Browsing by Author "Li, Yinxiao"

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Single bubble dynamics on superheated superhydrophobic surfaces
(2016-08)
Li, Yinxiao
;
Zhang, Ke
;
Lu, Ming-Chang
;
Duan, Chuanhua
;
機械工程學系
This work explores single bubble dynamics on superheated superhydrophobic surfaces and the corresponding heat transfer mechanism of water pool boiling. The superhydrophobic surfaces are prepared by coating a thin layer of polytetrafluoroethylene on top of silicon substrates with electroless etched silicon nanowires. It is observed that a vapor film covers the whole superhydrophobic surface when it is immersed in water. Once the surface is superheated, single bubble forms and departs from the vapor film. We find that the bubble growth rate shows little dependence on surface superheat and applied heat flux at large superheats due to the presence of the vapor layer, which seriously limits the heat transfer from the superheated superhydrophobic surface. The bubble departure diameter is mainly determined by the static force equilibrium, but local disturbance and surface superheat also play important roles. There is a plateau of bubble departure frequency at large superheats, which is a result of the constant departure velocity limit. Accompanying the continuous bubble formation and departure, a smooth boiling curve is observed. The corresponding heat transfer coefficient (HTC) first increases and then slowly decreases as the superheat increases, showing a peak at a superheat of similar to 50 K. Such HTC change is explained by the heat flux partitioning model, suggesting latent heat transport due to water vaporization and bulk liquid water convection after bubble departure contribute together to the heat removal on superhydrophobic surfaces. Insights gained from pool boiling on superhydrophobic surfaces might provide design guidelines for new surfaces excellent in heat removal or hydrodynamic drag reduction. (C) 2016 Elsevier Ltd. All rights reserved.
Ultrafast Diameter-Dependent Water Evaporation from Nanopores
(2019-03-01)
Li, Yinxiao
;
Chen, Haowen
;
Xiao, Siyang
;
Alibakhshi, Mohammad Amin
;
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.