1. Dr.
Robin
Ras,
Aalto
University,
Finland
Non-­‐we9ng
surfaces:
Robustness
and
applica@ons
Dr.
Robin
Ras
Molecular
Materials
Dept.
Applied
Physics
Aalto
University
(formerly
Helsinki
Univ.
Technology)
Helsinki,
Finland
hJp://Ly.tkk.fi/molmat/
robin.ras@aalto.fi

2. Dr.
Robin
Ras,
Aalto
University,
Finland
Milestones
of
superhydrophobicity
• 1940’s-­‐1950’s
– Theory
• Wenzel
• Cassie-­‐Baxter
• 1977
(BarthloJ,
Univ.
Bonn)
– plant
systema@cs
– assessing
the
value
of
certain
surface
structures
for
taxonomic
differen@a@on
• 1997
(BarthloJ
&
Neinhuis)
– first
comprehensive
experimental
study
on
self-­‐cleaning
of
plant
surfaces
– results
pointed
to
a
structural
basis
of
effec@ve
self-­‐cleaning
“Superhydrophob*”
based
on
Web
of
Knowledge
-­‐
May
2011
0
100
200
300
400
500
600
700
#
publica@ons

3. Dr.
Robin
Ras,
Aalto
University,
Finland
A droplet takes up the dirt
while rolling downWater droplets roll down the
leaf of the Lotus flower
Glue rolls down the leaf
of the Lotus flowerhJp://www.youtube.com/watch?v=XXHSM8ePuZw
Lotus
leaf:
archetype
of
a
self-­‐cleaning
surface

4. Dr.
Robin
Ras,
Aalto
University,
Finland
Loss
of
non-­‐we9ng:
caused
by
damage
Remember
the
two
requirements
for
the
Cassie
state
of
superhydrophobicity:
1. Topography
at
nano/micronscale
2. Hydrophobic
surface
chemistry
Cassie
state:
• low
contact
angle
hysteresis
(Δθ)
• low
sliding
angle
Δθ
=
θadv
−
θrec
Damage
to
1.
or
2.
leads
to
significantly
reduced
θrec
and
thus
increased
hysteresis
The
maximum
lateral
force
Flat
that
a
distorted
pinned
droplet
can
build
up
depends
on
θadv
and
θrec
Flat
=
cos
θrec
−
cos
θadv
≅
Δθ
sinθ
(for
small
θ)
Droplet
pinning
Low
fric@on
Verho,
Ras
et
al.,
Adv.
Mater.
2011,
23,
673–678

5. Dr.
Robin
Ras,
Aalto
University,
Finland
Loss
of
non-­‐we9ng:
caused
by
we9ng
transi@ons
• The
Cassie
state
of
we9ng
is
in
general
most
desired.
• Droplet
is
in
contact
mostly
with
air
• However,
transi@ons
from
Cassie
to
Wenzel
state
of
we9ng
are
possible.
• e.g.
hydrosta@c
pressure,
dissolu@on
of
the
trapped
air,
a
drop
falling
from
a
certain
height
• This
also
leads
to
loss
of
non-­‐we9ng,
even
though
the
contact
angle
can
s@ll
be
high
• The
reverse
Wenzel-­‐to-­‐Cassie
transi@on
is
difficult,
though
possible
in
some
cases.
Not
only
damage
to
the
surface,
but
also
we9ng
transi@ons
can
lead
to
pinning
of
droplets
Important
for
underwater
applica@ons
(long-­‐@me
contact
with
water)
e.g.
Ship
hull
• prevent
bio-­‐fouling
(algae,
mussels,
…)
• drag
reduc@on
Wenzel
Cassie
transi@on

6. Dr.
Robin
Ras,
Aalto
University,
Finland
Damage
to
non-­‐we9ng
surfaces
(1)
Two
types
of
damage
• loss
of
roughness
(increases
the
area
of
contact
between
water
and
the
surface)
– Mechanical
abrasion
• intrinsic
hydrophobicity
of
the
surface
is
reduced
– Damage
to
a
hydrophobic
surface
layer
• Mechanical
abrasion
• Ultraviolet
radia@on
• …
– Contamina@on
(organic/bio)
As
a
consequence,
the
Cassie
state
may
become
unstable
or
contact
angle
hysteresis
may
increase
due
to
hydrophilic
defects.
Verho,
Ras
et
al.,
Adv.
Mater.
2011,
23,
673–678

