OPEN Immersion Condensation on Oil-Infused

SUBJECT AREAS:
Heterogeneous Surfaces for Enhanced
NANOSCIENCE AND
TECHNOLOGY Heat Transfer
ENGINEERING
Rong Xiao1, Nenad Miljkovic1, Ryan Enright1,2* & Evelyn N. Wang1
NANOSCALE MATERIALS
NANOSCALE DEVICES
1
Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA, 2Stokes Institute,
University of Limerick, Limerick, Ireland.
Received
13 March 2013
Enhancing condensation heat transfer is important for broad applications from power generation to water
Accepted harvesting systems. Significant efforts have focused on easy removal of the condensate, yet the other desired
30 May 2013 properties of low contact angles and high nucleation densities for high heat transfer performance have been
typically neglected. In this work, we demonstrate immersion condensation on oil-infused micro and
Published nanostructured surfaces with heterogeneous coatings, where water droplets nucleate immersed within the
13 June 2013 oil. The combination of surface energy heterogeneity, reduced oil-water interfacial energy, and surface
structuring enabled drastically increased nucleation densities while maintaining easy condensate removal
and low contact angles. Accordingly, on oil-infused heterogeneous nanostructured copper oxide surfaces,
Correspondence and
we demonstrated approximately 100% increase in heat transfer coefficient compared to state-of-the-art
dropwise condensation surfaces in the presence of non-condensable gases. This work offers a distinct
requests for materials approach utilizing surface chemistry and structuring together with liquid-infusion for enhanced
should be addressed to condensation heat transfer.
E.N.W. (enwang@mit.
edu)

C
ondensation is an essential process in a wide variety of industrial applications including building envir-
onment1, power generation2, and water harvesting systems3. Enhancing condensation heat transfer has the
* Current address: potential to significantly improve efficiency and reduce the cost of these applications. In practice, filmwise
Thermal Management condensation, where a thin liquid film covers the surface, is the most prevalent condensation mode due to the high
Research Group,
wettability of common heat transfer materials. In this condensation mode, the heat transfer coefficient is limited
by the thermal resistance associated with the condensate film which insulates the surface4. Accordingly, efforts
Efficient Energy
spanning eight decades have been devoted to the realization of non-wetting surfaces for dropwise condensation
Transfer (gET) Dept., where shedding droplets clear the surface for droplet re-nucleation/re-growth, leading to enhanced heat transfer
Bell Labs Ireland, rates5–10. One order of magnitude higher heat transfer coefficients compared to filmwise condensation have been
Alcatel-Lucent Ireland reported using dropwise condensation in pure vapor environments6,7,10,11. In order to maximize the heat transfer
Ltd., Blanchardstown coefficient, a high performance dropwise condensation surface should simultaneously achieve three properties:
Business & Technology low contact angle hysteresis to minimize droplet departure radii, low contact angle to reduce the conduction
Park, Snugborough Rd,
resistance of the droplet, and high nucleation density12, as shown in Fig. 1 (see Supporting Information for
detailed model derivation). Recently, investigations have focused on understanding how chemically modified
Dublin 15, Ireland.
micro/nanostructured surfaces can achieve superhydrophobicity to allow droplets to form in a stable Cassie
wetting state13, which further improves droplet mobility and reduces the departure radii (Fig. 1a)9,14,15. In certain
cases, these surfaces enable surface-tension-driven droplet jumping at micron length scales8,12,16. However, this
focus on increasing the apparent hydrophobicity to reduce droplet departure radii does not necessarily address
the other two aspects influencing condensation heat transfer rates. The high apparent contact angles of condens-
ing droplets on superhydrophobic surfaces lead to an increase in the conduction resistance through the drop-
let12,17, hindering the overall heat transfer performance (Fig. 1b). Moreover, the Cassie wetting state introduces a
vapor layer beneath the condensate droplet, which significantly increases the thermal resistance12. In addition,
hydrophobic surface chemistry increases the nucleation thermodynamic energy barrier, thus reducing the nuc-
leation density and limiting the heat transfer coefficient (Fig. 1c)18. Hydrophobic structured surfaces with well-
defined hydrophilic sites on the roughness features have also been explored to control the nucleation density19,
but strong droplet adhesion on such surfaces is likely to limit their applicability for condensation heat transfer
enhancement. More recently, composite surfaces have been proposed whereby hydrophobic structured surfaces
were infused with oil to simultaneously achieve easy droplet removal and low contact angles20–22. During

