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Surface engineering for phase change heat transfer: A review
- Daniel Attinger, Christophe Frankiewicz, Amy R. Betz, Thomas M. Schutzius, Ranjan Ganguly, Arindam Das, Chang-Jin Kim, Constantine M. Megaridis
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- Journal:
- MRS Energy & Sustainability / Volume 1 / 2014
- Published online by Cambridge University Press:
- 20 November 2014, E4
- Print publication:
- 2014
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Owing to advances in micro- and nanofabrication methods over the last two decades, the degree of sophistication with which solid surfaces can be engineered today has caused a resurgence of interest in the topic of engineering surfaces for phase change heat transfer. This review aims at bridging the gap between the material sciences and heat transfer communities. It makes the argument that optimum surfaces need to address the specificities of phase change heat transfer in the way that a key matches its lock. This calls for the design and fabrication of adaptive surfaces with multiscale textures and non-uniform wettability.
Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiency at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930s to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoveries.
Contact angle dynamics in droplets impacting on flat surfaces with different wetting characteristics
- ILKER S. BAYER, CONSTANTINE M. MEGARIDIS
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- Journal:
- Journal of Fluid Mechanics / Volume 558 / 10 July 2006
- Published online by Cambridge University Press:
- 04 July 2006, pp. 415-449
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An experimental study is presented on contact angle dynamics during spreading/recoiling of mm-sized water droplets impacting orthogonally on various surfaces with $\hbox{\it We}\,{=}\,O(0.1)-O(10)$, $Ca\,{=}\,O(0.001)-O(0.01)$, $\hbox{\it Re}\,{=}\,O(100)-O(1000)$, $Oh\,{=}\,O(0.001)$ and $Bo\,{=}\,O(0.1)$. In this impact regime, inertial, viscous and capillary phenomena act in unison to influence contact angle dynamics. The wetting properties of the target surfaces range from wettable to non-wettable. The experiments feature accelerating and decelerating wetting lines, capillary surface waves in the early impact stages, contact angle hysteresis, and droplet rebound under non-wetting conditions. The objective of the work is to provide insight into the dynamic behaviour of the apparent (macroscopic) contact angle $\theta$ and its dependence on contact line velocity $V_{\hbox{\scriptsize{\it CL}}}$ at various degrees of surface wetting. By correlating the temporal behaviours of $\theta$ and $V_{\hbox{\scriptsize{\it CL}}}$, the angle vs. speed relationship is established for each case examined. The results reveal that surface wettability has a critical influence on dynamic contact angle behaviour. The hydrodynamic wetting theory of Cox (J. Fluid Mech. vol. 357, 1998, p. 249) and the molecular-kinetic theory of wetting by Blake & Haynes (J. Colloid Interface Sci.) vol. 30, 1969, p. 421) are implemented to extract values of the corresponding microscopic wetting parameters required to match the experimentally observed $\theta$vs. $V_{\hbox{\scriptsize{\it CL}}}$ data. Application of hydrodynamic theory indicates that in the slow stage of forced spreading the slip length and the microscopic contact angle should be contact line velocity dependent. The hydrodynamic theory performs well during kinematic (fast) spreading, in which solid/liquid interactions are weak. Application of the molecular kinetic theory yields physically reasonable molecular wetting parameters, which, however, vary with impact conditions. The results indicate that even for a single liquid there is no universal expression to relate contact angle with contact line speed. Finally, analysis of the spreading dynamics on the non-wettable surfaces shows that it conforms to the Cassie-Baxter regime (only partial liquid/solid contact is maintained). The present results offer guidance for numerical or analytical studies, which require careful attention to the implementation of boundary conditions at the moving contact line, including the need to specify the dependence of contact angle on contact line speed.
In-situ Fluid Experiments in Carbon Nanotubes
- Yury Gogotsi, Joseph A. Libera, Almila GüvenÇ Yazicioglu, Constantine M. Megaridis
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- Journal:
- MRS Online Proceedings Library Archive / Volume 633 / 2000
- Published online by Cambridge University Press:
- 15 March 2011, A7.4
- Print publication:
- 2000
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Closed-end multi-wall carbon nanotubes, which contain an encapsulated aqueous multi-phase fluidunder high pressure, have been produced by hydrothermal synthesis. These nanotubes are leak-tight by virtue of holding the fluid at the high vacuum of a transmission electron microscope (TEM) and can be used as a testplatform for unique in-situ nanofluidic experiments in TEM. They form an experimental apparatus, which is at least two orders of magnitude smaller than the smallest capillaries used in fluidic experiments so far. Excellent wettability of the carbon tube walls by the liquid and a dynamic behavior similar to that in micro-capillaries demonstrates the possibility of use of nanoscale (<100 nm) tubes in nanofluidic devices.However, complex interface behavior that can potentially create hurdles to fluid transport is also demonstratedherein.