Experimental research on friction-reduction with super-hydrophobic ...

Journal of Marine Science and Application, Vol.6, No.3, September 2007, pp.58-61
DOI: 10.1007/s11804-007-7007-3
<strong>Experimental</strong> <strong>research</strong> on friction-reduction with
super-hydrophobic surfaces
ZHAO Jia-peng, DU Xiang-dang and SHI Xiu-hua
College of Marine Engineering, Northwestern Polytechnical University, Xi’an 710072, China
1 Introduction 1
Abstract: Many recent studies have confirmed the existence of liquid slip over particular types of solid surfaces,
and these so-called super-hydrophobic surfaces have been shown to generate effective liquid slip because of the
air trapped between the surface structures. In this paper, based on boundary layer theory, the microscopic
structure of the super-hydrophobic surface is analyzed. The liquid slip effect on friction-reduction over
super-hydrophobic surfaces under various flow conditions is investigated by experiments with a flume and water
tunnel. The experimental results show that the greatest amount of drag-reduction that can be achieved is 8.76% at
a low Re.
Key words: liquid slip; super-hydrophobic; boundary; friction-reduction; Reynolds number.
CLC number: 0353.5 Document code: A Article ID: 1671-9433(2007)03-0058-04
Many surfaces in nature are highly hydrophobic and
self-cleaning. Examples include the wings of
butterflies and the leaves of plants such as cabbage.
The well-known example of a hydrophobic
self-cleaning surface is the leaves of the lotus plant.
Electron microscopy of the surface of lotus leaves
shows protruding nubs about 20~40 µm apart each
covered with a smaller scale rough surface of
epicuticular wax crystalloids [1] . Numerous studies
have confirmed that this combination of
micrometer-scale and nanometer-scale roughness,
along with the low surface energy material leads to
apparent contact angle ( θ > 150 ), a low sliding
angle and the self-cleaning effect [2] . Surfaces with
these properties are called “super-hydrophobic”.
There’re many reports that certain hydrophobic
surfaces allow noticeable slip, with slip lengths in the
range of 30 nm to 1 µm, theoretically, and
experimentally. In particular, rough hydrophobic
surfaces, the so-called “super-hydrophobic” surfaces
have been shown to generate an “effective” liquid slip
because of the air trapped between the surface
structures. There is currently considerable work in
microfluidics aimed at utilizing liquid slip to reduce
viscous drag [3] . Therefore, in this paper, the impact of
fluid liquid slip on viscous drag is analyzed by theory
and experiments.
Received date: 2007-04-02.
2 The mechanism of friction-reduction
over super-hydrophobic surface
Chang-Hwan Choi [6] , etc. have proposed
super-hydrophobic surface fluid flow in the
micro-structure model shown in Fig.1, the Couette
flow in which an air layer separates liquid from a wall
by the sharp tips of the hydrophobic posts, riding
mainly over air, the liquid is expected to flow over the
solid surface experiencing little friction. A slip length
δ due to the pure air layer of thickness b can be
represented by
δ = b(
μ1μa− 1) , (1)
where µ 1 and µ a are the viscosities of liquid and
air, respectively, an obvious effective slip is expected
due to the sizable viscosity difference between liquid
and air. Contact angle θ on the side surfaces of the
posts, balancing with the liquid pressure. The posts
need to be tall enough so that the liquid does not touch
the bottom surface between posts and also need to be
populated densely enough. The pitch should be small
enough, so that the surface tension of the warped
meniscus withstands the pressure in the liquid.
The slip effect on the surface entails meaningful
friction-reduction under various flow conditions.
Chang-Hwan Choi also proposed the relationship
between friction and liquid slip length [6] .

ZHAO Jia-peng, et al: <strong>Experimental</strong> <strong>research</strong> on friction-reduction with super-hydrophobic surfaces 59
h
b
liquid
air
Hydrophobic
structures
τ
slip
1
|
Coquette
=
, (2)
τ
no-slip
1 + ( δ h)
where δ is the slip length, h is the boundary layer
thickness, τ
slip
andτ no-slip
are the shear stresses at a
wall when slip and no-slip boundary conditions are
applied respectively.
3 Experiment
δ
Fig.1 Concept of large effective slip by
super-hydrophobic surface in Coquette flow
3.1 Preparation of super-hydrophobic surface and
models
The preparation of super-hydrophobic surface of the
nano-structures requires two conditions: 1) Preparation
of nano-structures; 2) modification to the nano-structures
by the material of low surface energy.
1) There are two general methods for preparation of
nano-structures: anodic oxidation method and
chemical corrosion method. The anodic oxidation
method is using electrochemical methods to prepare
the nano-alumina structure on aluminum plate; the
chemical corrosion method is using chemical
corrosion on aluminum plate.
2) Modification of low surface energy materials can be
divided into FAS (fluorine-containing silane) and
fluoro-coatings. FAS solution plays a role of catalyst in
the hydrolysis. It could be the low energy surface, hich
is the level of molecular weight in the basement that
had been processed. Fluoro-coating is made of
fluoro-resin, curing agent, FAS, poly tetra fluoro
ethylene(PTFE) and organic solvent confected
according to a certain proportion.
θ
α
d
V
In this paper, firstly, nano-alumina structures were
prepared in anodic oxidation method, and then
modified by fluoro-coatings. The prepared
super-hydrophobic surface has greater contact angle
with water droplet, a low sliding angle, having good
wear-resistance.
3.2 <strong>Experimental</strong> program and devices
There is a great difference between laminar and
turbulent boundary layer in viscous-friction. Therefore,
experimental <strong>research</strong>es were done separately.
a) <strong>Experimental</strong> <strong>research</strong> of friction-reduction on
laminar boundary layer. The experiment was done in
the rotary flume at the Physics Laboratory of
Northwestern Polytechnical University. <strong>Experimental</strong>
methods: Install the rotor with super-hydrophobic
coatings separately, and measure torque values at
different speeds by using rotary viscosimeter (mode
NDJ-1). The rotor diameter is 26 mm, length is 91 mm.
375
Strain
Flat model
Fig.2 Concept of drag testing system
Chamfer 16
500
Signal acquisition system
360
Chamfer 40 250
Flow direction
Liquid
(Arc transition to the front, thickness: 10, unit: mm)
Fig.3 Flat model
b) <strong>Experimental</strong> <strong>research</strong> of friction-reduction on
turbulent boundary layer. The experiment was done in
the water tunnel at the College of Marine Engineering,
Northwestern Polytechnical University. Main
performance parameters for the water tunnel: the size
of working section is Φ 400 mm × 2 000 mm , the
speed of working section is 0~18m/s. <strong>Experimental</strong>
methods: made experimental models associate with