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considered. Furthermore, experimentally obtained heat transfer

enhancements with such surfaces have not been reported.
In this work, we demonstrate immersion condensation, a new
approach to enhance condensation heat transfer by introducing het-
erogeneous surface chemistry composed of discrete hydrophilic
domains on a hydrophobic background in oil-infused micro and
nanostructured surfaces. This approach allows water droplets to
nucleate immersed within the oil to achieve high nucleation densities
while maintaining easy droplet removal and low contact angles
(Fig. 2a & b). In contrast to the same surface not infused with oil,
nucleation densities were one order of magnitude larger due to the
combined effect of the high-surface-energy sites and the reduced oil-
water interfacial energy which, together, lower the thermodynamic
energy barrier for stable nuclei formation. Meanwhile, the contact
angle hysteresis was as low as 3u and the droplet apparent contact
angle was <110u. We demonstrate that the immersion of droplets in
the presence of the heterogeneous coating is essential to the high
water nucleation densities and significant heat transfer enhance-
ments. We first characterized the heterogeneous coating on flat sil-
icon surfaces using an Atomic Force Microscope (AFM). The scans
showed the presence of discrete high-surface-energy sites on a low-
surface-energy background. Well-defined micropillar arrays were
subsequently coated and then infused with oil to study the physics
of condensation behavior. Finally, with our increased understanding
of the phenomenon, we experimentally demonstrated heat transfer
enhancements of approximately 100% with oil-infused, heteroge-
neously coated copper oxide nanostructured surfaces in comparison
with state-of-the-art dropwise condensing surfaces, which suggests
the practicality of our approach. This work promises the develop-
ment of a scalable strategy for highly efficient condensation heat
transfer for industrial, building energy, electronics cooling, and
water-harvesting applications.

Results
Surface heterogeneity by self-assembled coatings. We deposited a
self-assembled coating (SAC) of (tridecafluoro-1,1,2,2-tetrahydrooctyl)
-1-trichlorosilane (TFTS) from the vapor phase (See Methods for the
deposition process). The SAC coating method is capable of forming
heterogeneity by agglomeration23. We chose the SAC method due to its
simplicity and scalability, but alternative methods are also available to
generate heterogeneity at the appropriate length scale, e.g., block
copolymer or nano-imprinting24,25. Height and phase AFM images of
the TFTS coating on a smooth silicon surface were obtained and are
shown in Fig. 2c & d, respectively, where the white spots are the
nanoscale agglomerates of TFTS (<200–500 nm in diameter). The
phase angle of the agglomerates was significantly higher than that of
the background, indicating that the agglomerates have higher surface
energy26. We determined the local contact angle of water on the high-
surface-energy agglomerates to be 60u 6 1.5u by measuring the
advancing and receding contact angle of a water droplet on the
smooth, coated surface in air (ha/hr 5 122u 6 1.3u/78u 6 1.3u) and
interpreting the data using a modified Cassie-Baxter model that
incorporates the effect of local contact line deformation27.
Figure 1 | Parameters affecting condensation heat transfer coefficient.
Model results showing influence of: (a) departure radius with advancing
Immersion condensation on silicon micropillars. Next, we depo-
contact angle ha 5 110u and nucleation density N 5 1010 m22,
sited the SAC on silicon micropillar arrays to fundamentally investi-
(b) advancing contact angle with nucleation density N 5 1010 m22 and
gate nucleation behavior on oil-infused surfaces. We fabricated
departure radius Rmax 5 800 mm, and (c) nucleation density with
silicon micropillar arrays with diameters, d, ranging from 0.4–
ha 5 110u and Rmax 5 800 mm. The results assume a vapor pressure of
5 mm, periods, l, ranging from 4–25 mm, and heights, h, ranging
2700 Pa and surface temperature of 20uC.
from 10–25 mm using contact lithography and deep reactive ion
etching (DRIE) processes. The geometries were chosen to satisfy
condensation, two-tier surface roughness was shown to enhance the the imbibition condition to enable oil spreading28 and to stabilize
removal of droplets suspended on top of the infused oil layer21. While the oil film20. The pillar surfaces were subsequently functionalized
these works showed significant potential for enhanced condensation with the TFTS SAC, and infused with a fluorinated oil, Krytox GPL
surfaces, achieving high nucleation densities has not previously been 100. The low surface tension of Krytox oil (<17–19 mN/m) allowed