7. Dr.
Robin
Ras,
Aalto
University,
Finland
Damage
to
non-­‐we9ng
surfaces
(2)
• Most
superhydrophobic
surfaces
work
well
in
controlled
laboratory
condi@ons
• But
fail
in
real-­‐life
applica@ons.
The
requirements
for
durability
depend
on
the
area
of
applica@on.
Different
kinds
of
durability
• Robustness
in
weather
condi@ons
(e.g.
windows
of
traffic
cameras,
coa@ng
of
weather
sta@ons)
– Fouling-­‐resistant
– UV-­‐resistant
• Robustness
against
skin
contact
(e.g.
touch
screens)
– Mechanically
durable
– Resistant
against
finger
grease
• Food
packaging
/
kitchen
utensils
– Resistant
against
oil-­‐contamina@on
– (Mechanically
durable)
• …

8. Dr.
Robin
Ras,
Aalto
University,
Finland
Hierarchical
roughness
=
topography
at
two
or
more
length
scales
Only
microroughness
is
present.
Abrasion
causes
the
bumps
to
wear
off,
making
the
Cassie
state
no
longer
stable.
One
length
scale
Two
length
scales
Microbumps
with
a
nanoroughness
on
them.
Most
of
the
nanoroughness
is
unaffected
by
wear
and
the
Cassie
state
remains
stable.

9. Dr.
Robin
Ras,
Aalto
University,
Finland
Hierarchical
roughness:
example
1
• PET
fabric
coated
with
nanofilaments
before
and
awer
a
wear
test
that
simulates
skin
contact.
• majority
of
the
filaments
are
protected
by
the
3D
microstructure
of
the
fabric
• Since
the
residual
layer
awer
abrasion
is
also
s@ll
hydrophobic,
the
overall
superhydrophobic
proper@es
of
the
tex@le
are
retained.
• Contact
angle
hysteresis
has
increased
slightly
Adv.
Funct.
Mater.
2008,
18,
3662–3669

10. Dr.
Robin
Ras,
Aalto
University,
Finland
Hierarchical
roughness:
example
2
Despite
an
increase
in
contact
angle
hysteresis,
the
surface
remained
superhydrophobic,
showing
that
the
microscale
pyramids
protected
the
nanoscale
features
on
the
walls
of
the
pyramids
Nanotechnology
21
(2010)
155705
Micropyramids
with
nanoscale
roughness
Abrasion
with
Technicloth
paper
Sand
abrasion
(6
min)
θ=168°
Δθ=2°
θ=167°
Δθ=13°
θ=161°
Δθ=70°
Hydrophilic
pinning
site
θrec(Si02)=0°

11. Dr.
Robin
Ras,
Aalto
University,
Finland
Hierarchical
roughness:
example
2
Nanotechnology
21
(2010)
155705
Micropyramids
with
nanoscale
roughness
Abrasion
with
Technicloth
paper
Sand
abrasion
(6
min)
θ=168°
Δθ=2°
θ=167°
Δθ=13°
θ=161°
Δθ=70°
Hydrophilic
pinning
site
• Hydrophilic
bulk
materials
lead
to
pinning
sites
when
worn
off
• Solu@on:
hydrophobic
bulk
material
Verho,
Ras
et
al.,
Adv.
Mater.
2011,
23,
673–678

12. Dr.
Robin
Ras,
Aalto
University,
Finland
Hydrophobic
bulk
material
polishing
with
sandpaper
increased
the
contact
angle
hysteresis
only
from
4°
to
10°
even
though
scanning
electron
microscopy
showed
that
the
surface
had
suffered
considerable
damage.
Applied
Physics
Express
(2009)
125003
An
organoclay-­‐polymer
nanocomposite
before
and
awer
abrading
with
sand
paper
hJp://www.youtube.com/watch?v=HxVnFlKiFRw

13. Dr.
Robin
Ras,
Aalto
University,
Finland
Weather
durability
(1)
Conven@onal
(A–D)
and
Lotus-­‐Effect®
(E–F)
façade
paint
specimens
awer
six
years
of
exposure
under
deciduous
trees.
Bioinsp.
Biomim.
2
(2007)
S126–S134

14. Dr.
Robin
Ras,
Aalto
University,
Finland
Weather
durability
(2)
Colloids
and
Surfaces
A:
Physicochem.
Eng.
Aspects
302
(2007)
234–240
12
months
exposure
Untreated
glass
Superhydrophobic
glass
Organic
contamina@on
Silicone
nanofilaments
Awer
12
months
exposure
to
weather
elements

