Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G22: Drops: Hydrophobic and Dynamic Surface Interactions |
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Chair: Susmita Dash, IISc Bangalore Room: 604 |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G22.00001: The role of humidity and pressure in preserving superhydrophobicity for droplet impact Henry Lambley, Thomas Schutzius, Dimos Poulikakos The design of robust superhydrophobic surfaces for impacting water droplets is typically achieved by creating surface structures with capillary (anti-wetting) pressures greater than that of the incoming droplet (dynamic, water hammer). Recent work has focussed on how competition between compression and drainage of air within the surface texture dictates the evolution of the intervening air layer between droplet and substrate and its role in promoting impalement under ambient conditions through local increases in the droplet curvature. However, little consideration has been given to the influence of the intervening air layer composition, and compressibility effects, when departing from ambient conditions on the impact outcome. Here, we explore the limits of the working envelope for robust superhydrophobic surfaces by varying the ambient pressure and water vapour content. By testing rationally engineered materials with both micro and nanoscale features, we are able to provide additional design constraints and propose solutions for future applications of superhydrophobic and icephobic technologies across a broad range of environmental conditions. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G22.00002: Multilayered synthetic feathers for enhanced underwater superhydrophobicity Zaara Dean, Farzad Ahmadi, Viverjita Umashankar, Brian Chang, Sunghwan Jung, Jonathan Boreyko Submerged superhydrophobic surfaces can dramatically reduce hydrodynamic drag and bio-fouling, but the enabling air pockets are prone to irreversible collapse. Inspired by ducks, we demonstrate that air pockets within stacked layers of porous superhydrophobic feathers can withstand up to five times more water pressure compared to a single feather. In addition to natural duck feathers, the multilayered effect was replicated with synthetic feathers created by laser cutting micrometric slots into aluminum foil and imparting a superhydrophobic nanostructure. The mechanism for the multilayered enhancement is the more tortuous pathway required for water impalement, which serves to pressurize the enclosed air pockets. This was validated by a probabilistic impalement model and also by filling the feathers with an incompressible oil, rather than air, to suppress the multilayered effect. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G22.00003: Surfactant droplets on hydrophobic microstructures Peichun Amy Tsai, Nadia Shardt, Masoud B. Bigdeli, Janet Elliott Surfactants—amphiphilic molecules—can easily adsorb at interfaces. Their presence can destroy the useful, gas-trapping (Cassie–Baxter, CB) wetting state of a drop sitting on a superhydrophobic surface. Here, we examine how surfactants alter the wetting state and contact angle of aqueous drops on hydrophobic microstructures of different surface roughness ($r$) and solid fraction ($\phi$). Experimentally, at low surfactant concentrations ($C_s$), some drops attain a homogeneous wetting state (Wenzel, W), while other drops are in the CB state. In contrast, all of our high $C_s$ drops attain the Wenzel state. To explain this observed transition, we develop a thermodynamic free energy analysis and find that, theoretically, for our surfaces the W state is always thermodynamically preferred, while the CB state is metastable at low C, consistent with experimental results. We further provide a beneficial blueprint for stable CB, gas-trapping states for applications exploiting superhydrophobicity. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G22.00004: Dropwise Condensation on Hierarchical Uncoated Metallic Surfaces Daniel Orejon, Alexandros Askounis, Daniel Attinger, Yasuyuki Takata Dropwise condensation is receiving increasing attention in the past decade. Traditionally, for the fabrication of hydrophobic and superhydrophobic surfaces able to perform in a dropwise condensation manner, the application of a conformal hydrophobic coating was required. In this work, we demonstrate that by passive exposure of our metallic hierarchical micro- and nano-structured copper oxide surface resembling the wetting behavior of a lotus leaf, superhydrophobicity and dropwise condensation can be achieved. The change in wettability from hydrophilic to hydrophobic is due to the adsorption of volatile organic compounds present in the ambient. On contrast, on a similar copper etched surface without copper oxide nanostructures resembling the wetting behavior of a rose petal, filmwise is the final condensation reported. Experimental observations at the micro- and at the macro-scale coupled with droplet morphology and surface coverage analysis, as well as a surface energy analysis are presented to support the different condensation behavior. We conclude on the feasibility of dropwise condensation by the coupling mechanisms of surface structure and hydrocarbon adsorption without the need for a manmade hydrophobic coating. The authors gratefully acknowledge WPI-I$^{\mathrm{2}}$CNER and ThermaSmart for their support. