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 D34: Drops: Superhydrophobic Surfaces |
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Chair: Jonathan Boreyko, Virginia Tech Room: Ballroom BC |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D34.00001: Self-Propelled Sweeping Removal of Dropwise Condensate on Two-Tier Superhydrophobic Surfaces Jonathan Boreyko, Xiaopeng Qu, Fangjie Liu, Rebecca Agapov, Nickolay Lavrik, Scott Retterer, James Feng, Patrick Collier, Chuan-Hua Chen Dropwise condensation can be enhanced by nanostructured superhydrophobic surfaces, on which the condensate drops spontaneously jump upon coalescence. However, the self-propelled jumping in prior reports is mostly perpendicular to the substrate. Here, we propose a substrate design with regularly spaced micropillars. Coalescence on the sidewalls of the micropillars leads to self-propelled jumping in a direction nearly orthogonal to the pillars and therefore parallel to the substrate. This in-plane motion in turn produces sweeping removal of multiple neighboring drops. The spontaneous sweeping mechanism may greatly enhance dropwise condensation in a self-sustained manner. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D34.00002: Wenzel to Cassie transition for droplet impingement Cristian Clavijo, Julie Crockett, Daniel Maynes Advantages posed by self-cleaning, superhydrophobic surfaces quickly diminish as the liquid penetrates gas-filled cavities resulting in the so-called Wenzel state. To prevent penetration, surfaces must exhibit nanoscale features since penetrating pressure increases significantly for decreasing feature size. However, certain applications require microscale roughness such as those seeking to relax the no-slip condition and thus penetration reversal in microscale features remains of interest. Unfortunately, recent efforts to accomplish such reversal are complicated or locally-disruptive to the flow such as electrically-tunable surfaces and boiling. Here, we show that a Wenzel-to-Cassie transition is possible with a modest surface temperature increase. Dynamics are discussed for a water droplet impinging (We$=$100) on a wide range of superhydrophobic surfaces with features varying in height from 4 microns to 18 microns and separation distance of 8 microns to 16 microns. Results reveal that dewetting rates increase with increasing feature height and temperature up to 30 mL/s. A first order model is constructed to validate our hypothesis that surface tension and triple line dissipation are the two dominating forces during dewetting. Good agreement is found between the model and experimental results. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D34.00003: Droplets on inclined super hydrophobic substrates: between ''sandwich,'' free sliding and jumping Julian Martinez Mercado, Claus-Dieter Ohl We seek to understand the effect of confinement on the transport properties of droplets on super hydrophobic surfaces. In a straightforward experiment, the droplet slides down an incline while being sandwiched between two plates. The dynamics is captured from two views to reveal centre of mass motion and the three dimensional motion. The range of Weber and Reynolds number based on the droplet radius are 0.6-4 and 260-680, respectively. The capillary number is of order $10^{-3}$. Three geometries are studied, confined between two plates, droplet release, and droplet capture. For the latter two geometries, some part of the incline consists of lower and upper plates. The experimental observations are that the acceleration of a ``sandwiched" droplet is considerably reduced to a free sliding one. Droplets being released jump off the substrate converting considerable amount of the surface energy into potential energy. Droplet capture obeys a limit kinetic energy, below that, they are reflected from the constriction. We hope to present detail of the flow within the sandwiched droplet by the time of presentation. [Preview Abstract] |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D34.00004: Drop impact dynamics on liquid-infused superhydrophobic surfaces Jeong-Hyun Kim, Jonathan Rothstein In this talk, we present a series of experiments investigating the drop impact dynamics on hydrophobic, air-infused and lubricant-infused superhydrophobic surfaces. To create the superhydrophobic surfaces, smooth Teflon (PTFE) surfaces were roughened by a 240-grit sandpaper. The immiscible and incompressible silicone oils with different viscosities were infused into features of the superhydrophobic surfaces by a skim coating technique. The spreading and retraction dynamics on a series of the tested surfaces will be presented. We will show that the maximal deformation of the drops on lubricant-infused surfaces grows with increasing viscosity ratio between a water drop and the infused oil. We will show that this increase in the maximal deformation with the viscosity ratio is consistent with increasing the velocity and the viscosity of the drops but the rims of the drops destabilize with increasing the drop velocity. Finally, we will demonstrate that increasing the viscosity of the infused oil induces higher viscous force at the contact line, resulting in reduction in the movement of the drops during retraction and corresponding increase in the final drop size. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D34.00005: Anti-fogging surfaces Timothée Mouterde, Antonio Checco, Charles Black, Atikur Rahman, Christophe Clanet, David Quéré Achieving an anti-fogging material is more challenging than achieving an anti-rain material. A relevant way to investigate the resistance to fog consists of depositing hot water on a cold surface. We show that classical superhydrophobic surfaces with micron-size microstructures lose their superhydrophobic behaviour due to vapour condensation. To understand this phenomenon, we measured the adhesion force of hot water drops on different substrates and propose a quantitative description of this force generated by condensation. Our main result is that reducing the scale of the structures can strongly promote antifogging properties. