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Hydrophobic surfaces: Effect of surface structure on wetting and interaction forces
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The use of hydrophobic surfaces is important for many processes both in nature and industry. Interactions between hydrophobic species play a key role in industrial applications such as water-cleaning procedures and pitch control during papermaking but they also give information on how to design surfaces like hydrophobic mineral pigments.

In this thesis, the influence of surface properties on wetting and interaction forces has been studied. Surfaces with close-packed particles, pore arrays, randomly deposited nanoparticles as well as reference surfaces were prepared. The atomic force microscope (AFM) was utilized for force and friction measurements while contact angles and confocal Raman microscopy experiments were mainly used for wetting studies.

The deposition of silica particles in the size range of nano- to micrometers using the Langmuir-Blodgett (LB) technique resulted in particle coated surfaces exhibiting hexagonal close-packing and close to Wenzel state wetting after hydrophobization. Force measurements displayed long-range interaction forces assigned to be a consequence of air cavitation. Smaller roughness features provided larger forces and interaction distances interpreted as being due to fewer restrictions of capillary growth. Friction measurements proved both the surface structure and chemistry to be important for the observed forces.

On hydrophobic pore array surfaces, the three-phase contact line of water droplets avoided the pores which created a jagged interface. The influence of the pores was evident in the force curves, both in terms of the shape, in which the three-phase contact line movements around the pores could be detected, as well as the depth of the pores providing different access and amount of air. When water/ethanol mixtures were used, the interactions were concluded to be due to ethanol condensation.

Confocal Raman microscopy experiments with water and water/ethanol mixtures on superhydrophobic surfaces gave evidence for water depletion and ethanol/air accumulation close to the surface. Force measurements using superhydrophobic surfaces showed extremely long-range interaction distances.

This work has provided evidence for air cavitation between hydrophobic surfaces in aqueous solution. It was also shown that the range and magnitude of interaction forces could, to some extent, be predicted by looking at certain surface features like structure,roughness and the overall length scales.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xii, 73 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:52
Keyword [en]
hydrophobic surface, superhydrophobic surface, atomic force microscopy, surface forces, capillary forces, cavitaion, surface roughness, friction, wetting, confocal Raman, contact angles, surface preparation, Langmuir-Blodgett
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-103409ISBN: 978-91-7501-506-4 (print)OAI: oai:DiVA.org:kth-103409DiVA: diva2:559961
Public defence
2012-11-02, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20121011

Available from: 2012-10-11 Created: 2012-10-11 Last updated: 2012-10-11Bibliographically approved
List of papers
1. Solvent segregation and capillary evaporation at a superhydrophobic surface investigated by confocal Raman microscopy and force measurements
Open this publication in new window or tab >>Solvent segregation and capillary evaporation at a superhydrophobic surface investigated by confocal Raman microscopy and force measurements
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2011 (English)In: SOFT MATTER, ISSN 1744-683X, Vol. 7, no 3, 1045-1052 p.Article in journal (Refereed) Published
Abstract [en]

Wetting of water, a 1 : 1 water/ethanol mixture and an aqueous dodecylbenzene sulfonic acid surfactant solution on hydrophobic and superhydrophobic surfaces were studied using confocal Raman microscopy. The superhydrophobic surfaces were prepared by immersion of a glass substrate in a silica particle/fluoropolymer formulation followed by silanization. Preparation of hydrophobic surfaces was done in the same way with the exception that the silica particles were excluded from the formulation. The hydrophobic and superhydrophobic surfaces were characterized with respect to surface roughness using AFM, and by contact angle measurements using different liquids. Confocal Raman microscopy measurements in a 1 : 1 water/ethanol mixture showed an enrichment of ethanol close to the superhydrophobic surface, which could not be observed for the hydrophobic surface. Unexpectedly, the Raman spectrum of a pure water film in close proximity to the superhydrophobic surface displayed some differences compared to that of bulk water and indicated a stronger hydrogen-bonding close to the superhydrophobic surface. Evidence for capillary evaporation next to the superhydrophobic surface was also found, and this results in very long-range capillary attraction between one superhydrophobic surface and a hydrophobic colloidal probe as shown by AFM colloidal probe force measurements. Addition of a surfactant or ethanol suppresses capillary evaporation.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-30500 (URN)10.1039/c0sm00704h (DOI)000286615500034 ()2-s2.0-79251504899 (Scopus ID)
Note