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G22.00005: Quantification of Wind-Driven Water Droplets over Surfaces with Different Wettabilities Hui Hu, Zichen Zhang, Liqun Ma In the present study, a comprehensive experimental study is performed to quantify the transient runback behavior of water droplet/rivulet flows as driven by boundary layer winds over the surfaces of test plates with different wettabilities. The experimental study is conducted in a low-speed wind tunnel available at Iowa State University (ISU-IRT) A suite of advanced flow diagnostic techniques, which include high-speed photographic imaging, digital image projection (DIP), particle image velocimetry (PIV), are used to quantify the transient runback behavior of water droplets over test plates as driven by the boundary layer winds. Water droplets with their volumes changing from 10 to 100 $\mu $L are tested under different incoming wind speed. In addition to measuring the airflow velocity field around the wind-driven water droplets/rivulets, dynamic shape changes and stumbling runback motion of the water droplets/rivulets are also measured in real time in terms of water film thickness distribution, contact line moving velocity and wet surface area over the test plates with different wettabilities. The findings derived from the present study would be very helpful to gain a better understanding about the important microphysical process, which could lead to improvements of icing accretion models for more accurate prediction of ice formation and accretion process as well as the development of effective anti-/de-icing strategies for aircraft icing mitigation. -/abstract- Zichen Zhang, Liqun [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G22.00006: Out-of-Plane Self-Propulsion of Droplets on Heated Lubricant-Impregnated Surfaces. Susmita Dash, Jolet de Ruiter, Kripa Varanasi The dynamics of droplets on heated surfaces is crucial for heat transfer applications such as spray cooling. Here we report on the behavior of millimetric water droplets on heated liquid impregnated surfaces (LIS) that are stable at high temperatures. Next to a gentle in-plane hopping motion, droplets can demonstrate one of two vigorous behaviors -- at temperatures far below the typical Leidenfrost temperature: either a trapped gas bubble expands to ``blow-up'' the droplet into a thin liquid shell, or the droplet ``jumps'' out-of-plane. While the in-plane motion of the droplet is on the order of 10 mm/s, the droplet is propelled vertically upwards to several times its diameter at a velocity of approximately 200 mm/s. We present the mechanics underlying this behavior of droplets, which is specific to lubricant-impregnated surfaces and crucially depends on the thermodynamic state of the impregnating liquid. [Preview Abstract] |
Sunday, November 24, 2019 5:06PM - 5:19PM |
G22.00007: Impact-induced jets of highly-viscous liquids using a simple structured syringe Hajime Onuki, Kyota Kamamoto, Yoshiyuki Tagawa In 3D manufacturing, the increment of the ejectable liquid viscosity is crucial. This study introduces a new method for generating a jet of a highly viscous liquid. In our method, a syringe is partially filled with a liquid. The meniscus near the tip of the syringe initially has a concaved shape, which induces the flow-focusing effect during the jet formation. The jet emerges when the short-time impact (e.g., collision with the rigid floor) is applied. Remarkably, the highly viscous liquid up to 1,000 cSt is ejected as a liquid jet. The speed of the jet can be controlled by the liquid height inside the syringe. We discuss the reason why the new method can eject the highly viscous liquid based on our previous theory (Onuki et al., Phys. Rev. Applied 2018). We find that, thanks to the geometrical relation, the gradient of the pressure impulse (time integration of the pressure) inside the tip of the syringe is strongly increased, resulting in increasing the jet speed. [Preview Abstract] |
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