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Figure 2 | Mechanism of immersion condensation. (a) Schematic showing water vapor diffusing through the thin oil film and forming immersed
droplets on the tips of micropillars. (b) Magnified schematic showing the nuclei formation on high-surface-energy sites on micropillar tips in the oil.
(c) and (d) Height and phase images of atomic force microscope (AFM) images of TFTS ((Tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane)
coating. The higher phase angle at the nanoagglomerates indicates local higher surface energy. (e) and (f) Environmental scanning electron microscope
(ESEM) images of TFTS-coated micropillar arrays before and after the oil-infusion. (g) and (h) Contact angle hysteresis on a superhydrophobic surface
without and with oil-infusion. The hysteresis is <3u on the oil-infused surface with a contact angle <110u. The microstructure geometries were the same
on both surfaces, with diameter of 5 mm, height of 20 mm, and period of 15 mm. (i) and (j) White-light optical microscope images of condensation on
micropillar arrays before and after oil-infusion. The micropillar geometries were the same as (g) and (h). The supersaturation in the experiments was S 5
1.6. (k) Nucleation rates predicted as a function of contact angle and interfacial energy.

it to spread on the surface and form a stable film via capillarity. A dry The increase in nucleation density on the oil-infused TFTS sur-
N2 stream was used to assist spreading and remove excess oil. Typical faces was achieved via the combination of the high-surface-energy
environmental scanning electron microscope (ESEM) images of the sites and reduced water-oil interfacial energy. Based on classical
coated pillar arrays without and with oil-infusion are shown in Fig. 2e nucleation theory, the nucleation rate can be determined as a func-
& f, respectively. On these TFTS-coated pillar arrays, the advancing tion of the contact angle and the surface energy of the condensate
and receding contact angles without oil-infusion were ha/hr 5 139u at a given supersaturation, as shown in Fig. 2k (see Supporting
6 3u/128u 6 3u, whereas those with oil-infusion were ha/hr 5 110u 6 Information for the detailed derivation)18,30–32. On the oil-infused
2u/107u 6 2u (Fig. 2g & h). Such low contact angle hysteresis is a key surface, the tips of the pillars were covered by oil due to its low surface
attribute for allowing droplets to be removed with a small departure tension. However, the tips were still visible in the ESEM images
radius under gravity during condensation20,21,29. Figures 2i & j show (Fig. 2f) because of the small thickness of the oil film. In these regions,
white light optical microscope images comparing the drastic the water vapor is able to diffuse through the thin oil layer and form
difference in nucleation density during condensation without and nuclei immersed in the oil layer on the high-surface-energy sites. The
with oil-infusion on the TFTS-coated micropillar arrays, respectively critical sizes of nuclei (,10 nm) were much smaller than the sizes of
(see Methods for the experimental procedure). Under the prescribed the high-surface-energy sites (<200–500 nm) so that the local con-
supersaturation of S 5 1.6 (S 5 pv/pw where pv is the water vapor tact angles of the nuclei are only determined by the high-surface-
pressure and pw is the water saturation pressure associated with the energy sites. With the introduction of oil, the local contact angle of
surface temperature), nucleation was rarely observed on the surface nuclei on those high-surface-energy domains can be bounded in the
without oil-infusion (nucleation density N < (4 6 2) 3 108 m22) range from 43u to 67u using Young’s equation (see Supporting
(Fig. 2i), but was observed on every tip of the pillars after oil-infusion Information). As a result, the energy threshold for nucleation was
(nucleation density N < (4.4 6 0.2) 3 109 m22) (Fig. 2j). Nucleation significantly decreased due the low local contact angle, in combina-
in the space between the pillars was not observed due to the large tion with the reduced interfacial energy between water and oil
thickness of oil coverage that limits water vapor diffusion to the SAC. (<49 mJ/m2)21 compared to that between water and vapor
Meanwhile, nucleation on the oil/vapor interface did not occur due (<72 mJ/m2). Accordingly, as shown in Fig. 2k, assuming a local
to the low interfacial energy. contact angle lower than 67u, the predicted nucleation rate increases