15. Dr.
Robin
Ras,
Aalto
University,
Finland
Laundering
Durability
of
Superhydrophobic
CoJon
Fabric
Adv.
Mater.
2010,
22,
5473–5477
1H,1H,2H,2H-­‐nonafluorohexyl-­‐1-­‐acrylate
grawed
onto
a
coJon
fabric.
Grawing
=
polymeriza@on
onto
a
solid
surface

16. Dr.
Robin
Ras,
Aalto
University,
Finland
Laundering
Durability
of
Superhydrophobic
CoJon
Fabric
Adv.
Mater.
2010,
22,
5473–5477
Fluorinated
groups
are
covalently
bonded
to
the
coJon
fabric

superhydrophobicity
s@ll
retained
its
superhydrophobicity
awer
50
accelerated
laundering
cycles
(=
equivalent
to
250
commercial
or
domes@c
launderings).

binding
between
the
coJon
fiber
and
the
fluorinated
graw
chains
is
strong
enough
to
withstand
the
shear
force
of
the
water
and
the
stainless
steel
balls.

17. Dr.
Robin
Ras,
Aalto
University,
Finland
Transparent,
Thermally
Stable
and
Mechanically
Robust
Superhydrophobic
Surfaces
Made
from
Porous
Silica
Capsules
The
coa@ng
retains
its
superhydrophobicity
under
adhesion
tape
peeling
and
sand
abrasion
Adv.
Mater.
(2011)
DOI:
10.1002/adma.201100410

18. Dr.
Robin
Ras,
Aalto
University,
Finland
SuperHYDROphobic

superOLEOphobic
or
superOMNIphobic
?
Young
equa@on
γsg
–
γsl
=
γlg
cos
θ
• The
interfacial
energy
for
water
•
γlg=72.8
mN/m
(high)
• The
interfacial
energy
for
oils
and
organic
maJer
much
lower
• hexadecane
γlg=27.5
mN/m
• decane
γlg=23.8
mN/m
• octane
γlg=21.6
mN/m
• Difficult
to
increase
contact
angle,
• Remember:
The
lowest
known
are
for
fluorinated
chemical
groups
•
γsg
=
6.7
mN/m
for
-­‐CF3,
a
bit
higher
for
–CF2-­‐
Superoleophobic
surfaces:
The
contact
angle
>
150°
for
oils
Three
requirements:
• Low
surface
energy
• Roughness
• Re-­‐entrant
curvature
e.g.
Science
2007,
318,
1618.

19. Dr.
Robin
Ras,
Aalto
University,
Finland
Self-­‐healing
superhydrophobicity
(1):
a
property
from
nature
Chem.
Commun.,
2011,
47,
2324–2326

20. Dr.
Robin
Ras,
Aalto
University,
Finland
Self-­‐healing
superhydrophobicity
(2)
Angew.
Chem.
Int.
Ed.
2010,
49,
6129-­‐6133

21. Dr.
Robin
Ras,
Aalto
University,
Finland
Self-­‐healing
superhydrophobicity
and
superoleophobicity
(3)
Chem.
Commun.,
2011,
47,
2324–2326

22. Dr.
Robin
Ras,
Aalto
University,
Finland
Superhydrophobicity
=
Water
repellency
Superhydrophobic
applica@ons
• Self-­‐cleaning
• No
water
absorp@on
(tex@le
remains
dry)
– Energy
efficient
• An@-­‐icing
• An@-­‐fogging
• Dew
collec@on
• Floata@on
– Locomo@on
• Drag
reduc@on
• Thermal
insula@on
• Gas
extrac@on
from
water
Superhydrophobicity
in
nature
• Plant
leaves
• Insect
wings
• Insect
eyes
• Desert
beetle
• Water
strider
• Breathing
by
underwater
insects
plastron

23. Dr.
Robin
Ras,
Aalto
University,
Finland
Staying
dry
Cicada
wings
Ras
et
al.
JACS
(2008)
130,
11253
Clothing
Adv.
Funct.
Mater.
2008,
18,
3662–3669
Silicone
nanofilaments

24. Dr.
Robin
Ras,
Aalto
University,
Finland
Superhydrophobic
Tracks
for
Low-­‐Fric@on,
Guided
Transport
of
Water
Droplets
• A
water
droplet
does
not
penetrate
through
a
hole/groove
in
a
superhydrophobic
surface
• Track
edge
keeps
the
drop
inline
with
the
track
Mertaniemi,
Ras
et
al.
Advanced
Materials
(2011)
in
press.
DOI:10.1002/adma.201100461
gravita@on
Electrosta@c
force
Superhydrophobic
knife

