid manufacturing) technology. The competition between fast spreading dynamics and freezing is particularly intriguing. In the current work we give an overview on the studies we have performed on the impact of millimetric sized droplets on supercooled surfaces. We visualize the impact of tin droplets from the side and from below at ultrafast time scale, providing us with unique views on the solidification patterns and their dynamics. To better understand the pattern formation, a novel method based on total internal reflection (TIR) is presented, which allows us to observe the formation and growth of solidification seeds and dendrites. We study the dependence of the phenomena on the control parameters, namely on impact velocity and surface temperature, both during the impact even and the competing solidification and on much longer time scales thereafter. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D26.00002: Evaporation of pointy ice drops and snowflakes: smoothing out the singularity Etienne Jambon-Puillet, Noushine Shahidzadeh, Daniel Bonn The evaporation (sublimation) of ice and snow has a major impact on global climate, since the amount of ice and snow determines Earth’s albedo. Yet, due to their complex geometry with several sharp regions which are singular for the evaporation, the precise evaporation dynamics of snow and ice crystals remains challenging to predict. Here, we study the sublimation of snowflakes and pointy ice drops. We show that the evaporation rates of water and ice drops are similar; they are both limited by the diffusive transport of the vapour. This allows us to predict ice and snowflake evaporation quantitatively by solving the diffusive free-boundary problem, which correctly predicts the rapid self-similar evolution of sharp edges and points. Beyond providing a conceptual picture to understand the sublimation of ice crystals, our results are more generally applicable to other diffusion problems such as the dissolution of salt crystals or pharmaceuticals. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D26.00003: From Condensation Frosting to Anti-Frosting Jonathan Boreyko, Farzad Ahmadi, Saurabh Nath, Caitlin Bisbano, Grady Iliff, Pengtao Yue Condensation frosting, where supercooled dew nucleates on a surface and subsequently freezes into ice, is how the foundational layer of frost forms on chilled surfaces. For perfectly wetting surfaces, the supercooled condensate simply forms as a continuous film that freezes over all at once. But for non-wetting surfaces, condensation frosting manifests itself as inter-droplet ice bridges that connect frozen droplets to neighboring liquid droplets in a chain reaction. First, we present scaling laws that rationalize the dynamics of the inter-droplet ice bridging events. Second, a universal scaling law is developed to capture the extent of a dry zone that stabilizes about the perimeter of an isolated frozen droplet in the absence of ice bridging. Finally, a scaling law and numerical model are used to optimize a passive anti-frosting surface comprised of a dilute array of ice stripes promoting overlapping dry zones. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D26.00004: Rational Design of Supericephobic Heat Exchangers based on Condensation Frosting Heat Transfer Studies on Structured Surfaces Nenad Miljkovic Understanding frost formation is essential to a variety of heat transfer applications. Previous work has shown that interfacial wetting states as well as inter-droplet ice-bridging govern frosting on superhydrophobic surfaces (SHS). Yet, a physics based understanding of these two frost governing mechanisms is limited. Furthermore, the utilization of frost-growth physics for nanostructure design and scale up is sorely needed. Here, we begin by elucidating the fundamental thermal physics governing condensation frosting of water slabs, droplets, and ice-bridges. At the macroscale, we show that the superhydrophobic state contributes negligible thermal resistance during freezing of thick (~mm) water slabs. At the microscale, we show that the velocity of the ice bridge formation is independent of the substrate thermal conductivity, indicating that adjacent droplet evaporation is governed solely by vapor pressure gradients. We use our fundamental insights to rationally choose metal based surface structures, and demonstrate frost formation inhibition on meter scale aluminum heat exchangers. Our experiments show that SHS surfaces can achieve a 3X slower frost formation rate and a 50% defrost energy savings when compared to uncoated or superhydrophilic heat exchangers. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D26.00005: Abstract Withdrawn
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Sunday, November 18, 2018 3:35PM - 3:48PM |
D26.00006: Frost prevention through Nanoengineered Integral Humidity Sink Effect Konrad Rykaczewski, Xiaoda Sun Prevention of ice and frost formation is important for transportation and power generation industries. While many passive coatings that are supposed to achieve this goal have been proposed, most loose functionality in frosting conditions. In this talk, we will discuss how responsive two-layer antifreeze infused coatings can address this issue. These bioinspired bi-layered coatings have an inner superhydrophilic ``dermis'' infused with antifreeze and an outer permeable superhydrophobic ``epidermis'' [1]. Besides outperforming any passive coatings in conditions that lead to glaze and rime formation (with major antifreeze use reduction), our coatings also provide intriguingly long condensation onset delay. This result stems from the integral humidity sink effect where in the periodically exposed hygroscopic liquid depresses vapor concentration below saturation level required for nucleation to occur on the exterior surface [2]. We will discuss basic scaling laws that govern this process and how the exterior surface porosity can be engineered to optimize the anti-frosting capability of the bi-layer coating [3]. [1] Sun et al. Adv. Mater. Inter. 