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Research Papers: Micro/Nanoscale Heat Transfer

How Nanostructures Affect Water Droplet Nucleation on Superhydrophobic Surfaces

[+] Author and Article Information
Abulimiti Aili, QiaoYu Ge

Department of Mechanical
and Materials Engineering,
Masdar Institute of Science and Technology,
P.O Box 54224,
Abu Dhabi, United Arab Emirates

TieJun Zhang

Department of Mechanical
and Materials Engineering,
Masdar Institute of Science and Technology,
P.O Box 54224,
Abu Dhabi, United Arab Emirates
e-mail: tjzhang@masdar.ac.ae

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received September 14, 2016; final manuscript received May 1, 2017; published online June 21, 2017. Assoc. Editor: C. A. Dorao.

J. Heat Transfer 139(11), 112401 (Jun 21, 2017) (10 pages) Paper No: HT-16-1581; doi: 10.1115/1.4036763 History: Received September 14, 2016; Revised May 01, 2017

Nucleation is the first stage of phase change phenomena, including condensation on nanostructured superhydrophobic surfaces. Despite plenty of theoretical studies on the effect of nanostructure density and shape on water droplet nucleation, not many experimental investigations have been reported. Here, we show both experimentally and theoretically that a moderate increase in the nanostructure density can lead to an increase in the nucleation density of water droplets because of the decreased energy barrier of nucleation in cavities formed between the nanostructures. Specifically, we observed droplets aligned in regions with denser nanostructures. The number and average volume of the aligned droplets in these regions were larger than that of the droplets in the surrounding areas. However, nucleation in cavities subsequently caused initial pinning of the droplet base within the nanostructures, forming a balloonlike, slightly elongated droplet shape. The dewetting transition of the pinned droplets from the Wenzel state to the unpinned Cassie state was predicted by quantifying the aspect ratio of droplets ranging from 3 to 30 μm. Moreover, the coalescence-jumping of droplets was followed by a new cycle of droplet condensation in an aligned pattern in an emptied area. These findings offer guidelines for designing enhanced superhydrophobic surfaces for water and energy applications.

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Figures

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Fig. 1

Schematic of the ESEM condensation experiment setup. A custom-made copper stub with a superhydrophobic sample was mounted on a tiltable Peltier cooling stage to conduct water condensation experiment in an environmental scanning electron microscope: 1—tiltable Peltier cooling stage, 2—custom-made copper stub, 3—superhydrophobic sample, 4—cooling water inlet or outlet, and 5—RS-232 cable for temperature control.

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Fig. 2

Surface morphology of the copper oxide nanostructures and wettability of the superhydrophobic surface after silane coating. (a) SEM image of the copper oxide nanostructures with sharp tips, flat side walls, and cavities formed between them. (b) Tilted ESEM image of microscopic condensate droplets on the superhydrophobic surface. (c) Apparent contact angles of the microscopic droplets measured from the ESEM image. It was obtained by fitting a circle to the droplets and measuring the droplet base. Droplets only in the range of 5–40 μm and in the focused region were measured in order to obtain higher accuracy. The average apparent contact angle was 160.9 ± 3.9 deg, slightly smaller than 163.1 ± 3.0 deg measured from macroscopic droplets with a goniometer (given in the inset). The error bars were determined from the propagation of error associated with the tilt angle and the measurement uncertainty of droplet dimensions due to the resolution limit of the ESEM image.

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Fig. 3

Time-lapse, tilted ESEM images of condensation showing the effect of surface wettability on droplet nucleation. A locally less hydrophobic site on a largely superhydrophobic surface was generated by pre-exposing the highlighted site to a focused electron beam at high magnification prior to condensation. The beam damaged the hydrophobic coating, thus increasing the surface energy and wettability of the focused site. As a result, droplets more easily nucleated and grew faster on this site. Conditions: sample surface temperature T ≈ 2.0 °C and vapor pressure P = 750.0 Pa, which corresponds to a supersaturation S ≈ 1.06.

