Browsing by Author "Lo, Ching-Wen"

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Control of Ice Formation
(2017-03-01)
Lo, Ching-Wen
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Sahoo, Venkataraman
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Lu, Ming-Chang
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機械工程學系
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Department of Mechanical Engineering
Ice formation is a catastrophic problem affecting our daily life in a number of ways. At present, deicing methods are costly, inefficient, and environmentally unfriendly. Recently, the use of superhydrophobic surfaces has been suggested as a potential passive anti-icing method. However, no surface is able to repel frost formation at a very cold temperature. In this work, we demonstrated the abilities of spatial control of ice formation and confinement of the ice-stacking direction. The control and confinement were achieved by manipulating the local free energy barrier for frosting. The V-shaped microgroove patterned surface, which possessed these abilities, exhibited the best anti-icing and deicing performances among the studied surfaces. The insight of this study can be applied to alleviate the impact of icing on our daily life and in many industrial systems.
Elongated Bouncing and Reduced Contact Time of a Drop in the Janus State
(2018-09-18)
Sahoo, Venkataraman
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Chou, Chu-Yao
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Lo, Ching-Wen
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Lu, Ming-Chang
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機械工程學系
Fast drop bouncing is desired in numerous applications. However, it has never been realized on a superheated surface with concurrent contact boiling and the Leidenfrost effect (the so-called Janus thermal state). This is presumably because of the increased drop adhesion as a result of bubbling on a Janus surface. Nevertheless, on a hydrophilic SiNx surface with v-shaped Si microgrooves in this thermal state, an elongated bouncing for a water drop was observed and a small drop contact time of 10.36 ms was obtained. We propose that the elongated bouncing was originated from an asymmetric momentum force created by the vapor bubbles at the rim of the drop. Analytical expressions for the contact time and the asymmetric momentum force were established and the predictions qualitatively agreed with the experimental results. The obtained contact time was the lowest value that has ever been reported in the Janus thermal state. The proposed approach has promising applications in state-of-the-art technologies requiring high liquid mobility.
Enhancing Condensation Heat Transfer on Three-Dimensional Hybrid Surfaces
(2019-11-20)
Lo, Ching-Wen
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Chu, Yu-Cheng
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Yen, Ming-Han
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Lu, Ming-Chang
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機械工程學系
In recent years, micro/nanostructured surfaces have been applied to enhance condensation heat transfer. However, condensation heat transfer is greatly deteriorated by the flooding phenomenon that occurs at high subcooling temperatures. Here, we propose a three-dimensional (3D) hybrid surface to enhance the condensation at high subcooling temperatures. The 3D hybrid surface consisted of superhydrophobic (SHB) Si nanowire (SiNW) arrays and hydrophilic microchannels. The microchannels could confine the liquid-film thickness, and the liquid bridges formed on the 3D hybrid surfaces could be self-removed. Both of these characteristics prevent the surfaces from flooding. In addition, liquid droplets formed in the SiNW regions were dragged into the microchannels, which also improved the heat transfer. The heat transfer coefficient on the 3D hybrid surface could be enhanced over a large subcooling range. More remarkably, a record high heat flux of 655 +/- 10 kW.m(-2) was obtained on the 3D hybrid surface.
Leidenfrost suppression and contact time reduction of a drop impacting on silicon nanowire array-coated surfaces
(2020-02-01)
Sahoo, Venkataraman
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Lo, Ching-Wen
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Lu, Ming-Chang
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機械工程學系
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Department of Mechanical Engineering
Drop impacting on a superheated surface is of practical concern in several thermal systems. When the surface temperature is higher than the Leidenfrost point (LFP), a stable vapor cushion forms on the solid surface and the drop hovers over the thin vapor layer. This results in a drastic reduction in heat and mass transfer due to the thermally insulating vapor layer. The contact time of an impacting drop is the amount of time during which the drop comes in contact with the solid surface. A short contact time enhances heat transfer. Thus, a high LFP and a short drop contact time are of interest to researchers. The present study revealed substantial Leidenfrost suppression and contact time reduction on a superheated superhy-drophilic silicon nanowire (SiNW) array-coated surface. For an impacting drop of 14 mu L with a Weber number (We) of 2.0, a LFP of 655 +/- 14 degrees C was obtained on the SiNW array-coated surface with a nanowire height of 90 mu m. This LFP was the highest reported value in the literature. A theoretical modeling revealed that the elevated LFP was caused by a superior capillary force and a strong vapor dispersion into the nanowire array. In addition, contact time was substantially reduced on the superheated SiNW surface in the jet-bouncing regime. The contact time of 1.3 +/- 0.1 ms was 91% lower than the capillary-inertia limit. It was also the lowest reported value in the literature. The contact time reduction on the super-heated SiNW surface resulted from the application of an upward thrust on the impacting drop generated by the vigorous bubbling on the SiNW surface. (C) 2019 Elsevier Ltd. All rights reserved.