2015. [2] Sun et al. Langmuir, 2016. [3] Sun et al. ACS Nano, 2017. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D26.00007: Using entropic lattice Boltzmann modeling to understanding impacting and evaporating droplets Dominique Derome, Ali Mazloomi, Jan Carmeliet Entropic lattice Boltzmann method for two-phase flow can be effectively used to predict and understand the behavior of droplets impacting or drying on different surfaces. We first look at drop impact on chemically heterogeneous flat surfaces consisting of a regular checkboard pattern with alternating hydrophilic and hydrophobic patches. An equivalent contact angle representing Cassie’s apparent contact angle is determined for the distorted impacting droplets. During impact, the equivalent contact angle value increases until the maximum spreading is reached, after the equivalent contact angle remains constant until final state. Second, we analyze the contact line dynamics of evaporating droplets deposited on a set of parallel micro-ribs. The evaporating droplet undergoes a series of pinning-depinning events, showing alternatively constant contact radius and constant contact angle modes. LBM allows us to compute the Gibbs free energy taking into account the interfacial energy, pressure terms as well as viscous dissipation due to drop internal flow. The mechanism that causes the unpinning of the contact line results from an excess in Gibbs free energy. The energy barrier required for the contact line to depin can be enlarged by increasing the spacing or the rib height. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D26.00008: On Fundamentals of Icephobic Surfaces Hadi Ghasemi Anti-icing surfaces play a critical role in human daily lives in cold climates by impacting a broad range of systems including infrastructures, transportation network and power generation systems. Icing in electricity transmission systems can lead to collapse of poles and towers and rupture of conductors. Icing in aircrafts results in increased drag and may lead to loss of lift force and potential catastrophic events. Icing in energy systems significantly drops the heat transfer rate leading to inefficient operation of these systems. Non-wetting, liquid-infused and hydrated surfaces have inspired routes for development of anti-icing surfaces. However, high freezing temperature, high ice adhesion strength (~50-100 kPa) and subsequent ice accretion, low mechanical durability, and high production cost have restricted their practical applications. The goal of this talk is to elucidate the underlying nano-scale physics of ice formation and adhesion on surfaces which involves studies of thermodynamics, heat transfer and mechanics of solid-ice interfaces. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D26.00009: Water and Ice in Graphene Nanovessels Seyed Mohammadreza Ghodsi, Sushant Anand, Reza Shahbazian Yassar, Tolou Shokuhfar, Constantine Megaridis Studying water behavior in nano-enclosures is of particular interest but requires fine spectroscopic probes with sub-micrometer resolution. Graphene liquid cells (GLCs) which feature two closely-spaced sheets of graphene wrapped around hydrated samples, facilitate high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) measurements of water and ice encased tightly in hydrophobic graphene nanovessels. We perform TEM and EELS measurements in GLCs maintained at either 25oC or -165oC to examine the nanoscale arrangement of water and ice molecules with sub-eV energy resolution. Simultaneous quantification of water and ice thickness leads to the conclusion that H-bonding strengthens under increased confinement. The present results offer new insight on water molecule arrangement under high-confinement conditions. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D26.00010: Creation of an icephobic coating using graphite powder and PTFE nanoparticles Joseph Gonzales, Daiki Kurihara, Maeda Tetsuro, Masafumi Yamazaki, Saruhashi Takahito, Shigeo Kimura, Hirotaka Sakaue Polytetrafluorethylene (PTFE) particles exhibit hydrophobic properties, which can be used for the creation of super-hydrophobic surfaces, and applied to ice prevention. Such surfaces prevent water from collecting on a surface, allowing for the removal of sheets of ice when the layer of ice in contact with a surface melts. However, many icephobic surfaces require heating elements in order to melt the base layer of ice. A passive coating is proposed, which combines hydrophobic properties of PTFE particles with the solar absorptivity of graphite powder. For this, a coating with a 4:1 mass ratio of PTFE particles to graphite powder was created. This coating maintains contact angles of greater than 140o and sliding angles of less than 4o at standard conditions. In addition, the solar absorptivity constant of the coating is approximately 0.9 in the visible range, more than a 75% increase over pure PTFE-based coatings. Higher absorption of solar radiation greatly reduces the need for active anti-icing techniques. Due to the high thermal conductivity of graphite, heat captured by the surface is also distributed more evenly through the coating, making it more effective at removing layers of ice.
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