Lotus Effect: Surfaces with Roughness- Induced ...

Lotus Effect: Surfaces with Roughness-
Induced Superhydrophobicity, Self-Cleaning

and Low Adhesion

Prof. Bharat Bhushan

[email protected]

Yong Chae Jung
(Collaborator – Dr Mike Nosonovsky)

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

© B. Bhushan 1

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Background

Biomimetics- examples from nature

B. Bhushan. Phil Trans. R. Soc. A 367, 1631 (2009) 2
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Superhydrophobicity – definition and its importance

• A surface is superhydrophobic if it has a water contact angle above 150°.

• These surfaces are water repellent. These surfaces with low contact angle
hysteresis (less than 10º) also have a self cleaning effect, called “Lotus Effect”.
Water droplets roll off the surface and take contaminants with them.

‰ The self cleaning surfaces are of interest in various applications, e.g., self cleaning
windows, windshields, exterior paints for buildings, navigation-ships and utensils,
roof tiles, textiles, solar panels and reduction of drag in fluid flow, e. g. in
micro/nanochannels. Also, superhydrophobic surface can be used for energy
conservation and energy conversion.

• When two hydrophilic surfaces come into contact, condensation of water
vapor from environment forms meniscus bridges at asperity contacts
which lead to an intrinsic attractive force. This may lead to high adhesion
and stiction. Therefore, superhydrophobic surfaces are desirable.

Superhydrophobic surfaces can be achieved either by selecting low surface energy

materials/coatings or by introducing roughness. 3
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Objective and Approach

Objective

• Develop roughness-induced superhydrophobic surfaces by
mimicing lotus effect.

Approach

• Use numerical model to develop optimized roughness distribution for a given
contact angle.

• Study superhydrophobic and hydrophilic leaves to understand mechanism
responsible for hydrophobicity

‰ Fully characterize the surface of the leaves (contact angle, roughness,
adhesion and friction)

‰ Separate out the effect of microbumps, nanobumps and wax on the
hydrophobicity of the leaves.

• Fabricate and characterize micro-, nano- and hierarchical structured surfaces

‰ Study the effect of micro-, nano- and hierarchical structures on contact
angle and ability to form air pockets for superhydrophobicity.

• Fabricate and characterize oleophobic surfaces

‰ Study philic/phobic nature in three phase interfaces on the surfaces for
water and oil droplets.

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Roughness optimization model for superhydrophobic and
self cleaning surfaces

Complete wetting Composite interface For fluid flow, another
property of interest -
Contact angle hysteresis (θH)

α : Tilt angle

θH = θadv − θrec ≈ R f 1 − fLA (cosθrec0 − cosθadv0 )
2(Rf cosθ0 + 1)
Cassie-Baxter equation:
Droplet of liquid in contact with for high contact angle
a smooth and rough surface cosθ = Rf fSL cosθ0 − fLA (θ Æ 180°)

Wenzel’s equation: = Rf cosθ0 − fLA (Rf cosθ0 +1) Increase in fLA and reduction
in Rf decrease θH
cosθ = Rf cosθo http://lotus-shower.isunet.edu/the_lotus_effect.htm

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Need for hierarchical structure for stability of air pockets

• Composite interface is metastable.
• Capillary waves may lead to destabilization of the composite interface.

Condensation and accumulation of nanodroplets and surface inhomogeneity
(with hydrophilic spots) may destroy the composite interface.

‰ Microstructure resists capillary waves present at the liquid-air interface.
‰ Nanostructure prevents nanodroplets from filling the valleys between

asperities and pin the droplet.

Hierarchical structure is required to resist these scale-dependent
mechanisms and enlarges the liquid-air interface, resulting into high static
contact angle and low contact angle hysteresis.

M. Nosonovsky and B. Bhushan, Microelectronic Eng. 84, 382 (2007); Ultramicroscopy 107, 969 (2007) 6
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Characterization of superhydrophobic and hydrophilic leaves

Many leaves exhibit superhydrophobic and hydrophilic properties.