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Fig. 4

The effect of nanostructure density on droplet nucleation and growth. (a) SEM image of the chemically etched surface with nonuniform nanostructure density. The inset is the SEM image of an unpolished bare copper plate surface with linear scratch lines used for chemical etching. (b) Magnified view of a linear region with denser nanostructures. (c) Tilted ESEM image of the early stage condensation on the superhydrophobic surface with nonuniform nanostructure density. Droplets aligned along several regions pointed by the arrows. (d) Number of droplets in 20 regions sectioned with an equal width of 11.65 μm along the horizontal width of the ESEM image in (c). The number of aligned droplets in the three pointed regions was larger than that of the droplets in the neighboring regions. (e) Mean droplet volume of droplets in each region. It was calculated by estimating the volume of each droplet in a region through curve-fitting with ImageJ and then dividing the total volume by the number of droplets in (d). The average volume of droplets in the three pointed regions was larger than surrounding regions due to higher droplet growth rate. The error bars were determined from the propagation of errors associated with the tilt angle of the stage, the resolution limit of the ESEM image and the uncertainty in the number of the droplets.

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Fig. 5

The nucleation model of water droplets on the copper-oxide nanostructures. (a) Tilted SEM image of the surface nanostructures with sharp tips, flat side walls, and cavities formed between them. The dimensions of the nanostructures were comparable with the calculated size of the droplet nucleus (r*≈20 nm). (b) Three types of nucleation site marked with a shape angle β: nucleation in a conical cavity formed by the nanostructures, nucleation on a flat side wall surface of a nanostructure, and nucleation on a conical tip of a nanostructure. θ is the intrinsic contact angle of the nanostructures. On the silane-coated surface in this work, θ = 115 deg.

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Fig. 6

The effect of intrinsic wettability and geometry of the nucleation site on the normalized energy barrier of droplet nucleation. (a) The effect of intrinsic contact angle θ. The dotted line (β = 180 deg) represents nucleation on the flat side walls of nanostructures. The solid lines represent nucleation in cavities with different shape angles, while the dashed lines represent nucleation on tips with different shape angles. The solid vertical line represents the hydrophobic silane coating with an intrinsic contact angle of 115 deg. (b) The effect of nucleation site geometry with the shape angle β, for an intrinsic contact angle of 115 deg. (c) The effect of nucleation site geometry on the normalized area of liquid–vapor and liquid–solid interfaces (dotted and dashed curves, respectively) and on the normalized total work that needs to be done on the total interfacial area (solid curve). The interfacial areas are normalized by 4πr*2, and the total work is normalized by the energy barrier of homogeneous nucleation (ΔGHo*=(4πσlvr*2/3)). Upon nucleation, the liquid–vapor and liquid–solid interfaces are created with external energy (positive work) while the solid–vapor interface is displaced by the liquid and releases energy (negative work, thus not plotted). When the nucleation occurs in extremely confined cavities (significantly small β), the solid–liquid interfacial area is large, increasing the total work. Similarly, when the nucleation occurs on extremely sharp tips (significantly large β), the liquid–vapor interfacial area is large, thus increasing the total work.

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Fig. 7

Droplet morphology after nucleation. (a) Tilted ESEM images of condensate droplets. Droplets smaller than 5 μm had a balloonlike shape (Wenzel state) because their base was trapped by the surface nanostructures. Droplets larger than 10 μm had a full spherical shape, implying a Cassie state due to the dewetting transition of their base as the droplets became larger. (a) Droplet height H and droplet diameter 2r measured from multiple droplets in a single ESEM image. The slope of the curve is 0.95, which is approximately equal to (1 − cos θapp)/2 ≈ 0.97. (b) The aspect ratio of the droplet height H to the droplet diameter 2r in (b). Several observable droplets of around 5 μm size had an aspect ratio larger than (1 − cos θapp)/2, implying an elongated, balloonlike shape in Wenzel state as shown in (a). The aspect ratio of droplets larger than 10 μm was almost a constant equal to (1 − cos θapp)/2. Although not observable, droplets less than 2 μm were considered to have an increasing aspect ratio [20]. The dotted, dashed, and solid sections of the line illustrate the three phases, numbered as ①, ②, and ③, of the aspect ratio evolution. (c) Diagram of the aspect ratio evolution and the ESEM droplet morphology corresponding to phases 2 and 3. The observation of phase 1 was challenging due to the resolution limit of ESEM.

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Fig. 8

Droplet removal and a new cycle of droplet nucleation. (a) Time-lapse ESEM images of jumping droplets upon coalescence. Droplet jumping was followed by a new cycle of nucleation. The dashed ovals highlight the droplets prior to coalescence jumping and emptied spots after jumping. The dashed rectangle highlights the newly formed droplets. The dewetting transition of the droplet base was crucial for coalescence jumping enforced by the excess surface free energy released upon coalescence. (b) Illustration of the droplet nucleation, growth, partial depinning, and coalescence jumping processes.

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Fig. 9

Droplet nucleation model

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