8/6/2019 2011 MEMS Betz Boiling SHPiSHPo Pattern
http://slidepdf.com/reader/full/2011-mems-betz-boiling-shpishpo-pattern 1/4
SIGNIFICANT BOILING ENHANCEMENT WITH SURFACES COMBINING
SUPERHYDROPHILIC AND SUPERHYDROPHOBIC PATTERNS
Amy Rachel Betz 1 , James R. Jenkins2 , Chang-Jin “CJ” Kim2 and Daniel Attinger 1 1Columbia University, New York, NY, USA
2University of California, Los Angles (UCLA), CA, USA
ABSTRACT
In this work we describe the manufacturing and
characterization of patterned surfaces with large spatial
contrast in wettability. We find drastic enhancement of pool
boiling performance in water. In comparison to a
hydrophilic SiO2 surface with a wetting angle of 7º, surfaces
combining superhydrophilic and superhydrophobic patterns
can quadruple the heat transfer coefficient (HTC).
Superhydrophilic surface with hydrophobic islands can
increase the critical heat flux (CHF) by 80%. This
performance enhancement is important for applications such
as electronics cooling, because the increased HTC allows a
greater amount of heat to be removed at a lower wall
superheat.
INTRODUCTION
Boiling is an efficient process to transfer large amounts
of heat at a prescribed temperature because of the large
latent heat of vaporization. The term flow boiling describes
the boiling of liquids forced to move along hot surfaces,
while in pool boiling, the topic handled in this paper, the
liquid is stagnant and in contact with a hot solid surface [1].
Pool boiling performance is measured with two parameters:
the heat transfer coefficient (HTC) and the critical heat flux
(CHF). The CHF is measured by increasing the surface
temperature until a transition from high HTC to very low
HTC occurs, which signifies the formation of a vapor filminsulating the liquid from the heated surface, a phenomenon
called dry out.
As of today, the performance of boiling surfaces has
been increased by using wicking structures to prevent dry
out [2], by increasing the surface area with fins or fluidized
bed [2-5], and by enhancing the wettability of the surface
[4-8]. The latter objective is justified by experiments of
Wang and Dhir [9], showing that the critical heat flux was
increased by enhancing surface wettability. Wettability can
be enhanced by either increasing the surface roughness or
with microstructure or nanostructure coatings. For instance,
Jones et al. [10] have shown that a well-chosen roughness
can double or triple the heat transfer coefficient. Significant
heat transfer enhancement has also been obtained withsurfaces coated with a µm-thick carpet of nanometer
diameter wires (nanowires) [4-6]. The CHF enhancement
was attributed to coupled effects such as the multi-scale
geometry [4, 6] and the superhydrophilicity of the nanowire
arrays [5, 6].
In previous work [11] we took advantage of
microlithography techniques to design surfaces combining
hydrophobic and hydrophilic zones for pool boiling
experiments. We showed that a hydrophilic network with
hydrophobic islands can increase CHF by 65% and HTC by
100% compared to a hydrophilic wafer with a wetting angle
of 7º. We also found that increasing the wettability contrast
increased the CHF. In [11] we varied the size of the patterns
as well as the connectivity of the hydrophobic and
hydrophilic patterns. Hydrophilic surfaces with hydrophobic
islands were called hydrophilic networks meaning that any
two hydrophilic regions could be joined without passing
over a hydrophobic zone. Hydrophobic surfaces with
hydrophilic islands were called hydrophobic networks. We
found that hydrophilic networks increased both the HTC
and CHF while hydrophobic networks only increased the
HTC at low values of superheat. In the present work we
focus on these promising hydrophilic networks, further exploring the effects of wettability contrast using nano-
fabrication to create superhydrophilic networks with
superhydrophobic islands. Our intuition is that the
superhydrophilic surfaces might improve rewetting [12] and
the superhydrophobic islands might improve nucleation
[13]. We characterize the pool boiling performance of these
surfaces and compare it to state-of-the-art enhanced
surfaces.
DESIGN AND MANUFACTURING
We designed our superhydrophilic surfaces with
superhydrophobic patterns on the basis of our recent work
[11] where micromanufactured surfaces with spatial
contrasts of wettability exhibited increased pool boiling
performance. All the surfaces investigated in the present
work are superhydrophilic with either hydrophobic islands
or superhydrophobic islands. We varied the size of the
superhydrophobic patterns based on the range of active
nucleation site sizes calculated by the theory in [14].