Characterize these to understand the mechanisms.

Superhydrophobic leaves

• Nelumbo nucifera (lotus) and colocasia
esculenta

• The leaf surface consists of microbumps
formed by convex papilla epidermal cells
covered with a 3-D epicuticular wax
(crystalline tubules composed of a mixture of
secondary alcohol nonacosan-10-ol and
nonacosanediols) on surface which creates
nanobumps.

• Combination of hierarchical structure of the Hydrophobic leaves
rough surface and wax creates a
superhydrophobic surface.

Hydrophilic leaves

• Fagus sylvatica and magnolia grandiflora

• Rather flat tabular cells with a 2-D thin wax

film (not continuous) on the surface Hydrophilic leaves

(Neinhuis and Barthlott, 1997; Wagner et al., 2003)

NY Times, 1/27/05; ABCNEWS.com, 1/26/05; Z. Burton and B. Bhushan, Ultramicroscopy 106, 709 (2006); B. Bhushan and Y. C. Jung, Nanotechnology 17,

2758 (2006); B. Bhushan and Y. C. Jung, J. Phys.: Condens. Matter 20, 225010 (2008)

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 7

Rolling off liquid droplet over superhydrophobic Lotus leaf
with self cleaning ability

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Measured and calculated contact angles

• Measured contact angles for leaves with and without wax
• Using the calculated Rf, contact angles of flat surfaces were calculated.

Contact angle for various leaves before and after removing
surface layer and calculated values of flat leaves

Z. Burton and B. Bhushan, Ultramicroscopy 106, 709 (2006); B. Bhushan and Y. C. Jung, Nanotechnology 17, 2758 (2006); B. Bhushan and Y. C.
Jung, J. Phys.: Condens. Matter 20, 225010 (2008)

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Summary

Contributions of bumps and wax

• A 3-D epicuticular wax exists on superhydrophobic leaves and a very thin
wax layer (not continuous) exists on hydrophilic leaves.

• Superhydrophobic and self cleaning leaf surfaces have an intrinsic
hierarchical structure.

• The lotus leaf surface consists of microbumps formed by convex papilla
epidermal cells covered with a 3-D wax tubules composed of a mixture of
secondary alcohol nonacosan-10-ol and nonacosanediols on surface
which creates nanobumps.

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Fabrication and characterization of nanopatterned polymers

Study the effect of nano- and microstructure on superhydrophobicity

Nanopatterns Micropatterns

Low aspect ratio (LAR) – 1:1 height to diameter

High aspect ratio (HAR) – 3:1 height to diameter Lotus patterned

• Materials
‰ Sample
ƒ Poly(methyl methacrylate) (PMMA) (hydrophilic) for nano- and
micropatterns and polystyrene (PS) (hydrophobic) for micropatterns
‰ Hydrophobic coating for PMMA
ƒ Perfluorodecyltriethyoxysilane (PFDTES) (SAM)

Z. Burton and B. Bhushan, Nano Letters 5, 1607 (2005); Y. C. Jung and B. Bhushan, Nanotechnology 17, 4970 (2006); B. Bhushan and Y. C. Jung,

J. Phys.: Condens. Matter 20, 225010 (2008)N; anoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 11

Contact angle on micro-/nanopatterned polymers

• Different surface structures: film, Lotus, LAR, HAR
• Hydrophobic film, PFDTES, on PMMA and PS surface structures

LAR HAR Lotus

Rf 2.1 5.6 3.2

• In hydrophilic surfaces, contact angle decreases with roughness and in
hydrophobic surfaces, it increases.

• The measured contact angles of both nanopatterned samples are higher

than the calculated values using Wenzel equation. It suggests that

nanopatterns benefit from air pocket formation. Furthermore, pining at top

of nanopatterns stabilizes the droplet.
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Fabrication and characterization of micropatterned silicon

Transition for Cassie-Baxter to Wenzel regime depends upon the roughness
spacing and radius of droplet. It is of interest to understand the role of roughness
and radius of the droplet.