25. Dr.
Robin
Ras,
Aalto
University,
Finland
An@-­‐Icing
Superhydrophobic
Coa@ngs
Langmuir
2009,
25(21),
12444–12448
Langmuir
2011,
27(1),
25–29
hJp://www.youtube.com/watch?v=mxQy73rL3a8
Note:
also
robustness
is
a
problem
here,
as
the
growing
ice
crystals
may
damage
the
nano/micronscale
topography

26. Dr.
Robin
Ras,
Aalto
University,
Finland
Delayed
Freezing
on
Water
Repellent
Materials
Ini@al
water
temperature
25°C
Copper
plate
at
-­‐7°C
Figure
1.
Comparison
between
two
water
drops
(Ω
=
1200
μL)
deposited
on
microtextured
superhydrophobic
(black)
copper
(lew)
and
flat
(orange)
copper
(right),
both
at
a
temperature
T
=
-­‐7
C.
First
row:
the
drops
were
just
deposited;
their
colors
reflect
the
substrates.
Second
row:
the
drop
on
flat
copper
has
frozen.
Third
row:
both
drops
are
frozen.
There
is
no
difference
in
contact
angle
between
the
drops,
because
a
thin
ring
(of
radius
R
=
10
mm)
has
been
etched
in
both
plates,
providing
pinning
for
the
contact
line
and
allowing
us
to
compare
the
freezing
of
drops
of
same
volume
and
same
surface
area.
Langmuir
2009,
25(13),
7214–7216
Roughened
fluorinated
copper
=superhydrophobic
Smooth
fluorinated
copper
Normal
copper
The
drop
on
a
superhydrophobic
surface
contacts
more
air
than
solid

Insula@ng
proper@es

27. Dr.
Robin
Ras,
Aalto
University,
Finland
An@-­‐fogging
Adv.
Mater.
2007,
19,
2213–2217
Prevents
moisture
from
nuclea@ng

28. Dr.
Robin
Ras,
Aalto
University,
Finland
Harvesting of water
by a desert beetle
10
µm
Superhydrophobic
Hydrophilic peaks
Applica@on:
Fog
harves@ng
Tent
fabrics
and
roof
@les
to
collect
moisture
in
arid
areas.
Nature
(2001)
414,
33

29. Dr.
Robin
Ras,
Aalto
University,
Finland
Floata@on
on
water
using
surface
tension
forces
Advances
in
Insect
Physiology
(2008)
34,
117

30. Dr.
Robin
Ras,
Aalto
University,
Finland
Floata@on
on
water
using
surface
tension
forces
Hydrophilic
claws
to
grab
the
water
surface
Dimple:
stretching
of
the
water
surface
Advances
in
Insect
Physiology
(2008)
34,
117

31. Dr.
Robin
Ras,
Aalto
University,
Finland
Meniscus-­‐climbing
Nature
(2005)
437,
733

32. Dr.
Robin
Ras,
Aalto
University,
Finland
Water
strider
look-­‐alikes:
water-­‐
walking
devices
Exp
Fluids
(2007)
43:769–778
IEEE
TRANSACTIONS
ON
ROBOTICS,
VOL.
23,
NO.
3,
JUNE
2007
hJp://www.youtube.com/watch?v=756Tk9y0aNg
hJp://nanolab.me.cmu.edu/projects/waterstrider/

33. Dr.
Robin
Ras,
Aalto
University,
Finland
Content
Superhydrophobic
and
Superoleophobic
Nanocellulose
Aerogel
Membranes
as
Bioinspired
Cargo
Carriers
on
Water
and
Oil
Chemical
vapor
deposi@on
of
perfluorinated
trichlorosilane
• Low-­‐surface-­‐energy
coa@ng
• Roughness
from
nano-­‐
to
microscale
• Overhangs
Jin,
KeJunen,
Laiho,
Pynnönen,
Paltakari,
Marmur,
Ikkala,
Ras,
Langmuir
(2011)
1930.
Nanocellulose
aerogel

34. Dr.
Robin
Ras,
Aalto
University,
Finland
TiO2-­‐coated
nanocellulose
aerogel
KeJunen
(née
Pääkkö),
Silvennoinen,
Houbenov,
Nykänen,
Ruokolainen,
Sainio,
Pore,
Kemell,
Ankerfors,
Lindström,
Ritala,
Ras,
Ikkala,
Adv.
Funct.
Mater.
(2011)
510.
Nanocellulose aerogel
(highly porous solvent-free
network)
TiO2-coated nanocellulose aerogel
(coated by chemical vapor deposition
CVD or atomic layer deposition ALD)
Precursor:
TiO2 thickness ca. 7 nm
on nanocellulose fibril
ALD
or
CVD
Korhonen,
Hiekkataipale,
Malm,
Karppinen,
Ikkala,
Ras,
ACS
Nano
(2011)
1967.