The manufacturing process is shown in figure 1. The
test surfaces were made on a double-sided polished and
oxidized 500 µm thick silicon wafer. On the back side we
deposited thin film resistive heaters. The heaters were made
from sputtered Indium Tin Oxide (ITO) approximately 300
nm thick. The target resistance for the ITO heaters was 50 Ω
per square. Copper electrodes were thermally evaporatedover the ITO, leaving 1 cm2 of ITO exposed. The copper
electrodes were 1 µm thick to minimize their resistance and
therefore the power loss in the system. A 100 nm layer of
SiO2 was deposited to electrically passivate the heater. On
the top surface of the wafer, the silicon dioxide was first
removed from the top side only using CF4 gas in a reactive-
ion etching (RIE) machine. Next, a random array of silicon
nanostructures (figure 2a) was formed using the black
978-1-4244-9633-4/11/$26.00 ©2011 IEEE 1193 MEMS 2011, Cancun, MEXICO, January 23-27, 2011
8/6/2019 2011 MEMS Betz Boiling SHPiSHPo Pattern
http://slidepdf.com/reader/full/2011-mems-betz-boiling-shpishpo-pattern 2/4
Figure 1. Process flow for the fabrication of the resistive
heater (backside) and the combination of superhydrophilicand superhydrophobic patterns (front side). Not drawn to
scale.
Figure 2. (a) SEM image of nanostructured black silicon
surface, (b, c) chrome masks of the fractal and regular arrays of circles, and (d) the completed surface (fractal)
submerged in water; bubbles promptly form over the
superhydrophobic spots making them visible to the naked
eye. The light green is (super)hydrophobic and the darker
green is superhydrophilic.
in a deep reactive-ion etching (DRIE) machine. The height
of the nanostructures was usually less than 1 µm. It should
also be noted that nanostructured surfaces similar to the
ones we use here have been shown to enhance pool boiling
heat transfer in [4, 15]. Then, to ensure the hydrophilic
wettability of the surface, the silicon structures were
oxidized by exposure to oxygen plasma in the RIE machine
for 30 minutes, resulting in a silicon dioxide layer
approximately 30 nm thick. A layer of Teflon®fluoropolymer or Cytop® was then spin-coated at 2500 rpm
for 30 seconds and annealed at 250 °C or 125 °C,
respectively. The thickness of the polymer coating was less
than 100 nm. Zonyl FSN surfactant was added to AZ5214
photoresist to improve its wettability on the hydrophobic
surface and was subsequently spin-coated at 3000 rpm for
30 seconds. The photoresist was patterned by
photolithography into regular arrays of 50 µm diameter
hexagons, circles ranging from 25 µm to 100 µm in
diameter, or fractal arrays of circles ranging from 10 to 540
µm in diameter, shown in figures 2b and c.With the
photoresist as a mask, the hydrophobic layer was etched by
oxygen plasma for 3 minutes, thus defining hydrophobicislands amidst the hydrophilic network of oxide-covered
black silicon microstructures, shown in figure 2d. Finally,
the photoresist etching mask was removed in acetone and
the wafer was cleaved into chips.
SETUP
During the heat transfer experiments the chip was
placed in a polycarbonate chamber open to the atmosphere,
filled with degassed and deionized water as shown in figure
3. For more details see [11]. The resistive heater and
electrodes were encased in a 5-10 mm thick layer of PDMS
for electrical and thermal insulation. The water was
maintained at the saturation temperature of 100 °C with
submerged cartridge heaters. A data acquisition system
(OMEGA DAQ-55) was used to record the temperature
measured on the back of the wafer, Tmeas. From that
temperature, the temperature at the wafer-water interface
Tw= Tmeas-q”t/k was determined using Fourier’s law, where
q”, t and k are the respective heat flux, wafer thickness and
silicon thermal conductivity. For each data point the
temperature is obtained by averaging three hundred readings
over about three minutes. A 750 W power supply (Agilent
N5750A) was used to apply a given heat flux to the heater.
The CHF is determined as the heat flux corresponding to the
last observed stable temperature, beyond which a sudden
dramatic increase in temperature is observed.
The maximum combined uncertainty on the heat flux
was estimated as ±1.5 W/cm2, caused by the measurementof the heater area and the measurement of the electrical
power. The maximum uncertainty on the superheat was
estimated as ±1.5 K, due to the thermocouple uncertainty,
temperature acquisition and heater/wafer thickness
measurement uncertainties. For superheat values less than 1
K the uncertainty on the HTC can be greater than 100 %.
This error decreases as the superheat increases and is less
than 20 % of the HTC at superheats above 5 K.
