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/ [email protected]
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
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Dr. Robin Ras, Aalto University, Finland
A droplet takes up the dirt while rolling down Water droplets roll down the
leaf of the Lotus flower
Glue rolls down the leaf of the Lotus flower hJp://www.youtube.com/watch?v=XXHSM8ePuZw
Lotus leaf: archetype of a self-‐cleaning surface
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
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
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
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)
• …
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.
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
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°
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
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
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
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
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
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.
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
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.
Dr. Robin Ras, Aalto University, Finland
Self-‐healing superhydrophobicity (1): a property from nature
Chem. Commun., 2011, 47, 2324–2326
Dr. Robin Ras, Aalto University, Finland
Self-‐healing superhydrophobicity (2)
Angew. Chem. Int. Ed. 2010, 49, 6129-‐6133
Dr. Robin Ras, Aalto University, Finland
Self-‐healing superhydrophobicity and superoleophobicity (3)
Chem. Commun., 2011, 47, 2324–2326
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
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
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
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
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
Dr. Robin Ras, Aalto University, Finland
An@-‐fogging
Adv. Mater. 2007, 19, 2213–2217
Prevents moisture from nuclea@ng
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
Dr. Robin Ras, Aalto University, Finland
Floata@on on water using surface tension forces
Advances in Insect Physiology (2008) 34, 117
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
Dr. Robin Ras, Aalto University, Finland
Meniscus-‐climbing
Nature (2005) 437, 733
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/
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
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.
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.
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.
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
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
Dr. Robin Ras, Aalto University, Finland
Superhydrophobic Copper Tubes with Possible Flow Enhancement and Drag Reduc@on
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.
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
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
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 [email protected] hJp://Ly.tkk.fi/molmat/