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Author:Verho, Tuukka
Title:A Hard Superhydrophobic Surface With Hierarchical Roughness: Towards Robust Non-Wettability
Hierarkkisesti karhennettu kova superhydrofobinen pinta: kohti kestäviä kastumattomia pintoja
Publication type:Master's thesis
Publication year:2010
Pages:67      Language:   eng
Department/School:Informaatio- ja luonnontieteiden tiedekunta
Main subject:Optiikka ja molekyylimateriaalit   (Tfy-125)
Supervisor:Ikkala, Olli
Instructor:Ras, Robin
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark Aalto  55   | Archive
Keywords:superhydrophobic
non-wetting
hierarchical roughness
abrasion
Taber Abraser
hydrophobization
sessile drop method
superhydrofobinen
kastumaton
hierarkkinen karheus
kuluminen
Taber Abraser
hydrofobisointi
Abstract (eng): The practical usability of superhydrophobic surfaces is hampered by the mechanical fragility of their microscopic roughness patterns.
In this thesis we establish a concept of a robust, hierarchically rough hard superhydrophobic surface.
The hardness of the substrate material provides mechanical durability while the hierarchical roughness ensures the stability of the Cassie wetting state i.e. keeps the surface non-wetting.
The Taber Abraser is evaluated as a potential standard method for testing the wear resistance of non-wetting surfaces.

A prototype for a hard surface pattern was fabricated by plasma etching silicon with a lithographically created mask.
For hierarchical roughness, a vapour deposition setup was built for growing silicone nanofilaments on silicon.
The vapor deposition setup was also used for preparing hydrophobic monolayer coatings for superhydrophobic surfaces with non-hierarchical roughness.
The wettability of the prepared surfaces was measured with sessile drop contact angle measurements and the rough surfaces were imaged with scanning electron microscopy.
Monolayer coatings were characterized with atomic force microscopy.
The effect of abrasion was studied with contact angle measurements and optical microscopy.

Nanofilament coated patterned silicon surfaces were found to be more nonwetting than patterned silicon coated with just a hydrophobic layer.
Furthermore, a transition between two non-wetting states was found on hierarchically rough surfaces that has not been previously reported.
The durability of the non-wettability of the hierarchically rough surfaces suffered from the poor mechanical stability of the nanofilaments.
Moreover, loosening material from rubbing objects caused challenges.
It was found that a harder substrate material is needed for good scratch resistance.

The Taber Abraser was not found to be an ideal method for testing the durability of superhydrophobic surfaces because of too hard an abrading action of most of the abrading wheels and because the soft rubber wheel leaves a lot of debris on surfaces, rough and smooth alike.
Abstract (fin): Kastumattomien superhydrofobisten pintojen mikroskooppisten pintakuvioiden mekaaninen hauraus vähentää niiden käytännön sovellettavuutta.
Tässä työssä esitellään lähestymistapa, jossa hierarkkinen karheus tehdään kovalle pinnalle.
Kovuus parantaa mekaanista kestävyyttä ja hierarkkisuus tekee vesipisaroiden Cassie-tilasta stabiilin, eli varmistaa että pisarat vierivät helposti pois.
Työssä tutkitaan myös Taber Abraser -kulutustestauslaitteen soveltuvuutta kastumattomien pintojen kestävyyden testaamiseen.

Kova pintakarheus tehtiin etsaamalla piitä litografiaa hyödyntäen.
Hierarkkinen karheus saatiin muodostettua kasvattamalla siloksaaninanofilamentteja piikuvion päälle käyttäen tarkoitusta varten rakennettua höyrypäällystyslaitteistoa.
Samalla laitteistolla tehtiin myös hydrofobisia molekyylikerroksia ei-hierarkkisten kastumattomien pintakuvioiden tuottamiseksi.
Pintojen kastumisominaisuuksia tutkittiin kontaktikulmamittauksilla ja karheita pintoja kuvattiin pyyhkäisyelektronimikroskoopilla.
Hydrofobisia molekyylikerroksia karakterisoitiin atomivoimamikroskoopilla.
Hankauksen vaikutusta tutkittiin kontaktikulmamittauksilla ja valomikroskoopilla.

Nanofilamenteilla päällystetyt karhennetut piipinnat olivat kastumattomampia kuin vain hydrofobisella kerroksella päällystetyt.
Lisäksi hierarkkisesti karheilta pinnoilla havaittiin transitio kahdenlaisen Cassie-tilan välillä, jota ei ole aiemmin raportoitu.
Hierarkkisesti karheat pinnat menettivät kuitenkin kastumattomuutensa helposti koska nanofilamentit irtosivat kulutuspinnoilta.
Lisäksi kuluttavasta esineestä irtoava materiaali aiheutti haasteita.
Jatkossa hyvään naarmunkestävyyteen tarvitaan piitä vielä kovempia materiaaleja.

Kävi ilmi, että Taber Abraser -kulutustestauslaite ei ole paras mahdollinen menetelmä kastumattomien pintojen kestävyyden testaamiseen.
Useimmat tarjolla olevat kulutuspyörät aiheuttivat mikrotasolla liian kovaa hankausta ja toisaalta kevyin kumipyörä jätti niin tasaisille kuin karheillekin pinnoille runsaasti kumipartikkeleita.
ED:2010-11-19
INSSI record number: 41341
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