Optical profiler surface height maps of patterned Si with PF3

• Different surface structures with flat-top cylindrical pillars:

‰ Series 1: Diameter (5 µm) and height (10 µm) pillars with different pitch values
(7, 7.5, 10, 12.5, 25, 37.5, 45, 60, and 75 µm)

‰ Series 2: Diameter (14 µm) and height (30 µm) pillars with different pitch values
(21, 23, 26, 35, 70, 105, 126, 168, and 210 µm)

• Materials
‰ Sample – Single-crystal silicon (Si)
‰ Hydrophobic coating – 1, 1, -2, 2, -tetrahydroperfluorodecyltrichlorosilane (PF3) (SAM)

B. Bhushan and Y. C. Jung, Ultramicroscopy 107, 1033 (2007); J. Phys.: Condens. Matter 20, 225010 (2008); B. Bhushan, M. Nosonovsky,

and Y. C. Jung, J. R. Soc. Interf. 4, 643 (2007); Y. C. Jung, and B. Bhushan, Scripta Mater. 57, 1057 (2007); J. Microsc. 229, 127 (2008);

Langmuir 24, 6262 (2008); M. Nosonovsky and B. Bhushan, Ultramicroscopy 107, 969 (2007); Nano Letters 7, 2633 (2007); J. Phys.:

Condens. Matter 20, 225009 (2008); Mater. Sci. Eng.:R 58, 162 (2007) ; Langmuir 24, 1525 (2008)

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Cassie-Baxter to Wenzel Regime Transition criteria for
patterned surfaces

• The curvature of a droplet is governed by Laplace eq. which relates pressure
inside the droplet to its curvature. The maximum droop of the droplet

δ≈( 2P − D)2
R

If δ ≥ H Transition from Cassie-Baxter regime to Wenzel regime

• Geometry (P and H) and radius R govern transition. A droplet with a large radius
(R) w.r.to pitch (P) would be in Cassie-Baxter regime.

Y. C. Jung, and B. Bhushan, Scripta Mater.57, 1057(2007); Y. C. Jung, and B. Bhushan, J.Microsc. 229, 127 (2008); B. Bhushan and Y. C. Jung,

J. Phys.: Condens. Matter 20, 225010 (2008)Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 14

Static contact angle, contact angle hysteresis, and tilt angle on patterned Si
surfaces with PF3

Droplet size = 1 mm in radius

• For the selected droplet, the transition occurs from Cassie-Baxter regime
to Wenzel regime at certain pitch values for a given pillar height.

B. Bhushan, and Y. C. Jung, Ultramicroscopy 107, 1033 (2007); Y. C. Jung, and B. Bhushan, J. Microsc. 229, 127 (2008); B. Bhushan and Y. C. Jung,

J. Phys.: Condens. Matter 20, 225010 (2008)Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 15

Flat Si surface Patterned Si surface with 5-µm diameter,
10-µm height, and 37.5-µm pitch

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Evaporation of a droplet on patterned Si surfaces with PF3
to study the effect of radius of droplet on transition

• During the droplet evaporation, the transition occurs from Cassie-Baxter
regime to Wenzel regime at a certain radius of droplet.

• Air pocket are visible below the droplet in Cassie-Baxter regime.

Y. C. Jung, and B. Bhushan, Scripta Mater. 57, 1057(2007); Y. C. Jung, and B. Bhushan, J. Microsc. 229, 127 (2008); B. Bhushan and Y. C. Jung,

J. Phys.: Condens. Matter 20, 225010 (2008)Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 17

Patterned Si surface with 14-µm diameter, 18
30-µm height, and 105-µm pitch

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Transition from Cassie-Baxter regime to Wenzel regime

• The critical radius of droplet for the transition increases with the pitch
based on both the transition criterion and the experimental data.