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from 0.2 m22s21 to greater than 1014 m22s21 due to the encapsulating Immersion condensation on scalable copper oxide nanostruc-
oil phase in comparison with the same surface without oil-infusion. tures. Next, we studied the overall heat transfer performance of an
The oil encapsulation is essential in reducing the energy barrier for immersion condensation surface. While studies on well-defined
nuclei formation and enhancing nucleation density, which is distinct silicon micropillar arrays can provide physical insight into
from previous work where the encapsulating oil phase was consid- immersion condensation behavior, they are not practical due to
ered as unfavorable for condensation21. The calculated nucleation cost and challenges in interfacing the silicon substrate and our heat
rate allows the nucleation density to be orders of magnitude larger transfer measurement apparatus with minimum uncertainties.
than the density of the high-surface energy domains. As a result, Therefore, we performed immersion condensation heat transfer
multiple nuclei could form on each tip of the pillars where the oil measurements on oil-infused copper oxide (CuO) nanostructures
layer is thin enough for effective vapor diffusion. However, due to the functionalized with TFTS, which promises a scalable, low cost
resolution limits of our imaging experiments, only a single droplet platform for condensation surfaces33. ESEM images of
was apparent on each pillar tip. Therefore, we only determined an representative copper oxide nanostructures without and with
order of magnitude increase in the observed nucleation density, Krytox oil-infusion are shown in Fig. 3a & b, respectively.
which was equal to the density of the pillars (Fig. 2j). We also per- Condensation experiments were performed on the CuO surfaces
formed control condensation experiments on oil-infused micropillar without and with oil-infusion in an environmental SEM with 1 ,
arrays with dimethyldicholorosilane (DMCS), which is a homogen- S , 1.29 for visualization (see Methods for detailed imaging
eous hydrophobic coating with advancing and receding contact process)12. The Figures 3c & d show an order of magnitude
angles of ha/hr 5 103.8u 6 0.5u/102.7u 6 0.4u (see Supporting increase in nucleation density on the oil-infused surface, as
Information for details of the control experiments). We found no similarly observed on the silicon-based microstructures. To
observable change in nucleation density after oil-infusion on the capture the condensation heat transfer behavior, we formed the
DMCS coated surfaces, as predicted by theory (Fig. 2k). These results oil-infused heterogeneous CuO surfaces on copper tubes (see
further support the idea that a high performance condensation sur- Methods for detailed fabrication process). Figures 3e & f show
face can be achieved through the combination of local high-surface- condensation on a typical dropwise hydrophobic surface and an
energy sites and oil-infusion, which has not been demonstrated prev- oil-infused heterogeneous immersion condensation surface,
iously. However, the overall surface needs to be hydrophobic to respectively. Significantly higher droplet densities were observed
prevent the spreading of the condensate beneath the oil film and on the oil-infused surface. Meanwhile, the average shedding radius
of droplets was reduced from RbDHP 5 1.83 6 0.31 mm on the typical
maintain easy droplet removal. Otherwise, the condensate would
dropwise hydrophobic surfaces to R bIC 5 0.98 6 0.13 mm on the
wet the substrate, disrupting the oil film and resulting in droplet
pinning. immersion condensation surfaces (see Supporting Information for
details on determining the droplet shedding radii). Prior to droplet
departure, the droplets grew orders of magnitude larger than the
characteristic length scale of the nanostructures, thus high
apparent contact angles of the droplet (<110u) were observed,
consistent with the low surface energy of the solid-oil composite
surface.
Overall heat transfer coefficients were measured to evaluate the
performance on three different Cu-based surfaces: a hydrophobic
surface for typical dropwise condensation, a superhydrophobic
TFTS-coated CuO surface, and a Krytox oil-infused, TFTS-coated
CuO surface (Fig. 4) (see Methods and Supporting Information for
detailed experimental process). The Krytox GPL 100 oil evaporates
completely when the test chamber is evacuated to pressures lower

