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Research Papers

Experimental and Numerical Study of Single Bubble Dynamics on a Hydrophobic Surface

[+] Author and Article Information
Youngsuk Nam, Jinfeng Wu, Gopinath Warrier, Y. Sungtaek Ju

Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90095

J. Heat Transfer 131(12), 121004 (Oct 15, 2009) (7 pages) doi:10.1115/1.3216038 History: Received September 28, 2007; Revised March 28, 2008; Published October 15, 2009

The growth and departure of single bubbles on two smooth surfaces with very different wettabilities are studied using high-speed video microscopy and numerical simulations. Isolated artificial cavities of approximately 10μm diameter are microfabricated on both a bare and a Teflon-coated silicon substrate to serve as nucleation sites. The bubble departure diameter is observed to be almost 3 times larger and the growth period almost 60 times longer for the hydrophobic surface than for the hydrophilic surface. The waiting period is practically zero for the hydrophobic surface because a small residual bubble nucleus is left behind on the cavity from a previous ebullition cycle. The experimental results are consistent with our numerical simulation results. Bubble nucleation occurs on nominally smooth hydrophobic regions with root mean square roughness (Rq) less than 1 nm even at superheat as small as 3°C. Liquid subcooling significantly affects bubble growth on the hydrophobic surface due to increased bubble surface area. Fundamental understanding of bubble dynamics on heated hydrophobic surfaces will facilitate the development of chemically patterned surfaces with enhanced boiling heat transfer performance and novel phase-change based micro-actuators and energy harvesters.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic diagram of the experimental setup: (a) the whole setup and (b) the silicon wafer instrumented with strain gauge heaters and thermocouple wires

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Figure 2

Surface topography of Teflon-coated wafer: (a) 3D surface topography and the water droplet placed on this surface and (b) the line profile at Y=5.5 μm

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Figure 3

Temperature distributions on the silicon wafer surface

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Figure 4

Selected images of the single bubble during one ebullition cycle (heating surface: bare silicon wafer, ΔTw=4.0°C, ΔTsub=1.5°C, p=1 atm, t∗=t/t0, and t0=1.6×10−2 s)

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Figure 5

Selected images of the single bubble during one ebullition cycle (heating surface: Teflon-coated silicon wafer, ΔTw=2.9°C, ΔTsub=0°C, p=1 atm, t∗=t/t0, and t0=1.6×10−2 s)

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Figure 6

Bubble growth history during one ebullition cycle (heating surface: bare silicon wafer, ΔTw=4.0°C, ΔTsub=1.5°C, p=1 atm, t∗=t/t0, t0=1.6×10−2 s, Deq∗=Deq/l0, DB∗=DB/l0, Dd∗=Dd/l0, and l0=2.5×10−3 m)

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Figure 7

Bubble growth history during one ebullition cycle (heating surface: Teflon-coated silicon wafer, ΔTw=2.9°C, ΔTsub=0°C, p=1 atm, t∗=t/t0, t0=1.6×10−2 s, DB∗=DB/l0, H∗=H/l0, Dd∗=Dd/l0, and l0=2.5×10−3 m)

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Figure 8

Temporal change in capillary number during one ebullition cycle (t∗=t/t0, t0=1.6×10−2 s, and logarithmic scale)

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Figure 9

Selected images of the single bubble in subcooled liquid (heating surface: Teflon-coated wafer, ΔTw=2.9°C, ΔTsub=2.5°C, and p=1 atm)

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Figure 10

Selected images of the single bubble in subcooled liquid (heating surface: Teflon-coated wafer, ΔTw=4.6°C, ΔTsub=3.5°C, and p=1 atm)

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Figure 11

Bubble shapes, temperature distributions, and velocity vectors (φ=120 deg, ΔTw=2.9°C, ΔTsub=0°C, p=1 atm, t∗=t/t0, t0=1.6×10−2 s, t1∗=experimental data, and t2∗=numerical data)

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Figure 12

Bubble shapes and pressure distributions (φ=120 deg, ΔTw=2.9°C, ΔTsub=0°C, p=1 atm, t∗=t/t0, and t0=1.6×10−2 s)

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