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Dust trace after droplet evaporation to determine the footprint of the
droplet at the transition

• Dust trace after droplet evaporation on the
patterned Si with PF3 in which the transition
occurred at 360 µm radius of droplet. The
footprint size is about 450 µm.

• Dust trace after droplet evaporation on the
patterned Si with PF3 in which the transition
occurred at about 20 µm radius of droplet. The
footprint size is about 25 µm.

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Condensation and evaporation of microdroplets (~20 µm) on patterned Si
surfaces with PF3 in an ESEM

Process of growing droplets on patterned Si surfaces in an ESEM

Y. C. Jung, and B. Bhushan, J. Microsc. 229, 127 (2008); B. Bhushan and Y. C. Jung, J. Phys.: Condens. Matter 20, 225010 (2008)

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Contact angle hysteresis on patterned Si surfaces with PF3

Droplet size = about 20 µm in radius in ESEM

• Contact angle hysteresis for the microdroplet with about 20 µm radius
show the same trends with those for the droplet with 1 mm radius.

• The contact angle hysteresis decreases with pitch because the contact
area between the patterned surface and the droplet decreases as
increasing pitch.

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Dynamic effects of bouncing water droplets on patterned Si surfaces with PF3

• Dynamic effects such as bouncing of a droplet can destroy the composite
solid-air-liquid interface.

• The relationship between the impact velocity of a droplet and geometric
parameters affects the transition from solid-air-liquid interface to solid-liquid
interface. Therefore, it is necessary to study the dynamic effect of droplets
under various impact velocities.

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Bouncing at impact velocity of 0.44 m/s Wetting at impact velocity of 0.88 m/s

• As the droplet hits the surface, the droplet bounces off at an impact
velocity of 0.44 m/s, and the wetting of the surface occurs at an impact
velocity of 0.88 m/s.

Y. C. Jung, and B. Bhushan, Langmuir 24, 6262 (2008) 24
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Transition criteria for bouncing water droplet on patterned surfaces

Laplace pressure: p = 2γ / R = 16γδ /( 2P − D)2
L
1
Dynamic pressure: p = 2 ρV 2
d

• The critical velocity at which the droplet touches the bottom is obtained by
equating the Laplace pressure to the dynamic pressure. The critical velocity of
the droplet

V≈ 32γH
ρ( 2P − D)2

• The critical velocity increases by a decrease of pitch (P) or an increase of

Height (H).

Y. C. Jung, and B. Bhushan, Langmuir 24, 6262 (2008) 25
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Contact angles before and after transition

θ0 : 109° (Contact angle measured on flat Si coated with PF3)

• The static contact angle of the droplet after hitting at 0.44 m/s is lower than
that of the droplet gently deposited, which can be interpreted as an abrupt
increase of solid-liquid surface area by dynamic impact.

• After the droplet hits the surface, the liquid-air interface can be changed to
the solid-liquid interface due to dynamic impact on the surface with lower
pitch.

Y. C. Jung, and B. Bhushan, Langmuir 24, 6262 (2008) 26
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Transition from Cassie-Baxter regime to Wenzel regime

• The critical impact velocity at which pinning occurs is in good quantitative
agreement with the predictions.

• The critical impact velocity of the droplet decreases with the geometric
parameter (pitch). For the surface with small pitch, the critical impact
velocity of droplet can be large.

Y. C. Jung, and B. Bhushan, Langmuir 24, 6262 (2008) 27
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Ideal surfaces

Structure of ideal hierarchical surface

• As stated earlier, hierarchical surface is needed to develop composite
interface with high stability.

• Proposed transition criteria can be used to calculate geometrical parameters for
a given droplet radius. For example, for a droplet on the order of 1 mm or larger,
a value of H on the order of 30 μm, D on the order of 15 μm and P on the order
of 130 μm is optimum.