1194
8/6/2019 2011 MEMS Betz Boiling SHPiSHPo Pattern
http://slidepdf.com/reader/full/2011-mems-betz-boiling-shpishpo-pattern 3/4
Polycarbonate chamber
Thin film K-type
thermocouplePower Supply
Submerged
cartridge heaters+ -
Silicon
Silicon dioxide
Indium tin oxide
Copper
Teflon®
PDMS
Silicon
Silicon dioxide
Indium tin oxide
Copper
Teflon®
PDMS
Polycarbonate chamber
Thin film K-type
thermocouplePower Supply
Submerged
cartridge heaters+ -
Silicon
Silicon dioxide
Indium tin oxide
Copper
Teflon®
PDMS
Silicon
Silicon dioxide
Indium tin oxide
Copper
Teflon®
PDMS
Figure 3. Pool boiling setup for heat transfer experiments.
RESULTS AND ANALYSIS
Figure 4 shows the results of our manufacturing
process. The structured silicon oxide surface shown in
figure 2a and 4a was perfectly wetting, in the sense that a
drop several millimeters in diameter placed on the surface
spread over the entire chip with an area around 10 cm2
.Figure 4b shows the wetting of a drop on the structured
surface coated with Cytop® an amorphous fluoropolymer.
Figure 4c shows the wetting of a drop on the structured
surface coated with Teflon®. Indeed we have achieved both
superhydrophilic regions and superhydrophobic regions
with a wetting angle above 150º.
a b ca b c
Figure 4. spreading of water drops on the three different surfaces: (a) spreading on the superhydrophilic surface, the
drop will continue to spread outside the field of view,indicating a wetting angle close to 0º (b) hydrophobic
surface made with Cytop® has a wetting angle of 120º and
(c) a superhydrophobic surface made from Teflon® shows
wetting angles over 150º.The results from our heat transfer measurements are
shown in figure 5. In this work we tested three types of
surfaces that we refer to in figure 5 using the wetting angle
values for the base surface and islands, in parentheses:
superhydrophilic surfaces with hydrophobic islands
(0º/120º), a superhydrophilic surface made from nano-
structured oxidized silicon (0º), and superhydrophilic
surfaces with superhydrophobic islands (0º/150º). We also
compared the boiling performance of these surfaces to our
previous work using micropatterned hydrophilic and
hydrophobic networks [11] and to state-of-the-art
nanostructured surfaces [4, 15] for pool boiling.
The results for the HTC are plotted in figure 5a. We
find that superhydrophilic surface with superhydrophobic
islands (0º/150º) can quadruple the HTC, while the (0º/120º)
and the (0º) surfaces both have a moderate increase in HTC
compared to a hydrophilic SiO2 (7º) surface. Figure 5b
a
b
a
b
Figure 5. (a) The Heat Transfer Coefficient (HTC) versus
heat flux for the current work is higher than the best HTC values to date for flat nanoengineered surfaces [4, 11, 15].
Note, however, at low heat flux < 30W/cm2, that
thermocouple reading error induces a large variance of the HTC. As the heat flux and superheat increase, the error
becomes negligible. (b) Boiling curves for this work
compared to the previous work of [4, 11, 15].
shows that the superhydrophilic wafer with hydrophobic
islands (0º/120º) can increase CHF by 80 % compared to a
hydrophilic SiO2 (7º) surface. The (0º/150º) and (0º)
surfaces did not show any significant change in CHF. There
was no significant difference in the performance between
the regular and fractal arrays.
1195
8/6/2019 2011 MEMS Betz Boiling SHPiSHPo Pattern
http://slidepdf.com/reader/full/2011-mems-betz-boiling-shpishpo-pattern 4/4
We would also like to note that that the surfaces with
superhydrophobic islands (0º/150º) show a HTC 100%
higher than the best state-of-the-art nanoengineered surfaces
[4, 11, 15].
All the surfaces tested in this work facilitated bubble
nucleation. For comparison the hydrophilic SiO2 (7º)
surface, tested in [11], required a superheat over 15 K
before significant nucleation occurred and the surface was
not saturated with bubbles until the heat flux was close toCHF. The (0º) and (0º/120º) surfaces showed significant
nucleation with a superheat of 10 K and the surfaces with
superhydrophobic islands (0º/150º) promoted nucleation at
an even lower superheat and heat flux. For this surface
(0º/150º) the entire heated area was saturated with bubbles
at heat flux q” = 25 W/cm2 and superheat ΔT = 3 K, which
is consistent with the idea that an increase in hydrophobicity
increases the number of nucleation sites.