Figure 3 | Scalable copper oxide (CuO) surfaces for immersion

condensation. (a) Field emission SEM (FESEM) image of CuO
nanostructures. (b) Environmental SEM (ESEM) image of CuO
nanostructures infused with Krytox oil. (c) ESEM image of nucleation on Figure 4 | Experimental immersion condensation heat transfer
TFTS-coated CuO surface. (d) ESEM image of oil-infused TFTS-coated measurement. Comparison of overall heat transfer coefficient during
CuO surface. An order of magnitude higher nucleation density was condensation on the hydrophobic surface, TFTS-coated
observed compared to (c). (e) Image of dropwise condensation on a superhydrophobic surface, and oil-infused composite surface with an
hydrophobic copper tube surface. (f) Image of condensation on an oil- initial chamber pressure of 30 Pa (primarily non-condensable gases). The
infused TFTS-coated CuO surface. Significantly higher droplet density was supersaturation was varied in the range 1 , S , 1.6. The heat transfer
observed on the oil-infused surface while a low departure radius of coefficient on the oil-infused surface increased by approximately 100%
0.98 6 0.13 mm was maintained. compared to the dropwise and superhydrophobic surfaces.

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than 1 Pa. Therefore, we set the initial chamber pressure as high as Methods
30 Pa (primarily composed of non-condensable gases, NCG) to Surface fabrication. The silicon micropillar arrays were fabricated using contact
lithography followed by deep reactive ion etching. For copper oxide surfaces, we used
avoid the evaporation of oil with steam pressures ranging from 2
commercially available oxygen-free Cu tubes (99.9% purity) with outer diameters,
to 3 kPa (1 , S , 1.6) in the experiments. This is consistent with DOD 5 6.35 mm, inner diameters, DID 5 3.56 mm, and lengths L 5 131 mm as the
actual condenser systems where NCG partial pressures are typically test samples for the experiments. Each Cu tube was cleaned in an ultrasonic bath with
found in the range of 30 Pa and significantly affect the condensation acetone for 10 minutes and rinsed with ethanol, isopropyl alcohol and de-ionized
(DI) water. The tubes were then dipped into a 2.0 M hydrochloric acid solution for 10
heat transfer performance34–37. Accordingly, with these experimental minutes to remove the native oxide film on the surface, then triple-rinsed with DI
conditions, we were able to emulate a more realistic condensation water, and dried with clean nitrogen gas.
environment and demonstrate the practical significance of the Nanostructured CuO films were formed by immersing the cleaned tubes into a hot
immersion condensation mode. While the superhydrophobic sur- (96 6 3uC) alkaline solution composed of NaClO2, NaOH, Na3PO4?12H2O, and DI
water (3.75555105100 wt.%)33,39. During the oxidation process, a thin (,200 nm)
face is more hydrophobic than the typical dropwise hydrophobic Cu2O layer was formed that then re-oxidized to form sharp, knife-like CuO structures
surface, flooding and strong pinning of the condensate was observed with heights of h < 1 mm, solid fraction39 Q < 0.023 and roughness factor39 r < 10. To
due to the high supersaturation conditions (S as high as 1.6), leading verify the independence of oxide thickness on chemical oxidation time40, four sepa-
to similar heat transfer coefficients with the typical dropwise hydro- rate samples were made using oxidation times, t 5 5, 10, 20, and 45 minutes. The
sharp CuO structures were then coated with a silane SAC to create SHP surfaces.
pohobic surfaces. Note that these results are distinct from previous In addition to SHP surfaces, cleaned copper tubes were also immersed into
literature where jumping of droplets on superhydrophobic surfaces hydrogen peroxide solutions at room temperature to form a thin smooth layer of
increased heat transfer coefficients at lower saturation conditions (S Cu2O. The smooth surfaces were also coated with TFTS to achieve typical hydro-
, 1.12)16,38. In addition, the overall heat transfer coefficients on DHP phobic surfaces for dropwise condensation (DHP).
surfaces in this work (h < 2–7 kW/m2K) are much lower compared Surface coating deposition. The self-assembled coatings (SAC) were formed using a
to pure vapor conditions (h < 12–13 kW/m2K)16 due to the presence vapor deposition process. First, the silicon surfaces were cleaned using a Piranha
of NCGs acting as a diffusion barrier to the transport of water vapor solution (H2O2:H2SO4 5 153) to remove possible organic contamination and to
towards the condensing surface. In comparison to the typical hydro- create a large number of –OH bonds on the surface, which enables the bonding
between silane molecules and the silicon surface. For the copper oxide surfaces, the
phobic surfaces, the Krytox oil-infused TFTS-coated CuO surface surfaces were cleaned by intensive plasma (<1 hr). The samples were then placed in a
demonstrated approximately a 100% improvement in heat transfer desiccator (Cole-Palmer) together with a small petri dish containing <1 mL of the
coefficient over the entire range of supersaturations tested (1 , S , silane liquid. The desiccator was pumped down to <10 kPa. The pump was then shut
1.6) with the existence of NCGs. While the available condensation off and the valve was closed so that the silane liquid could evaporate in the low-
pressure environment of the desiccator and attach to the surfaces to form the SAC via
area was reduced due to the oil coverage, the significant improve- the following reaction,
ment in the overall heat transfer coefficient highlights the collective Si{OHzR{Si{Cl?Si{O{Si{RzHCl:
role of enhanced nucleation density, more frequent droplet removal,
During the self-assembly process, the silane molecules form nanoscale agglomer-
and lower droplet contact angle (Fig. 1). ates with diameters of <200–500 nm shown in Figure 2c and d, as reported prev-
iously23. After 30 minutes of reaction, the desiccator was vented and the samples were
rinsed using de-ionized (DI) water. Such vapor deposition process was used for both
Discussions TFTS and Dimethyldicholorosilane (DMCS) coatings, but in dedicated desiccators to
Oil-impregnated surfaces have been recently reported as a promising avoid cross-contamination of the different silane molecules.
approach to enhance condensation heat transfer surfaces due to the
ultra-low droplet adhesion20–22. However, easy droplet removal is not Surface characterization. Advancing and receding contact angles for all samples
were measured and analyzed using a micro-goniometer (MCA-3, Kyowa Interface
the only desired property for high heat transfer performance. Low Science Co., Japan). Field emission electron microscopy was performed on a Zeiss
contact angle and high nucleation densities are also essential to fur- Ultra Plus FESEM (Carl Zeiss GMBH) at an imaging voltage of 3 kV.
ther enhance condensation heat transfer. In this work, we have
demonstrated that by combining surface heterogeneity and oil-infu- OM imaging procedure. The samples were horizontally mounted on a thermal stage
inside an enclosure and cooled to Tw 5 283.1 6 0.1 K in a dry nitrogen atmosphere.
sion, the nucleation density in condensation can be increased by over Following thermal equilibration (<5 minutes), nucleation was initiated by flowing
an order of magnitude via immersion condensation while maintain- water-saturated nitrogen into the enclosure. The humidity of the gas flow was
ing low droplet adhesion, which has not been observed previously. measured using a humidity probe located 1 cm above the sample to determine the
The increase in nucleation densities via the combined effect of het- supersaturation, S, defined as the ratio of the vapor pressure to the saturation pressure
at the stage temperature (S 5 pv/pw). Typical values of supersaturation were S < 1.6.
erogeneity and the reduced oil-water interfacial tension was The nucleation density and subsequent growth behavior was recorded at a frame rate
explained by our model based on classical nucleation theory, and of 10 frames per second using a high speed camera (Phantom V7.1, Vision Research)
was also corroborated with control experiments using silane-coated attached to the optical microscope. The observable nucleation density during each
silicon micropillar arrays. With improved understanding of the experiment was determined by counting the number of nuclei in the captured images
and dividing the number of nuclei by the imaging area. Multiple experiments were
physics, we investigated oil-infused superhydrophobic copper oxide performed to determine the average nucleation densities on the different surfaces.
surfaces as a platform for condensation enhancement in practical
systems. We demonstrated that the condensation heat transfer coef- ESEM imaging procedure. Condensation nucleation and growth were studied on
ficient on such oil-infused heterogeneous surfaces can be enhanced these fabricated surfaces using an environmental scanning electron microscope (EVO
55 ESEM, Carl Zeiss GMBH). Backscatter detection mode was used with a high gain.
by approximately 100% compared to state-of-the-art dropwise sur- The water vapor pressure in the ESEM chamber was 800 6 80 Pa. Typical image
faces in the presence of non-condensables gases. Based on previous capture was obtained with a beam potential of 20 kV and variable probe current
condensation heat transfer models shown in Fig. 1, an order of mag- depending on the stage inclination angle. To limit droplet heating effects, probe
nitude increase in nucleation density could contribute to approxi- currents were maintained below 2.0 nA and the view area was kept above 400 mm 3
300 mm41. A 500 mm lower aperture was used in series with a 100 mm variable
mately 80% increase in the overall heat transfer coefficient. pressure upper aperture to obtain greater detail. The sample temperature was initially
Meanwhile, the low departure radii and low contact angle also set to 4 6 1.5uC and was allowed to equilibrate for 5 minutes. The surface temperature
assisted in the total improvement. Achieving the three key aspects was subsequently decreased to 3 6 1.5uC, resulting in nucleation of water droplets on
of condensation simultaneously is essential to realize heat transfer the sample surface. Accordingly, the supersaturation, S, during the imaging process
was in the range of 1 , S , 1.29. Images and recordings were obtained at an
enhancement by as high as 100%. Further work is needed to tailor oil inclination angle of 45u from the horizontal to observe droplet growth. Cu tape was
and coating properties, as well as surface geometry to minimize oil used for mounting the sample to the cold stage to ensure good thermal contact.
loss during operation and maximize condensing surface area. With
continued development, immersion condensation promises to be an Heat transfer measurements. The test samples, 6.35 mm diameter tubes with
different surface treatments, were placed in an environmental chamber (Kurt J.
important condensation mode for a variety of heat transfer and Lesker) for the heat transfer measurements. A water reservoir, which was connected
resource conserving applications. to the chamber via a vapor valve, was heated to .95uC to produce steam. The vapor