QC 20110317

Available from: 2011-03-17 Created: 2011-02-28 Last updated: 2012-10-11Bibliographically approved
2. Robust Hydrophobic Surfaces Displaying Different Surface Roughness Scales While Maintaining the Same Wettability
Open this publication in new window or tab >>Robust Hydrophobic Surfaces Displaying Different Surface Roughness Scales While Maintaining the Same Wettability
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2011 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 27, no 13, 8153-8159 p.Article in journal (Refereed) Published
Abstract [en]

A range of surfaces coated with spherical silica particles, covering the size range from nanometer to micrometer, have been produced using Langmuir-Blodgett (LB) deposition. The particles were characterized both in suspension and in the Langmuir trough to optimize the surface preparation procedure. By limiting the particle aggregation and surface layer failures during the preparation steps, well-defined monolayers with a close-packed structure have been obtained for all particle sizes. Thus, this procedure led to structured surfaces with a characteristic variation in the amplitude and spatial roughness parameters. In order to obtain robust surfaces, a sintering protocol and an AFM-based wear test to determine the stability of the deposited surface layer were employed. Hydrophobization of the LB films followed by water contact angle measurements showed, for all tested particle sizes, the same increase in contact angle compared to the contact angle of a flat hydrophobic surface. This indicates nearly hexagonal packing and gives evidence for nearly, complete surface wetting of the surface features.

Keyword
LANGMUIR-BLODGETT-FILMS, AIR/WATER INTERFACE, SILICA PARTICLES, WENZEL, CASSIE, NANOPARTICLES, DEPOSITION, MONOLAYERS, CRYSTALS, WATER
National Category
Other Basic Medicine
Identifiers
urn:nbn:se:kth:diva-36230 (URN)10.1021/la201121p (DOI)000292124000025 ()2-s2.0-79959788273 (Scopus ID)
Note

QC 20110711

Available from: 2011-07-11 Created: 2011-07-11 Last updated: 2017-12-11Bibliographically approved
3. Influence of Surface Topography on the Interactions between Nanostructured Hydrophobic Surfaces
Open this publication in new window or tab >>Influence of Surface Topography on the Interactions between Nanostructured Hydrophobic Surfaces
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2012 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 28, no 21, 8026-8034 p.Article in journal (Refereed) Published
Abstract [en]

Nanostructured particle coated surfaces, with hydrophobized particles arranged in close to hexagonal order and of specific diameters ranging from 30 nm up to 800 nm, were prepared by Langmuir-Blodgett deposition followed by silanization. These surfaces have been used to study interactions between hydrophobic surfaces and a hydrophobic probe using the AFM colloidal probe technique. The different particle coated surfaces exhibit similar water contact angles, independent of particle size, which facilitates studies of how the roughness length scale affects capillary forces (previously often referred to as "hydrophobic interactions") in aqueous solutions. For surfaces with smaller particles (diameter < 200 nm), an increase in roughness length scale is accompanied by a decrease in adhesion force and bubble rupture distance. It is suggested that this is caused by energy barriers that prevent the motion of the three-phase (vapor/liquid/solid) line over the surface features, which counteracts capillary growth. Some of the measured force curves display extremely long-range interaction behavior with rupture distances of several micrometers and capillary growth with an increase in volume during retraction. This is thought to be a consequence of nanobubbles resting on top of the surface features and an influx of air from the crevices between the particles on the surface.