35. Dr.
Robin
Ras,
Aalto
University,
Finland
Op@cally
controlled
water
absorp@on
within
TiO2-­‐coated
cellulose
aerogel
No illumination Ultraviolet illumination
λ = 350 nm
After ultraviolet
illumination
Rejects water Absorbent Rejects water
High
contact
angle on
surface
Water
expelled
from the
pores
High
contact
angle on
surface
Water
expelled
from the
pores
Zero contact
angle on
surface
Water
absorbed in
the pores:
16 x water
vs the
aerogel
weight
Recovering
slowly
KeJunen
(née
Pääkkö),
Silvennoinen,
Houbenov,
Nykänen,
Ruokolainen,
Sainio,
Pore,
Kemell,
Ankerfors,
Lindström,
Ritala,
Ras,
Ikkala,
Adv.
Funct.
Mater.
(2011)
510.

36. Dr.
Robin
Ras,
Aalto
University,
Finland
Humidity
sensing
using
TiO2
nanotube
aerogels
Korhonen,
Hiekkataipale,
Malm,
Karppinen,
Ikkala,
Ras,
ACS
Nano
(2011)
1967.
Nanotube
films
act
as
fast
resis@ve
humidity
sensors.

37. Dr.
Robin
Ras,
Aalto
University,
Finland
Plastron:
a
thin
layer
of
trapped
air
at
the
surface
of
an
immersed
superhydrophobic
surface
SoL
MaMer,
2010,
6,
714
Angew.
Chem.
Int.
Ed.
2007,
46,
1710
–1712
Mirror-­‐like
silvery
appearance
Reflec@vity
96%
Bioinsp.
Biomim.
2
(2007)
S126–S134

38. Dr.
Robin
Ras,
Aalto
University,
Finland
Slip
and
drag
reduc@on:
lower
fric@on
of
flowing
water
To
analyze
con@nuum
liquid
flows,
a
so-­‐called
“no-­‐slip”
boundary
condiUon
is
typically
made.
This
condiUon
implies
that
the
flow
velocity
of
a
given
fluid
at
a
solid
wall
is
zero.
True
for
most
surfaces,
not
for
superhydrophobic
surfaces

39. Dr.
Robin
Ras,
Aalto
University,
Finland
Superhydrophobic
Copper
Tubes
with
Possible
Flow
Enhancement
and
Drag
Reduc@on

40. Dr.
Robin
Ras,
Aalto
University,
Finland
Underwater
breathing:
plastron
func@ons
as
external
lung
O2
CO2
J.
Fluid
Mech.
(2008),
vol.
608,
pp.
275–296.

41. Dr.
Robin
Ras,
Aalto
University,
Finland
Gas
extrac@on
from
water
APPLIED
PHYSICS
LETTERS
89,
104106
(2006)
A
sphere
of
3m
diameter
would
provide
enough
oxygen
for
a
human
to
survive

42. Dr.
Robin
Ras,
Aalto
University,
Finland
Conclusion
• Robustness
of
superhydrophobic
surfaces
was
long
@me
ignored
• Last
two
years
progress
made
towards
robust
superhydrophobic
surfaces
• Some
promising
routes,
but
more
work
needed
• We
can
learn
a
lot
from
nature
(=biomime@cs)
• Wide
range
of
applica@ons
beyond
self-­‐cleaning
for
non-­‐we9ng
surfaces

43. Dr.
Robin
Ras,
Aalto
University,
Finland
Acknowledgements
Aalto
Univ.
(Finland)
• O.
Ikkala,
H.
Mertaniemi,
T.
Verho,
H.
Jin,
M.
KeJunen
(née
Pääkkö),
J.
Korhonen,
P.
Hiekkataipale,
A.
Laiho.,
M.
Karppinen,
J.
Malm,
S.
Franssila,
V.
Jokinen,
L.
Sainiemi.
Technion
(Israel)
• A.
Marmur
Nokia
Research
Center
-­‐
Cambridge
(UK)
• P.
Andrew
and
C.
Bower
Funding
• Nokia
Research
Center,
UPM
Kymmene,
TEKES,
Acad.
Finland.
Dr.
Robin
Ras
Aalto
University,
Helsinki,
Finland
robin.ras@aalto.fi
hJp://Ly.tkk.fi/molmat/