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Author contributions
R.X., R.E., N.M. and E.N.W. conceived the initial idea of this research. E.N.W. guided the
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Condensation on Lubricant Impregnated Nanotextured Surfaces. Acs Nano 6, work. R.X. and N.M. carried out the experiments and collected data. R.X., R.E. and N.M.
10122–10129 (2012). analyzed the data. N.M. carried out the theoretical analysis. R.X., N.M., R.E. and E.N.W.
22. Mishchenko, L. et al. Design of Ice-free Nanostructured Surfaces Based on were responsible for writing the paper.
Repulsion of Impacting Water Droplets. Acs Nano 4, 7699–7707 (2010).
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Self-Assembled Monolayers. Langmuir 16, 7742–7751 (2000). Supplementary information accompanies this paper at http://www.nature.com/
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Copolymer Lithography: Periodic Arrays of ,1011 Holes in 1 Square Centimeter.
Science 276, 1401–1404 (1997). Competing financial interests: The authors declare no competing financial interests.
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Channels by Nanoimprinting and Its Application for DNA Stretching. Nano Lett Condensation on Oil-Infused Heterogeneous Surfaces for Enhanced Heat Transfer.
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27. Raj, R., Enright, R., Zhu, Y., Adera, S. & Wang, E. N. Unified Model for Contact
NonCommercial-NoDerivs Works 3.0 Unported license. To view a copy of this
Angle Hysteresis on Heterogeneous and Superhydrophobic Surfaces. Langmuir
license, visit http://creativecommons.org/licenses/by-nc-nd/3.0
28, 15777–15788 (2012).

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