Browsing by Author "Sahoo, Venkataraman"

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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.
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.