Sirris Smart Coating workshop - Easy-to-clean and Self cleaning Coatings - 19 May 2011 - Non-wetting surfaces: robustness and applications - Robin Ras, Aalto University Finland

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]


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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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


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


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


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.



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)

•  …


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.


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



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°



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



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


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


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


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


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


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.


Self-‐healing superhydrophobicity (1): a property from nature

Chem. Commun., 2011, 47, 2324–2326


Self-‐healing superhydrophobicity (2)

Angew. Chem. Int. Ed. 2010, 49, 6129-‐6133


Self-‐healing superhydrophobicity and superoleophobicity (3)

Chem. Commun., 2011, 47, 2324–2326


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


Staying dry

Cicada wings

Ras et al. JACS (2008) 130, 11253

Clothing

Adv. Funct. Mater. 2008, 18, 3662–3669

Silicone nanofilaments


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


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


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


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


Floata@on on water using surface tension forces

Advances in Insect Physiology (2008) 34, 117


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


Meniscus-‐climbing

Nature (2005) 437, 733


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/


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.


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.


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


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


Superhydrophobic Copper Tubes with Possible Flow Enhancement and Drag Reduc@on


Underwater breathing: plastron func@ons as external lung

O2

CO2

J. Fluid Mech. (2008), vol. 608, pp. 275–296.


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


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


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/