60
3-component internal strain gage balance and installed
in working section (Fig.2).Drag values were measured
at different water velocities. There are two models for
this experiment, aluminum alloy plate (Fig. 3) and
rotary model (Fig. 4).
66mm
842mm
Head and tail are two-parameter elliptic curves,
sticky wet area: 0.162 0 m 2
Fig.4 Rotary model
4 <strong>Experimental</strong> results and discussion
4 4
1) Under laminar flow condition (Re= 10 ~ 2 × 10 ),
Fig.5 shows that the greatest amount of drag-reduction
on super-hydrophobic surface can be achieved by
8.76% at low Re.
Fig5. <strong>Experimental</strong> results of laminar flow
6 6
2) Under Turbulent flow condition (Re= 2× 10 ~5× 10 ),
Fig. 6 shows, to the flat model, there was not
1
friction-reduction at 4~8m s −
⋅ speed, but there
1
was a little drag-reduction after the 8 m s −
⋅ speed.
Fig.7 shows, to the rotary model, there was not
drag-reduction but appeared a little more
drag-increasing. And the measure of drag-increasing
increased with water speed growth. For these results,
the turbulent boundary is the main part of the whole
boundary layer at large Reynolds number to the flat
and rotary models. People have <strong>research</strong>ed on the
structure of turbulent boundary layer for a long time
and drawn a conclusion that near wall region (the
region between viscous sublayer and transition layer),
the process of low-speed stream busting into outer
turbulent boundary layer is the initial burst to the
entire turbulence. This process has created a cycle of
Journal of Marine Science and Application, Vol.6, No.3, September 2007
bursting the turbulent boundary layer and consumed
the energy of outer flow. Therefore, the burst in the
near wall region is an essential process to the whole
turbulent boundary layer. For drag-reduction, the most
important thing is that it reduced turbulent burst in the
boundary layer and restricted the three-dimensional
boundary layer flow. Based on the theory and
experiments, it is of no effect to weaken either the
strength of the vortex flow to the super-hydrophobic
surfaces, or the frequency and intensity of turbulent
burst. Instead, because of this non-wetting property
and the smoothness surface of super-hydrophobic
surfaces, the instability and disturbance of flow in the
near wall region were increased, so the frequency and
intensity of low-speed stream busted into outer
turbulent boundary layer in near wall region were also
increased. Thus, increasing the development of
turbulent boundary layer and the process of the
boundary layer momentum exchange, accordingly, the
increase of pulse of pressure and rate led to the
increase of friction.
Fig.6 <strong>Experimental</strong> results of turbulence flow with flat plate model
Fig.7 <strong>Experimental</strong> results of turbulence flow with rotary model
5 Conclusions
1) Under laminar flow condition, it has been proved

ZHAO Jia-peng, et al: <strong>Experimental</strong> <strong>research</strong> on friction-reduction with super-hydrophobic surfaces 61
that there is direct friction-reduction on the
super-hydrophobic surface from theory and
experiments, achieving the purpose of reducing
viscous resistance.
2) Under turbulent flow condition, because of this
non-wetting property and smoothness surface of
super-hydrophobic surfaces, the instability and
disturbance of flow in the near wall region are
increased, accordingly, the increase of pulse of
pressure and rate lead to the increase of friction.
References
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[3] YANG J, KWOK D Y. Effect of liquid slip in electrokinetic
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2003, 260:225–33.
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Institution of Mechanical Engineers , Part J: Journal of
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[5] SPIKES H A. The half-wetted bearing, part I—extended
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ZHAO Jia-peng was born in 1981. He is a
doctoral student at Northwestern Polytechnical
University. His current <strong>research</strong> interests include
multidisciplinary optimization design and its
application in underwater vehicles.
SHI Xiu-hua was born in 1945. She is a
Professor of Northwestern Polytechnical
University, Doctoral Supervisors. Her main
<strong>research</strong>
weapons.
area is <strong>research</strong> on underwater