Keyword
Adhesion, Contact angle, Hydrophobicity, Surface chemistry, Thin film transistors
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-98328 (URN)10.1021/la300628m (DOI)000304492700011 ()2-s2.0-84861619050 (Scopus ID)
Note

QC 20120625

Available from: 2012-06-25 Created: 2012-06-25 Last updated: 2017-12-07Bibliographically approved
4. Frictional forces between hydrophilic and hydrophobic particle coated nanostructured surfaces
Open this publication in new window or tab >>Frictional forces between hydrophilic and hydrophobic particle coated nanostructured surfaces
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(English)Manuscript (preprint) (Other academic)
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-103406 (URN)
Note

QS 2012

Available from: 2012-10-11 Created: 2012-10-11 Last updated: 2012-10-11Bibliographically approved
5. Effect of surface depressions on wetting and interactions between hydrophobic pore array surfaces
Open this publication in new window or tab >>Effect of surface depressions on wetting and interactions between hydrophobic pore array surfaces
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2012 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 28, no 30, 11121-11130 p.Article in journal (Refereed) Published
Abstract [en]

The surface structure is known to significantly affect the long-range capillary forces between hydrophobic surfaces in aqueous solutions. It is, however, not clear how small depressions in the surface will affect the interaction. To clarify this, we have used the AFM colloidal probe technique to measure interactions between hydrophobic microstructured pore array surfaces and a hydrophobic colloidal probe. The pore array surfaces were designed to display two different pore spacings, 1.4 and 4.0 ÎŒm, each with four different pore depths ranging from 0.2 to 12.0 ÎŒm. Water contact angles measured on the pore array surfaces are lower than expected from the Cassie-Baxter and Wenzel models and not affected by the pore depth. This suggests that the position of the three-phase contact line, and not the interactions underneath the droplet, determines the contact angle. Confocal Raman microscopy was used to investigate whether water penetrates into the pores. This is of importance for capillary forces where both the movement of the three-phase contact line and the situation at the solid/liquid interface influence the stability of bridging cavities. By analyzing the shape of the force curves, we distinguish whether the cavity between the probe and the surfaces was formed on a flat part of the surface or in close proximity to a pore. The pore depth and pore spacing were both found to statistically influence the distance at which cavities form as surfaces approach each other and the distance at which cavities rupture during retraction.

Keyword
AFM, Capillary force, Close proximity, Colloidal probe techniques, Colloidal probes, Confocal Raman microscopy, Force Curve, Hydrophobic pore, Hydrophobic surfaces, Pore arrays, Pore depth, Pore spacing, Solid/liquid interfaces, Surface depressions, Three-phase contact line, Water contact angle, Contact angle, Magnetic bubbles, Probes, Surface chemistry, Hydrophobicity
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-101563 (URN)10.1021/la302036d (DOI)000309199900021 ()2-s2.0-84864423872 (Scopus ID)
Note

QC 20120831

Available from: 2012-08-31 Created: 2012-08-30 Last updated: 2017-12-07Bibliographically approved
6. Hydrophobic pore array surfaces: Wetting and interaction forces in water/ethanol mixtures
Open this publication in new window or tab >>Hydrophobic pore array surfaces: Wetting and interaction forces in water/ethanol mixtures
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2013 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 396, 278-286 p.Article in journal (Refereed) Published
Abstract [en]

Interactions between and wetting behavior of structured hydrophobic surfaces using different concentrations of water/ethanol mixtures have been investigated. Silica surfaces consisting of pore arrays with different pore spacings and pore depths were made hydrophobic by silanization. Their static and dynamic contact angles were found to be independent of the pore depth while fewer pores on the surface, i.e. a closer resemblance to a flat surface, gave a lower contact angle. As expected, a higher amount of ethanol facilitated wetting on all the surfaces tested. Confocal Raman microscopy measurements proved both water and ethanol to penetrate into the pores. AFM colloidal probe force measurements clearly showed that formation of air cavitation was hindered between the hydrophobic surfaces in presence of ethanol, and an increase in ethanol concentration was followed by a smaller jump-in distance and a weaker adhesion force. On separation, an immediate jump-out of contact occurred. The measured forces were interpreted as being due to capillary condensation of ethanol between the surfaces giving rise to very unstable cavities immediately rupturing on surface separation.

Keyword
Surface force, AFM, Capillary condensation, Ethanol, Pore array surfaces, Hydrophobic interaction, Wetting
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-103408 (URN)10.1016/j.jcis.2013.01.040 (DOI)000316372400038 ()2-s2.0-84875420239 (Scopus ID)
Funder
Swedish Foundation for Strategic Research
Note

QC 20130506. Updated from manuscript to article in journal.

Available from: 2012-10-11 Created: 2012-10-11 Last updated: 2017-12-07Bibliographically approved

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