• Nanoasperities should have a small pitch to handle nanodroplets, less than 1
mm down to few nm radius. The values of h on the order of 10 nm, d on the
order of 100 nm can be easily fabricated.

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Fabrication and characterization of hierarchical surfaces

Study the effect of hierarchical structure on superhydrophobicity

• Fabrication of microstructure
‰ Replication of Lotus leaf and micropatterned silicon surface using
an epoxy resin and then cover with the wax material

B. Bhushan et al., Soft Matter 4, 1799 (2008); Appl. Phys. Lett. 93, 093101 (2008); Ultramicroscopy (in press); Langmuir 25, 1659 (2009); Phil. Trans. R.
Soc. A. 367, 1631 (2009); Koch et al., Soft Matter 5, 1386 (2009)

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Fabrication of nanostructure and hierarchical structure

Recrystallization of wax tubules

• Nanostructure

‰ Self assembly of the Lotus wax deposited by thermal evaporation

ƒ Expose to a solvent in vapor phase for the mobility of wax molecules

• Hierarchical structure

‰ Lotus and micropatterned epoxy replicas and covered with the tubules of Lotus wax

B. Bhushan et al., Soft Matter 4, 1799 (2008); Appl. Phys. Lett. 93, 093101 (2008); Ultramicroscopy (in press); Langmuir 25, 1659 (2009); Phil. Trans. R.

Soc. A. 367, 1631 (2009); Koch et al., Soft MNatatenro5p,r1o3b8e6L(a2b0o09ra)tory for Bio- & Nanotechnology and Biomimetics 30

Nanostructures of nonacosan-ol wax tubules

Tubules of Lotus wax (0.8 μg/mm2)
After seven days with ethanol vapor (50° C)

• Nanostructure is formed by tubules of Lotus wax.

• Tubules are hollow structures and randomly orientated on the surface.

• The tubular diameter varies between 100 and 150 nm and their length varies
between 1500 and 2000 nm.

• The created nanostructures are comparable to the wax crystal morphology
found on superhydrophobic Lotus leaf.

K. Koch, B. Bhushan, Y. C. Jung and W. Barthlott, Soft Matter 5, 1386 (2009) 31
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Static contact angle, contact angle hysteresis, tilt angle and adhesive force on
various structures

• Nano- and hierarchical structures with tubular wax led to high static contact
angle of 167º and 173º and low hysteresis angle on the order of 6º and 1º.

• Compared to a Lotus leaf, hierarchical structure showed higher static
contact angle and lower contact angle hysteresis.

K. Koch, B. Bhushan, Y. C. Jung and W. Barthlott, Soft Matter 5, 1386 (2009) 32
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Self-cleaning efficiency of various surfaces

• As the impact pressure of the droplet is zero or low, most of particles on
nanostructure were removed by water droplets, resulting from geometrical
scale effects.

• As the impact pressure of the droplet is high, all particles which are sitting

at the bottom of the cavities between the pillars on hierarchical structure

were removed by the water droplets.
, B. Bhushan, Y. C. Jung and K. Koch, Langmuir 25, 3240 (2009)
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Dynamic effects of bouncing water droplets on various surfaces

• As the droplet hits the microstructure, the droplet bounces off at an impact
velocity of 0.44 m/s, and the wetting of the surface occurs at an impact
velocity of 0.77 m/s.

• As the droplet hits the hierarchical structure, the droplet always bounces
off during applying impact velocity of up to 1.5 m/s.

Y. C. Jung and B. Bhushan, Langmuir (in press) 34
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Transition from Cassie-Baxter regime to Wenzel regime

• For all microstructures with n-hexatriacontane and Lotus wax, the critical
impact velocities of the droplet are lower than those on nano- and
hierarchical structures due to the larger distance between the pillars.

• Nano- and hierarchical structures have superior to resist the dynamic
impact for stable composite solid-air-liquid interface.