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D34.00006: Orientation Dependence of Jumping Droplet Condensation Austin Berrier, Jonathan Boreyko On nanostructured superhydrophobic surfaces, microscopic condensate exhibits out-of-plane jumping that minimizes the average droplet size for maximal phase-change heat transfer. This jumping-droplet phenomenon occurs independently of gravity and is due to surface energy being partially converted to kinetic energy upon coalescence events. Although the initial departure of the jumping droplets is independent of gravity, the subsequent trajectories exhibit a dependence upon the orientation of the substrate. The drop size distribution of jumping-droplet condensation growing on a superhydrophobic substrate was characterized for both horizontal and vertical surface orientations. With the horizontal orientation, jumping condensate returns to the substrate by gravity. While this can result in chain reactions with other droplets to trigger further jumping events, eventually the rebounding droplets become too large to jump and are stuck on the surface. In contrast, droplets jumping off a vertically oriented surface do not return to the substrate. For this reason, the maximum droplet diameters during condensation growth were found to be significantly larger on the horizontally oriented superhydrophobic surface than on the vertical orientation. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D34.00007: On the origin of surface fraction scaling for receding contact angles on textured superhydrophobic surfaces Etienne Barthel, Jeremie Teisseire, Marco Rivetti It has long been recognized that surface fraction is a relevent parameter to rationalize the receding contact angle on textured superhydrophobic surfaces [1]. This notion can easily be rationalized from a simple surface energy averaging procedure, which leads to the Cassie relation. The concept has recently been challenged because it is unclear how surface averaging could apply to a line, and line averaging often provides a better fit to the data [2]. We have revisited this problem by exploring strongly anisotropic surfaces for which surface fraction and line fraction scalings are clearly differentiated. Our experimental and simulation results suggest that surface fraction scaling originates from line defects. Since these defects straddle rows, they probe both lattice dimensions, whereby surface fraction scaling emerges. However, our results also show that strict proportionality as predicted by the Cassie relation does not hold: a much more singular behavior is found at low surface fractions, in keeping with the near-threshold behaviour expected from a depinning process [3]. [1] QU\'{E}R\'{E}, D. Annu. Rev. Mater. Res. 38 (2008) 71 [2] CHOI, W.; A. et al. J. Colloid Interf. Sci. 339 (2009) 208 [3] RIVETTI et al. Phys. Rev. Lett. 115 (2015) 016101 [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D34.00008: Layers of Porous Superhydrophobic Surfaces for Robust Water Repellency Farzad Ahmadi, Jonathan Boreyko In nature, birds exhibit multiple layers of superhydrophobic feathers that repel water. Inspired by bird feathers, we utilize porous superhydrophobic surfaces and compare the wetting and dewetting characteristics of a single surface to stacks of multiple surfaces. The superhydrophobic surfaces were submerged in water in a closed chamber. Pressurized gas was regulated to measure the critical pressure for the water to fully penetrate through the surfaces. In addition to using duck feathers, two-tier porous superhydrophobic surfaces were fabricated to serve as synthetic mimics with a controlled surface structure. The energy barrier for the wetting transition was modeled as a function of the number of layers and their orientations with respect to each other. Moreover, after partial impalement into a subset of the superhydrophobic layers, it was observed that a full dewetting transition was possible, which suggests that natural organisms can exploit their multiple layers to prevent irreversible wetting. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D34.00009: A computational study of the impingement of water droplets onto freezing superhydrophobic surfaces WEN JIN, Behrooz Amirzadeh, Mazdak Tootkaboni, Mehdi Raessi We present computational simulations of the impingement of micron-size water droplets onto freezing superhydrophobic surfaces at various Weber numbers, droplet initial temperatures, and surface temperatures. The simulation results are from an in-house volume-of-fluid based, free-surface flow solver with phase change. The objective is to investigate the conditions under which the droplets bounce off the surface or stick to the surface and freeze. The transition between the bouncing and sticking regimes is shown. Then, using a dimensional analysis of the timescales for droplet freezing and drop-surface contact, a theoretical model is proposed for predicting the above transition. Finally, the predictions of the theoretical model are compared to the transition conditions observed in the computational simulations. [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D34.00010: Transferring heat during a bounce Samira Shiri, James Bird When a hot liquid drop impacts a cold non-wetting surface, the temperature difference drives heat transfer. If the drop leaves the surface before reaching thermal equilibrium, the amount of heat transfer may depend on the contact time. Past studies exploring finite-time heat exchange with droplets focus on the Leidenfrost condition where heat transfer is regulated by a thin layer of vapor. Here, we present systematic experiments to measure the heat transferred by a bouncing droplet in non-Leidenfrost conditions. We propose a physical model of this heat transfer and compare our model to the experiments. [Preview Abstract] |
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