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Research Papers: Two-Phase Flow and Heat Transfer

Droplet Impingement and Vapor Layer Formation on Hot Hydrophobic Surfaces

[+] Author and Article Information
Ji Yong Park

Materials Research Laboratory and
Department of Materials Science
and Engineering,
University of Illinois,
Urbana, IL 61801
e-mail: park98@illinois.edu

Andrew Gardner, William P. King

Department of Mechanical Science
and Engineering,
University of Illinois,
Urbana, IL 61801

David G. Cahill

Materials Research Laboratory and
Department of Materials Science
and Engineering,
University of Illinois,
Urbana, IL 61801
Department of Mechanical Science
and Engineering,
University of Illinois,
Urbana, IL 61801

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received May 18, 2013; final manuscript received June 10, 2014; published online June 27, 2014. Assoc. Editor: Cila Herman.

J. Heat Transfer 136(9), 092902 (Jun 27, 2014) (7 pages) Paper No: HT-13-1249; doi: 10.1115/1.4027856 History: Received May 18, 2013; Revised June 10, 2014

We use pump–probe thermal transport measurements and high speed imaging to study the residence time and heat transfer of small (360 μm diameter) water droplets that bounce from hydrophobic surfaces whose temperature exceeds the boiling point. The structure of the hydrophobic surface is a 10 nm thick fluorocarbon coating on a Si substrate; the Si substrate is also patterned with micron-scale ridges using photolithography to further increase the contact angle. The residence time determined by high-speed imaging is constant at ≈1 ms over the temperature range of our study, 110 < T < 210 °C. Measurements of the thermal conductance of the interface show that the time of intimate contact between liquid water and the hydrophobic surface is reduced by the rapid formation of a vapor layer and reaches a minimum value of ≈0.025 ms at T>190 °C. We tentatively associate this time-scale with a ∼1 m s 1 velocity of the liquid/vapor/solid contact line. The amount of heat transferred during the impact, normalized by the droplet volume, ranges from 0.028 J mm 3 to 0.048 J mm 3 in the temperature range 110 < T < 210 °C. This amount of heat transfer is ≈1–2% of the latent heat of evaporation.

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Figures

Grahic Jump Location
Fig. 4

(a) Time evolution of the effective thermal conductance for the hydrophobic, CFx-coated surface at 110 °C (solid circles) and 170 °C (open circles). (b) Residence time plotted as a function of surface temperature. Error bars describe the standard deviation of the mean for three sets of 320 repetitions of each experiment. Error bars are omitted if the extent of the error bars is smaller than the size of the data points. Symbols are labeled by the surface used in the experiment: Pt-coated (filled triangles) and CFx-coated (filled circles). The residence time acquired from high-speed imaging for CFx-coated (open circles) is included for comparison. The residence time of the hydrophilic surface with an attached water droplet is compared to the correlation of Makino and Michiyoshi [26] (solid line).

Grahic Jump Location
Fig. 3

Individual frames from high-speed imaging of droplet rebounds from (a) hydrophilic Pt-coated surface and (b) hydrophobic CFx-coated surface at 110 °C. The volume of the water droplet is 25 nl. The water droplet attaches to the hydrophilic surface and bounces off the hydrophobic surface. (c) Images of the droplet dynamics for the hydrophobic surface at 190 °C.

Grahic Jump Location
Fig. 2

Characterization of the time-resolution of the experiment. In (a), the input of the rf lock-in is an amplitude modulated 12 MHz square wave where the amplitude modulation is a square wave of various durations (100–500 μs). The output of the rf lock-in is measured by a digital-to-analog data acquisition board with 10 kS/s sampling rate. The time-dependence of the signals are not fully resolved but, as expected, in (b), the integrated areas under the curves shown in panel (a) scale linearly with the duration of the square wave used to modulate the signal.

Grahic Jump Location
Fig. 1

(a) Schematic diagram of the sample area. Pump and probe beams (λ = 1550 nm) are used for TDTR measurements of the effective thermal conductance of the water/sample interface and TPA measurements of the temperature changes of the sample. A He–Ne laser (λ = 632 nm) and detector is used to trigger the data acquisition and reduce the timing jitter of the droplet arrival time. The top layer of the sample is either a hydrophobic fluorocarbon (CFx) layer or a hydrophilic Pt layer. (b) Scanning electron microscope image of the patterned surface of the sample acquired at a tilt angle of 30 deg. The square pattern has a 40 μm periodicity, 3 μm ridge width, and 1 μm ridge height.

Grahic Jump Location
Fig. 5

The amount of thermal energy transferred between the sample and the water droplet plotted as a function of surface temperature. The energy transferred is scaled by the volume of the water droplet, Ω = 0.025 mm3. Error bars describe the standard deviation of the mean for three sets of 320 repetitions of each experiment. Error bars are omitted if the extent of the error bars is smaller than the size of the data points. Symbols are labeled by the type of surfaces: Pt-coated (filled triangles) and CFx-coated (filled circles). Dashed line is an estimate of the thermal energy transfer due to conduction alone for residence times (CFx-coated in Fig. 4(b)), see Eq. (4).

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