Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D36: Drops: Superhydrophobic Surfaces |
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Chair: Pierre Chantelot, Ecole Polytechnique Room: Portland Ballroom 251 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D36.00001: Ring bouncing Pierre Chantelot, Anais Gauthier, Christophe Clanet, David Quere Point like superhydrophobic macrotextures attached to a flat substrate of same repellency can modify the dynamics of impacting water droplets and lead to shorter bouncing times than on a flat substrate. We investigate the contact time reduction for centered and off-centered impacts on a single texture and show that the effect is robust. We discuss how a macrotextured substrates modifies the impact figure compared to a regular substrate and link it to the reduction of the bouncing time. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D36.00002: Superhydrophobic immersion Martin Coux, Adrien Mathis, Christophe Clanet, David Quere A superhydrophobic object is an object on which water doesn't spread. We can think conversely, such an object should be covered by air when immersed in water. The film of air that is formed in this case is visible at the naked eye owing to its brightness. Natural questions that arise from the observation of this phenomenon are how much air stays trapped between the liquid and the solid, \textit{ie} what is \quad the thickness of the film, and how this quantity can be modified. In this study, we describe an experimental setup that allows us to easily control the velocity of immersion of an object into a liquid bath and to access the volume of dragged air, from which we can deduce the thickness of the air film. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D36.00003: Tuning Superhydrophobic Nanostructures to Enhance Jumping-Droplet Condensation Megan Mulroe, Bernadeta Srijanto, Patrick Collier, Jonathan Boreyko It was recently discovered that condensation growing on a nanostructured superhydrophobic surface can spontaneously jump off the surface when two or more droplets coalesce together. The minimum droplet size for jumping to occur is of order 10 microns, but it is unclear whether this is the true lower limit of jumping droplets or simply a limitation of current superhydrophobic surfaces. Here, we analyze the dynamics of jumping droplets on six different superhydrophobic surfaces where the topography of the nanopillars was systematically varied. The critical diameter for jumping to occur was observed to be highly dependent upon the height and diameter of the nanopillars; surfaces with very tall and slender nanopillars enabled jumping droplets at a smaller critical size of order 1 micron. An energetic model of the incipient growth of condensate shows that the nanostructure topology affects the rate of increase of a growing droplet's apparent contact angle, with jumping being enabled at very large angles. These findings indicate that the true upper limit to the performance of jumping-droplet condensers has not yet been reached and can be further improved using advanced nanofabrication techniques. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D36.00004: More Puddle Jumping Babak Attari, Mark Weislogel, Andrew Wollman, Yongkang Chen, Trevor Snyder Large droplets and puddles jump spontaneously from sufficiently hydrophobic surfaces during routine drop tower tests. The simple low-cost passive mechanism can in turn be used as an experimental device to investigate dynamic droplet phenomena for drops up to 10,000 times larger than their normal terrestrial counterparts. We provide or confirm quick and qualitative design guides for such `drop shooters' as employed in drop tower tests including relationships to predict droplet ejection durations and velocities as functions of drop volume, surface texture, surface contour, wettability pattern, drop volume, and fluid properties including contact angle. The latter are determined via profile image comparisons with numerical equilibrium interface computations. Water drop volumes of 0.04 to 400 mL at ejection speeds of --0.007 to 0.12 m/s are demonstrated. An example application of the puddle jump method is made to the classic problem of regime mapping for low-gravity phase change heat transfer for large impinging drops. Many other candidate problems might be identified. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D36.00005: Viscous Puddle Jump Taif Al jubaree, mark Weislogel, tan hua The phenomena of spontaneous droplet jump from hydrophobic surfaces during low-g drop tower tests was recently reviewed (Wollman et al., Experiments in Fluids, 2016). Such drops may be over 10,000 times larger than typical terrestrial drops and are more akin to puddles than drops. In this work we investigate the effect of viscosity on the puddle jump process for drop/puddle volumes up to 100 mL and dynamic viscosities up to 950 cSt. The large low-cost hydrophobic surfaces are created using PTFE-coated 320 grit sand paper. We adopt a scaling approach to evaluate the relevant terms of the momentum equation before performing an energy balance for both driving and dissipation terms. A scaling law is corroborated by the experimental data for viscous puddle jump time and puddle recoil velocity. Numerical solutions are also conducted for comparisons. We demonstrate highly damped puddle jumps which may be exploited in turn to study further drop dynamics phenomena such as vanishingly small Weber number drop-wall impacts, over-damped oblique impacts and rebounds, and viscous wall-bound droplet boiling in low-gravity environments. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D36.00006: Capillary migration of large confined super-hydrophobic drops in wedges Logan Torres, Mark Weislogel, Sam Arnold When confined within an interior corner, drops and bubbles migrate to regions of minimum energy by the combined effects of surface tension, surface wetting, and corner geometry. Such capillary phenomena are exploited for passive phase separation operations in micro-fluidic devices on earth and macro-fluidic devices aboard spacecraft. Our study focuses on the migration of large inertial-capillary drops confined between two planar super-hydrophobic surfaces. In our experiments, the near weightless environment of a drop tower produces Bo \textless \textless 1 for drop volumes $O$(10mL) with migration velocities up to 10 cm/s. We observe transient power law behavior as a function of drop volume, wedge angle, initial confinement, and fluid properties including contact angle. We then further demonstrate how the experiment method may be employed as a large horizontal quiescent droplet generator for studies ranging from inertial non-wetting moving contact line investigations to large geyser-free horizontal drop impacts. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D36.00007: Robust superhydrophobic PDMS/camphor based composite coatings with self-cleaning and self-healing properties Sushanta Mitra, Bichitra Sahoo, Sonil Nanda, Janusz Kozinski We report a novel process for the preparation of self-cleaning polymer composite with self-healing ability to self-repair from chemical and mechanical damages using readily available materials like Polydimethylsiloxane (PDMS) and camphor soot particles. When the camphor soot particles loading attained a critical level, the composite coating on glass and stainless steel surfaces reveals self-cleaning property with water contact angle of 171$^{\mathrm{0}}$. We also demonstrate that any degradation of its surface energy under the oxygen plasma etching can be recuperated, illustrating that the obtained superhydrophobic surface has a good self-healing ability. The fabricated PDMS/Camphor soot hybrid coating exhibited excellent retention of superhydrophobicity against impact of sand particles from a height of 10-70 cm. In addition, after being damaged chemically by strong acid treatment (2M HNO$_{\mathrm{3}}$ solution), the coating can also restore its properties after a short thermal cycle. Such versatile superhydrophobic surfaces can have wide applications ranging from under-water marine vessels to coating for surfaces to protect them from moisture and unwanted penetration of water. [Preview Abstract] |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D36.00008: Self-Cleaning Properties on Superhydrophobic Surfaces via Condensation David Miller, Julie Crockett, Daniel Maynes Superhydrophobic (SH) surfaces have many unique capabilities, one of which is self-cleaning. When a water droplet rolls on a contaminated SH surface, particulates can adhere to the droplet and roll away with the droplet, creating a self-cleaning effect. Another unique characteristic of SH surfaces is the promotion of dropwise condensation when cooled in a humid environment. These droplets may engulf particulates on the surface as they are generated and coalesce. This research seeks to understand the potential cleaning efficiency SH surfaces have when water vapor is condensed on a dirty SH surface and allowed to roll off. Multiple condensation cycles with common particulates deposited on SH surfaces oriented vertically are explored. Sliding and contact angles are measured to approximate the cleaning efficiency of the condensed, rolling droplets after each condensation cycle. Results are compared with the cleaning efficiency of water droplets placed on the surface to roll. [Preview Abstract] |
Sunday, November 20, 2016 4:41PM - 4:54PM |
D36.00009: Any material becomes superhydrophobic, if you can make it rough enough at multiple scales. daniel attinger, Christophe Frankiewicz Superhydrophobic surfaces with the self-cleaning behavior of lotus leaves are sought for drag reduction and phase change heat transfer applications. These superrepellent surfaces have traditionally been fabricated by random or deterministic texturing of a hydrophobic material, either as the base material or as a coating on technically relevant base materials. Recently, superrepellent surfaces have also been made from hydrophilic materials, by deterministic texturing using photolithography, without low-surface energy coating. Here, we show that hydrophilic materials can also be made superrepellent [1] to water by chemical texturing, a stochastic rather than deterministic process. These metallic surfaces [2] are the first analog of lotus leaves, in terms of wettability, texture and repellency. A mechanistic model is also proposed to describe the influence of multiple scales of roughness on wettability and repellency. These superrepellent surfaces made of hydrophilic materials are also able to switch between a metastable Cassie-Baxter state and a hydrophilic wetting state. Related opportunities for controlling phase change heat transfer will be discussed. [1] S. Herminghaus, "Roughness-induced non-wetting," \textit{EPL (Europhysics Letters), }vol. 52, p. 165, 2000; [2] C. Frankiewicz and D. Attinger, "Texture and wettability of metallic lotus leaves," Nanoscale, DOI: 10.1039/c5nr04098a. [Preview Abstract] |
Sunday, November 20, 2016 4:54PM - 5:07PM |
D36.00010: Droplets on porous hydrophobic surfaces perfused with gas: An air-table for droplets Nikolaos Vourdas, Vassilis Stathopoulos Wetting phenomena on porous hydrophobic surfaces are strongly related to the volume and the pressure of gas pockets resided at the solid-liquid interface. When the porous medium is perfused with gas by means of backpressure an inherently sessile pinned droplet undergoes various changes in its shape, contact angles and mobility. This provides an alternative method for active and controlled droplet actuation, without use of electricity, magnetism, foreign particles etc. Superhydrophobicity is not a prerequisite, electrode fabrication is not needed, the liquid is not affected thermally or chemically etc. In this work we explore this method, study the pertinent underlying mechanisms, and propose some applications. The adequate backpressure for droplet actuation has been measured for various hydrophobic porous surfaces. Backpressure for actuation may be as low as some tens of mbar for some cases, thus providing a rather low-energy demanding alternative. The droplet actuation mechanism has been followed numerically; it entails depinning of the receding contact line and movement, by means of a forward wave propagation reaching on the front of the droplet. Applications in valving water plugs inside open- or closed- channel fluidics will be provided. [Preview Abstract] |
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