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Vibration of a droplet on various surfaces to study the effect of frequency
and amplitude on transition

Wetting on microstructure Bouncing off on hierarchical structure

• After vibrating at amplitude of 2.0 mm for microstructure, the transition
occurred because air pockets do not exist below the droplet as a result of
droplet impalement.

• Hierarchical structure showed that the transition did not occur but the
droplet started to bounce off the surface from amplitude 0.8 mm.

Y. C. Jung and B. Bhushan, Langmuir (in press) 36
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Transition criteria for vibrating water droplet on the surface

Resonance frequency:

fr = n(n −1)(n + 2)γ
3πρV

Adhesion force: FA = 2R sinθγ (cosa − cosθr )
Inertia force force: FI = ρVAω2

• If the inertia force of the droplet vibrated on the surface can overcome the
adhesion force between the droplet and surface, which ΔF is positive value, the
droplet can be vertically separated from the surface (bouncing off).

ΔF = FI − FA

Y. C. Jung and B. Bhushan, Langmuir (in press) 37
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Adhesion and inertia forces on vibrating droplet

• The transition occurred as a result of droplet impalement by the structures
by increasing the inertia force of droplet on the surfaces.

• Hierarchical structures have the positive value of the difference between
the inertia force and adhesive force for droplet bounce off responsible for
superior resistance to the dynamic effects.

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Fabrication and characterization of oleophobic surfaces

Philic/phobic nature in three phase interfaces on the surfaces

• Superoleophobic surfaces are needed for self-cleaning and anti-fouling from
biological and organic contaminants both in air and underwater, e.g., in marine ships.

• A model surface for superoleophobicity and self-cleaning is provided by fishes which
are protected from oil pollution although they are wetted by water.

• The surface tension of oil and organic liquids is lower than that of water. So it is
difficult to create a superoleophobic surface.

• For a hydrophilic surface,
an oleophobic surface in
the solid-water-oil interface

can be created if γOAcosθo
is lower than γWAcosθw.

Y. C. Jung and B. Bhushan, Langmuir (in press) 39
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Philic/phobic nature in three phase interfaces on the surfaces

• For a hydrophobic surface
and an oleophobic surface in
solid-air-oil interface, an
oleophobic surface in solid-
water-oil interface can be

created if γOAcosθo is higher
than γWAcosθW.

Y. C. Jung and B. Bhushan, Langmuir (in press) 40
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Droplets in three phase interfaces on phobic/philic surfaces

• Experiments performed with water and oil in air and oil in water.

• Both hydrophilic and hydrophobic and oleophilic surfaces in air became
oleophobic in solid-water-oil interface.

• To study the surfaces with some oleophobicity, n-perfluoroeicosane (C20F42),
which has lower surface tension than that of oil, was deposited on the surfaces.

• Hydrophobic and both oleophilic and oleophobic surfaces in air became

oleophilic in solid-water-oil interface. 41

Y. C. Jung and B. Bhushan, Langmuir (in press)
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Droplets in three phase interfaces on shark skin replica

• Fabrication of shark skin
‰ Replication of shark skin surface using an epoxy resin

• The shark skin replica shows only three ribs on each scale.

• V-shaped riblets’ height varies between 200-500 μm, and their space varies
between 100-300 μm.

• Shark skin replica was oleophilic in air but became oleophobic in solid-water-oil
interface.

Y. C. Jung and B. Bhushan, Langmuir (in press) 42
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

To study optimization of oleophobicity on static contact angle on
micropatterned surfaces with different pitch values

• For water droplet, the transition occurred • In solid-oil-water interface, as the pitch
at pitch value of about 30 μm because the increased up to 26 μm, the contact angle
air pocket does not exist. increased from 146º to 155º because the
oil droplet sits on water trapped in the
• For oil droplet, all surfaces were oleophilic pillars, and then it decreased rapidly due to
because γSO was lower than γSA. the transition.

Y. C. Jung and B. Bhushan, Langmuir (in press) 43
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