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Research Papers

Effects of Free Surface Evaporation on Water Nanodroplet Wetting Kinetics: A Molecular Dynamics Study

[+] Author and Article Information
Gui Lu

Key Laboratory for Thermal Science and
Power Engineering of MOE,
Tsinghua University,
Beijing 100084, China

Yuan-Yuan Duan

Key Laboratory for Thermal Science and
Power Engineering of MOE,
Tsinghua University,
Beijing 100084, China
e-mail: yyduan@tsinghua.edu.cn

Xiao-Dong Wang

State Key Laboratory of Alternate Electrical Power
System With Renewable Energy Sources,
North China Electric Power University,
Beijing 102206, China
Beijing Key Laboratory of Multiphase Flow and
Heat Transfer for Low Grade Energy,
North China Electric Power University,
Beijing 102206, China
e-mail: wangxd99@gmail.com

1Corresponding author.

Manuscript received November 4, 2013; final manuscript received August 12, 2014; published online May 14, 2015. Assoc. Editor: Yogesh Jaluria.

J. Heat Transfer 137(9), 091001 (May 14, 2015) (6 pages) Paper No: HT-13-1562; doi: 10.1115/1.4030200 History: Received November 04, 2013

The wetting kinetics of a water nanodroplet undergoing evaporation on a heated gold substrate were examined using molecular dynamics (MD) simulations. Various substrate and initial droplet temperatures were used to obtain different evaporation rates. The water molecule absorption–desorption behavior was analyzed in the vicinity of the contact line region to show the microscopic details of the spreading–evaporating droplet. Increasing substrate temperatures greatly affected the dynamic wetting process, while the initial water droplet temperature had very little effect. The effects of droplet size and substrate wettability on the droplet spreading–evaporating process were also examined. The radius versus time curves agree well with molecular kinetics theory (MKT) for spreading without evaporation but differ from MKT when the spreading induced evaporation. The enhancement of the wetting kinetics by the evaporation can be attributed to the reduction of the liquid–vapor surface tension and the increased water molecule motion in the contact line region and in the bulk droplet.

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References

Figures

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Fig. 2

Spreading radius for limited evaporating spreading (Tl = 300 K and Ts = 300 K): (a) spreading law fitting with R-t and (b) solid–liquid friction coefficient fitting with U-θ

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Fig. 3

Precursor layer from MD simulations (Tl = 300 K, Ts = 300 K, and t = 1 ns)

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Fig. 4

Spreading and evaporating snapshots (t = 4 ns) for various substrate temperatures

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Fig. 5

Dynamic wetting radii for various substrate temperatures (Tl = 300 K)

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Fig. 6

Surface tension calculation based on the excess free energy method: (a) water film with two free surfaces and (b) bulk water without free surfaces

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Fig. 7

Molecular tracing in the vicinity of the contact line region: (a) Ts = 300 K; (b) Ts = 333 K; and (c) Ts = 363 K

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Fig. 8

Replacement portion of dyed molecules shown in Fig. 7

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Fig. 9

Dynamic wetting radii for various initial droplet temperatures (Ts = 363 K)

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Fig. 10

Water droplet temperature evolution for various initial temperatures (Ts = 363 K): (a) 4500 water molecules and (b) 9000 water molecules

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Fig. 11

Effects of substrate wettability on dynamic wetting: (a) limited evaporation (Ts = 300 K and Tl = 300 K) and (b) with evaporation (Ts = 363 K and Tl = 300 K)

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