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Effect of surface depressions on wetting and interactions between hydrophobic pore array surfaces
YKI, Ytkemiska Institutet AB.
KTH, School of Information and Communication Technology (ICT), Material Physics.
KTH, School of Information and Communication Technology (ICT), Material Physics.ORCID iD: 0000-0002-5260-5322
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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.

Place, publisher, year, edition, pages
2012. Vol. 28, no 30, 11121-11130 p.
Keyword [en]
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
URN: urn:nbn:se:kth:diva-101563DOI: 10.1021/la302036dISI: 000309199900021ScopusID: 2-s2.0-84864423872OAI: diva2:548678

QC 20120831

Available from: 2012-08-31 Created: 2012-08-30 Last updated: 2012-11-01Bibliographically approved
In thesis
1. Hydrophobic surfaces: Effect of surface structure on wetting and interaction forces
Open this publication in new window or tab >>Hydrophobic surfaces: Effect of surface structure on wetting and interaction forces
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.
Trita-CHE-Report, ISSN 1654-1081 ; 2012:52
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
urn:nbn:se:kth:diva-103409 (URN)978-91-7501-506-4 (ISBN)
Public defence
2012-11-02, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)

QC 20121011

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

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