Scale Effect on Dropwise Condensation on Superhydrophobic Surfaces
(2014-08-27)
Lo, Ching-Wen
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Wang, Chi-Chuan
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Lu, Ming-Chang
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機械工程學系
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Department of Mechanical Engineering
Micro/nano (two-tier) structures are often employed to achieve superhydrophobicity. In condensation, utilizing such a surface is not necessarily advantageous because the macroscopically observed Cassie droplets are usually in fact partial Wenzel in condensation. The increase in contact angle through introducing microstructures on such two-tier roughened surfaces may result in an increase in droplet departure diameter and consequently deteriorate the performance. In the meantime, nanostructure roughened surfaces could potentially yield efficient shedding of liquid droplets, whereas microstructures roughened surfaces often lead to highly pinned Wenzel droplets. To attain efficient shedding of liquid droplets in condensation on a superhydrophobic surface, a Bond number (a dimensionless number for appraising drop-wise condensation) and a solid-liquid fraction smaller than 0.1 and 0.3, respectively, are suggested.
Spatial Control of Heterogeneous Nucleation on the Superhydrophobic Nanowire Array
(2014-03-01)
Lo, Ching-Wen
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Wang, Chi-Chuan
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Lu, Ming-Chang
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機械工程學系
;
Department of Mechanical Engineering
Condensation is a common phenomenon and is widely exploited in power generation and refrigeration devices. Although drop-wise condensation offers high heat and mass transfer rates, it is extremely difficult to maintain and control. In this study, the ability to spatially control heterogeneous nucleation on a superhydrophobic surface by manipulating the free energy barrier to nucleation through parameterizing regional roughness scale on the Si nanowire array-coated surface is reported. Water vapor preferentially condenses on the designed microgrooves on the Si nanowire surface and continuous shedding of the drop-wise condensate is observed on the surface. The nucleation site density can also be manipulated by tailoring the density of the microgroove on the surface. Moreover, the cycle time on the Si nanowire array with microgrooves is approximately ten times smaller than that on a plain Si surface. This suggests that potentially high heat and mass transfer rates can be achieved on the surface. The insight from this study has implications in enhancing energy efficiency in a wide range of thermal energy conversion systems.
Superhydrophobic Si nanowires for enhanced condensation heat transfer
(2017-08-01)
Lu, Ming-Chang
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Lin, Chien-Chang
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Lo, Ching-Wen
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Huang, Cheng-Wei
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Wang, Chi-Chuan
Condensation is an essential process in various industrial systems. Enhancing condensation by employing superhydrophobic (SHB) surfaces had drawn significant attention in recent years because of the emerging technology for surface engineering. However, the efficacy of SHB surfaces in condensation is controversy in the literature. The observed deteriorated heat transfer on SHB in condensation is presumably a result of highly pinned Wenzel droplets or flooding formed on the SHB surfaces. Si nanowire (SiNW) array-coated surface which can simultaneously provide a large number of nucleation sites and prevent condensate from penetrating into the nano-structure is a promising candidate for enhancing condensation. In this work, heat transfer on the SHB SiNW surface was investigated. At low subcooling, jumping of liquid droplets accompanied with a high droplet departure frequency resulted in a large heat transfer coefficient (HTC) of 88 16 kW/m(2) K on the SHB surface. This value is one of the highest reported condensation HTCs in the literature. It was 155% and 87% higher than those on the plain hydrophilic and hydrophobic surfaces, respectively. Heat transfer decreased with the rise of subcooling due to an increased condensate surface coverage ratio. However, condensate can still be rapidly shed away from the SHB SiNW surface at high subcooling, which render the comparatively larger HTC of 18.6 2 kW/m(2) K on the SHB SiNW surface as opposed to plain hydrophobic and hydrophilic surfaces. It was evidenced that SHB surface could have a superior heat and mass transfer performance than hydrophobic surface provided that the liquid droplets on the SHB could be shed away efficiently. (C) 2017 Elsevier Ltd. All rights reserved.
Ultrafast Diameter-Dependent Water Evaporation from Nanopores
(2019-03-01)
Li, Yinxiao
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Chen, Haowen
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Xiao, Siyang
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Alibakhshi, Mohammad Amin
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Lo, Ching-Wen
Evaporation from nanopores plays an important role in various natural and industrial processes that require efficient heat and mass transfer. The ultimate performance of nanopore-evaporation-based processes is dictated by evaporation kinetics at the liquid-vapor interface, which has yet to be experimentally studied down to the single nanopore level. Here we report unambiguous measurements of kinetically limited intense evaporation from individual hydrophilic nanopores with both hydrophilic and hydrophobic top outer surfaces at 22 degrees C using nanochannel-connected nanopore devices. Our results show that the evaporation fluxes of nanopores with hydrophilic outer surfaces show a strong diameter dependence with an exponent of nearly -1.5, reaching up to 11-fold of the maximum theoretical predication provided by the classical Hertz-Knudsen relation at a pore diameter of 27 nm. Differently, the evaporation fluxes of nanopores with hydrophobic outer surfaces show a different diameter dependence with an exponent of -0.66, achieving 66% of the maximum theoretical predication at a pore diameter of 28 nm. We discover that the ultrafast diameter-dependent evaporation from nanopores with hydrophilic outer surfaces mainly stems from evaporating water thin films outside of the nanopores. In contrast, the diameter-dependent evaporation from nanopores with hydrophobic outer surfaces is governed by evaporation kinetics inside the nanopores, which indicates that the evaporation coefficient varies in different nanoscale confinements, possibly due to surface-charge-induced concentration changes of hydronium ions. This study enhances our understanding of evaporation at the nanoscale and demonstrates great potential of evaporation from nanopores.