Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session A31: Focus Session: Interfacial Engineering in Thermal-Fluids I |
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Chair: Neelesh Patankar, Northwestern University Room: 33B |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A31.00001: Droplet Interaction with Lubricant Impregnated Surfaces Rajeev Dhiman, Jonathan David Smith, Sushant Anand, Robert Cohen, Gareth McKinley, Kripa Varanasi The interaction of water drops with lubricant impregnated surfaces was studied experimentally under static and dynamic conditions. Such surfaces contain microscopic roughness features into which a liquid lubricant is impregnated and held by virtue of capillary forces. The remarkable feature of such a construction is that droplets, immiscible to the lubricant, experience negligible resistance to movement on the surface, provided the system is designed carefully under static and dynamic considerations. We describe these considerations and present their implications to droplet mobility. We also study impacting droplets and observe shedding characteristics that are unique to lubricant impregnated surfaces. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A31.00002: ABSTRACT WITHDRAWN |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A31.00003: The Leidenfrost transition for drops on micro- and nano-textures Hyuk-Min Kwon, James Bird, Kripa Varanasi When a liquid drop contacts a sufficiently hot surface, the drop can float on its own vapor in a process kwon as the Leidenfrost effect. Although it has been observed that the Leidenfrost transition temperature varies with the physical properties of the heated surfaces, the precise mechanisms that set this transition are still not fully understood. Here, we examine the mechanisms for drops contacting a heated surface with well defined micro and nano-textures. We rationalize our experimental results with a scaling model, and subsequently use this model to make predictions that we test experimentally. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A31.00004: ABSTRACT WITHDRAWN |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A31.00005: Hotspot Cooling with Self-Propelled Jumping Condensate Xiaopeng Qu, Jonathan B. Boreyko, Fangjie Liu, Chuan-Hua Chen Dynamic hotspots are prevalent in electronic systems including microprocessors and power electronics with constantly changing computing tasks or payloads. Here, we report a new adaptive hotspot cooling technique that rapidly responds to moving hotspots in a passive manner independent of external forces. The hotspot cooling is based upon the self-propelled jumping of dropwise condensate, which directly returns the working fluid from a superhydrophobic condenser to an opposing superhydrophilic evaporator. The adaptive thermal management is accomplished by the preferential evaporation of water at the hotspots and the rapid jumping return of the condensate across the very short inter-plate distance. The proof-of-concept for this hotspot cooling technique will be demonstrated by the adaptive response to hotspots at increasing heat fluxes. [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A31.00006: Direct visualization and self-similarity of contact line depinning Adam Paxson, Kripa Varanasi We report a novel technique to observe the microscale three-dimensional geometry of the contact line of a drop of water on a superhydrophobic surface as it recedes and depins from roughness features. We measure the local receding contact angle at the base of the capillary bridges at the contact line and find them to be equivalent to the macroscale receding contact angle observed on a chemically equivalent smooth surface, providing experimental validation of the Gibbs criterion at the microscale. We use this technique on a dual-scale hierarchically textured surface and reveal a self-similar depinning mechanism that explains how the geometry of the roughness at each length scale affects the adhesion of the contact line. This mechanism allows us to propose a model for predicting the adhesion force of a macroscopic drop, and we use a tensiometer to experimentally verify the model's applicability to both synthetic and natural surfaces. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A31.00007: Biphilicity and Superbiphilicity for Wettability Control of Multiphase Heat Transfer Daniel Attinger, Amy Rachel Betz, T. M. Schutzius, J. Jenkins, C.-J. Kim, C.M. Megaridis Multiphase energy transport, such as in boiling, suggests contradictory requirements on the wettability of the solid surfaces coming into contact with the working fluid. On the one hand, a hydrophobic wall promotes nucleation. On the other hand, a hydrophilic wall promotes water contact and enhances the critical heat flux. An analogous situation appears in the opposite thermodynamic process, i.e. condensation. These apparently contradictory requirements can be accommodated with biphilic surfaces, which juxtapose hydrophilic and hydrophobic regions. Biphilic surfaces were first manufactured in 1964 by Young and Hummel, who sprayed Teflon drops onto a smooth steel surface: they showed enhanced heat transfer coefficient during boiling of water. Our recent work has revisited the manufacturing of biphilic surfaces using micro- and nanofabrication processes (Betz et al. 2010, Schutzius et al. 2012); for instance, we fabricated the first superbiphilic surfaces, which juxtapose superhydrophobic and superhydrophilic areas. Using these surfaces, we measured significant enhancement during pool boiling of both the heat transfer coefficient and the critical heat flux. This enhanced performance can be explained by the inherent ability of the surfaces to control multiphase flow, decreasing nucleation energies and shaping drops, bubbles and jets, to maximize transport and prevent instabilities. [Preview Abstract] |
Sunday, November 18, 2012 9:31AM - 9:44AM |
A31.00008: Staying dry under water Paul Jones, Eduardo Cruz-Chu, Constantine Megaridis, Jens Walther, Petros Koumoutsakos, Neelesh Patankar Lotus leaves are known for their non-wetting properties due to the presence of surface texture. The superhydrophobic behavior arises because of the prevention of liquid water from entering the pores of the roughness. Present superhydrophobic materials rely on air trapped within the surface pores to avoid liquid permeation. This is typically unsustainable for immersed bodies due to dissolution of the air, especially under elevated pressures. Here, molecular dynamics simulations are used to demonstrate the non-wetting behavior of an immersed ten-nanometer pore. This is accomplished by establishing thermodynamically sustained vapor pockets of the surrounding liquid medium. Over 300,000 atoms were used to construct the nanopore geometry and simulate SPC/E water molecules. Ambient pressure was varied along two isotherms (300 K, and 500 K). This approach for vapor-stabilization could offer valuable guidance for maintaining surfaces dry even in a submerged state without relying on trapped air. The approach may be extended to control general phase behavior of water adjacent to textured surfaces. [Preview Abstract] |
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