Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session GW: Mini-Symposium on Fluid Dynamics at Super-repellent Surfaces |
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Chair: Eric Lauga, University of California, San Diego, Constantine Megaridis, Univ. of Ill. at Chicago, Lyderic Bocquet, Université de Lyon Room: 208A-D |
Monday, November 23, 2009 8:00AM - 8:26AM |
GW.00001: Unusual dynamical properties of water repellent materials Invited Speaker: The reason why water repellent materials have been attracting such an attention for about ten years is mainly related to the remarkable dynamical properties they generate. Paradoxically, apart from the question of slip on such surfaces, only a few quantitative studies were devoted to these properties. In our talk, we plan to describe different ways of reaching superhydrophobicity, by texturing the underlying solid or the deposited liquid, or by heating the substrate. Then we list and describe the different specific dynamical behaviours which are observed, such as ultra-low hysteresis, (fast) running, bouncing or self-motion [Preview Abstract] |
Monday, November 23, 2009 8:26AM - 8:52AM |
GW.00002: Drag reduction in laminar and turbulent flows past superhydrophobic surfaces Invited Speaker: A series of experiments and direct numerical simulations (DNS) will be presented which demonstrate significant drag reduction for both laminar and turbulent flows of water through channels using superhydrophobic surfaces with well-defined micron-sized surface roughness. The surfaces are fabricated from PDMS to incorporate precise patterns of ridges or posts that can support a shear-free air-water interface. A flow cell is used to measure the pressure drop and velocity profile as a function of the flow rate for a series of channel geometries and superhydrophobic surface designs. DNS are performed for flow past superhydrophobic surfaces which both complement and extend the range of geometries and Reynolds number obtained in the experiments. We will show that drag reductions up to 75\% and slip lengths up to 150$\mu$m can be obtained in turbulent flows past superhydrophobic surfaces. Additionally, we will show that slip along the air water interface forestalls the transition from laminar to turbulent flow. The drag reduction is found to increases with increasing post/ridge spacing and the fraction of air-water interface. In turbulent flows, the drag reduction increases with Reynolds number before eventually reaching a plateau. These results suggest that in turbulent flows, the drag reduction scales with the thickness of the viscous sublayer and not the overall channel height as in laminar flows. [Preview Abstract] |
Monday, November 23, 2009 8:52AM - 9:18AM |
GW.00003: Controlling inertia dominated flows with super-repellent surfaces Invited Speaker: The possibility to affect liquid flows through surface properties was naturally put forward by the recent emergence of small-scales fluidic devices, as downsizing invariably emphasizes the role of surfaces, with respect to bulk properties. Such strategy of flow modification by surface effects is \textit{a priori} restricted to the natural scales setting the interactions between the surface and the nearby liquid that is, essentially to nanometric scales. In this context, super-repellent surfaces have emerged as possessing not only remarkable (non-)wetting properties but also unique dynamical properties. The latter manifest on their ability to promote large boundary slippage, characterized by slip lengths from 1 to hundreds of microns, that make them capable of modifying flows up such micro-scales. More fundamentally, this raises the question of how far this strategy of flow control through surfaces can be pushed, and of how deep the modification of liquid flows close to super-repellent surface is: can it persist at large scales or large velocities? After briefly going through the properties of super-repellent surfaces in laminar viscous flows, I will discuss their impact on different macro-scale experimental configurations involving inertia-dominated flows. Focusing on splashing and dripping phenomena - the latter being associated to the well-known teapot effect- I will show that although surface effects are usually ignored in such situations, in view of the large values of the Weber number, it is still possible to shape the liquid flows by tailoring surface properties, with optimized effects obtained for super-repellent surfaces. [Preview Abstract] |
Monday, November 23, 2009 9:18AM - 9:44AM |
GW.00004: Promoting Giant Liquid Slip on Omniphobic Surfaces with Re-entrant Textures Invited Speaker: It is now well-known that by controlling the surface chemistry and topographic details of a textured surface one can generate composite air-liquid-solid interfaces or ``Cassie-Baxter states.'' If the surface topography becomes \textit{re-entrant} (i.e. multi-valued) and very low surface energy coatings are employed, then it becomes possible to create superoleophobic surfaces that are not wetted even by low-tension liquids such as oils and alcohols. Such robustly liquid-repellent (or \textit{omniphobic}) surfaces lead to very high apparent contact angles, low contact angle hysteresis and the possibility of ``giant liquid slip'' over the microscopic air pockets or ``plastron film'' trapped in the re-entrant textured surface. Lithographic fabrication approaches have been proposed for developing such re-entrant textured surfaces - a major challenge with such approaches is to develop viable manufacturing protocols that can be readily extended to larger areas. In the present work we use periodic woven fiber meshes of controlled feature size and weave, coupled with a simple elastomeric fluoropolymer dipcoating protocol, to prepare a series of model re-entrant and friction-reducing surfaces. We use parallel-plate rheometry to explore the degree of friction reduction that can be achieved as the geometric details of the meshes are varied and compare the experimental results with recent scaling theories. Apparent slip lengths of greater than 500$\mu$m are observed for optimal textures and coatings. By varying the thickness and viscosity of the sheared fluid layer, the robustness of the plastron air film to increasing pressure differentials can also be explored in parallel. [Preview Abstract] |
Monday, November 23, 2009 9:44AM - 10:10AM |
GW.00005: Development of Surface Structures for Large Effective Slip: How Much Slip Is Possible in Ideal, Lab and Real Conditions? Invited Speaker: An ideal condition to reduce the drag of a liquid flowing on a solid surface is maintaining a lubricating gas layer between the solid and the liquid. For water flowing on a 1 or 10 $\mu$m-thick air layer, for example, the slip length is calculated to be roughly 50 or 500 $\mu$m, respectively - large enough to benefit a wide range of engineering applications. Unfortunately, however, the above ideal water-levitating condition is only imaginary, because such a liquid-gas meniscus cannot be sustained in nature. Instead, water-repelling structured surfaces bring us closer to the imaginary condition by minimizing the liquid-solid interface and keeping the water mostly on a layer of air. The underlying goal in developing a large-slip surface is, therefore, to create a condition as close as possible to the uniform air lubrication, which is often overlooked. For example, while a large contact angle on a superhydrophobic surface helps keep the liquid fakir, note that once levitated, the contact angle has little effect on increasing the slip length. Instead, the geometrical parameters of the surface structures, e.g., air fraction, pitch and depth of the structures, are the determining factors. A series of development efforts to create surfaces that bring us closer to the ideal air-lubricating condition will be presented, with the slip length currently measured as large as 400 $\mu$m. However, it will be also noted that they are valid only in laboratory conditions, where the sample is fabricated to near perfection and the pressure in the flowing liquid is under strict control. In real-life engineering conditions, which include high and fluctuating pressure, defective surfaces, and liquids full of impurities and particles, it remains to be seen if we will ever be able to create a slip surface that can be field-deployed - a millennium-old dream. [Preview Abstract] |
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