Explaining the observed trends is complex because pool
boiling is a transient, multiphase phenomenon; visualization
is difficult especially for the violent boiling near CHF, and
the geometry and wettability of these enhanced surfaces is
complex. We conjecture that the enhancement in HTC isdue to an increase in the number of nucleation sites from the
superhydrophobic and hydrophobic islands as well as from
cavities in the nanostructured surfaces. The HTC
enhancement could also be attributed to the increased
surface area from the nanostructures. The increase in critical
heat flux may be due to increased surface wettability or
wettability contrast, moderation of instabilities, or from
wicking in the nanostructures. More analysis with simpler
experiments and multiscale modeling are probably needed
to identify the mechanisms enhancing heat transfer.
CONCLUSIONS
In summary we have combined micro- andnanoengineering techniques to manufacture flat surfaces
with large spatial contrast in wettability. In comparison with
a very hydrophilic surface, these surfaces typically enhance
pool boiling performance. Superhydrophilic surfaces with
superhydrophobic islands quadruple the heat transfer
coefficient compared to a very hydrophilic surface, without
significantly improving the critical heat flux. We have also
shown that superhydrophilic surfaces with hydrophobic
islands can double the heat transfer coefficient and increase
the critical heat flux by 80 %, compared to a very
hydrophilic surface.
REFERENCES[1] Carey, V.P., Liquid-vapor-phase-change phenomena,
Taylor and Francis, 1992.
[2] Weibel, J.A., S.V. Garimella, and M.T. North,
"Characterization of evaporation and boiling from
sintered powder wicks fed by capillary action" ,
International Journal of Heat and Mass Transfer ,
vol.53, pp. 4204-4215, 2010.
[3] Chien, L.H. and R.L. Webb, "Visualization of pool
boiling on enhanced surfaces" , Experimental Thermal
and Fluid Science, vol.16, pp. 332-341, 1998.
[4] Li, C., Z. Wang, P.I. Wang, Y. Peles, N. Koratkar, and
G.P. Peterson, "Nanostructured copper interfaces for
enhanced boiling" , Small , vol.4, pp. 1084-1088, 2008.
[5] Chen, R., M.-C. Lu, V. Srinivasan, Z. Wang, H.H.
Cho, and A. Majumdar, "Nanowires for enhanced
boiling heat transfer" , Nano Lett , vol.9, pp. 548-53,
2009.
[6] Kim, S., H.D. Kim, H. Kim, H.S. Ahn, H. Jo, J. Kim,and M.H. Kim, "Effects of nano-fluid and surfaces
with nano structure on the increase of CHF" ,
Experimental Thermal and Fluid Science, vol.34, pp.
487-495, 2010.
[7] Dhir, V.K. and S.P. Liaw, "Framework for a unified
model for nucleate and transition pool boiling" , Journal of Heat Transfer , vol.111, pp. 739-746, 1989.
[8] Wang, C.H. and V.K. Dhir, "On the gas entrapment
and nucleation site density during pool boiling of
saturated water" , Journal of Heat Transfer-
Transactions of the ASME , vol.115, pp. 670-679,
1993.
[9] Wang, C.H. and V.K. Dhir, "Effect of surfacewettability on active nucleation site density during
pool boiling of water on a vertical surface" , Journal of
Heat Transfer-Transactions of the ASME , vol.115, pp.
659-669, 1993.
[10] Jones, B.J., J.P. McHale, and S.V. Garimella, "The
Influence of Surface Roughness on Nucleate Pool
Boiling Heat Transfer" , Journal of Heat Transfer-
Transactions of the Asme, vol.131, 2009.
[11] Betz, A.R., J. Xu, H. Qiu, and D. Attinger, "Do
surfaces with mixed hydrophilic and hydrophobic
areas enhance heat transfer?" ,Applied Physics Letters,
2010.
[12] Wang, C.H. and V.K. Dhir, "Effect of Surface
Wettability on Active Nucleation Site Density During
Pool Boiling of Water on a Vertical Surface" , Journal
of Heat Transfer , vol.131, pp. 659, 1993.
[13] Mikic, B.B. and W.M. Rohsenow, " A New
Correlation of Pool-Boiling Data Including Effect of
Heating Surface Characteristics" , Journal of Heat
Transfer , vol.91, pp. 245, 1969.
[14] Hsu, Y.Y., "On the Size Range of Active Nucleation
Cavities on a Heating Surface" , Journal of Heat
Transfer , vol.207-216, 1962.
[15] Chen, R., M.C. Lu, V. Srinivasan, Z. Wang, H.H.
Cho, and A. Majumdar, "Nanowires for enhanced
boiling heat transfer" , Nano Letters, vol.9, pp. 548-
553, 2009.
1196
