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Wetting hysteresis induced by temperature changes: supercooled water onhydrophobic surfaces
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials. SP Technical Research Institute of Sweden.ORCID iD: 0000-0003-0016-3398
(SP Technical Research Institute of Sweden)
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The state and stability of supercooled water on (super)hydrophobic surfaces is crucial for low temperature applications and for obtaining anti-icing and de-icing properties. Surface characteristics such as topography and chemistry are expected to affect wetting hysteresis during temperature cycling experiments, and also the freezing delay of supercooled water. We utilized stochastically rough wood surfaces that were further modified to render them hydrophobic or superhydrophobic. Liquid flame spraying (LFS) was utilized to create a multi-scale roughness by depositing titaniumdioxide nanoparticles. The coating was subsequently made non-polar by applying a thin plasma polymer layer. As flat reference samples modified silica surfaces with similar chemistries were utilized. With these sets of surfaces we test the hypothesis that superhydrophobic surfaces also should retard ice formation. Wetting hysteresis was evaluated using contact angle measurements during a freeze-thaw cycle from room temperature to freezing occurrence at -7 °C, and then back to room temperature. Further, the delay in freezing of supercooled water droplets was studied at temperatures of -4 °C and -7 °C. The hysteresis in contact angle observed during a cooling-heating cycle is found to be small on flat hydrophobic surfaces. However, significant changes in contact angles during a cooling-heating cycle are observed on the rough surfaces, with a higher contact angle observed on cooling compared to during the subsequent heating. This hysteresis is lower for hydrophobic wood samples with multi-scale roughness compared to those with predominantly micro-scale features. Condensation and subsequent frost formation at sub-zero temperatures induce the hysteresis. The freezing delay data suggests that the multi-scale roughness reduces the penetration of supercooled water into surface depressions, and enhances the freezing delay at low degrees of supercooling. However, the flat surface is even more efficient in enhancing the freezing delay than the rougher surfaces, which can be rationalized considering heterogeneous nucleation theory. Thus, our data suggests that molecular flat surfaces, rather than rough superhydrophobic surfaces, are beneficial for retarding ice formation under conditions that allow condensation and frost formation to occur.

Keyword [en]
wetting hysteresis, contact angle, supercooled water, morphology, hydrophobization, multi-scale roughness, wood, superhydrophobicity, liquid flame spray (LFS), plasma polymerization
National Category
Physical Chemistry
Research subject
URN: urn:nbn:se:kth:diva-175872OAI: diva2:862779

QS 2015

Available from: 2015-10-23 Created: 2015-10-23 Last updated: 2015-12-09Bibliographically approved
In thesis
1. Wettability of modified wood
Open this publication in new window or tab >>Wettability of modified wood
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Despite many excellent properties of wood which make it suitable for many applications, it suffers from a number of disadvantages limiting its use. For instance, modification is needed to reduce water sorption and to improve decay resistance, dimensional stability and weathering performance. In addition, wood/liquid interaction such as water wettability on wood plays an important role in design and characteristics of many processes and phenomena such as adhesion, coating, waterproofing, wood chemical modification, and weathering. This thesis focuses on enhancing the understanding of wetting of wood, with emphasis on modified wood. The influence of surface chemical composition of wood and its microstructural characteristics on wetting and swelling properties has also been studied.

A multicycle Wilhelmy plate technique has been developed to evaluate wetting properties of porous materials, such as wood, in which the samples were subjected to repeated immersions and withdrawals in a swelling liquid (water) and in a non-swelling liquid (octane). This method was utilized to dynamically investigate contact angle, sorption and swelling properties, as well as dimensional stability of unmodified, chemically and surface modified wood samples. Scots pine sapwood and heartwood samples were utilized to establish the principles of the technique. Acetylated and furfurylated wood samples with different level of modification were thereafter examined utilizing the developed technique for wetting measurements. A perimeter model based on a linear combination of the measured force and final change in sample perimeter was suggested to evaluate the dynamic dimensional stability of wood veneers. The feasibility of this method for studying dynamic wettability was investigated by measuring the changes of advancing and receding contact angles over repeated cycles on surface modified wood samples, created by combining liquid flame spray and plasma polymerisation methods. X-ray photoelectron spectroscopy (XPS) and X-ray computed tomography (XCT) were employed to study the surface chemical composition and microstructural properties of the samples, respectively.

Three different kinetic regimes were observed in the wetting measurements: i) fast wetting and spreading of the liquid on the wood surface, ii) void filling and wicking and iii) swelling, which was the slowest of the three. The multicycle Wilhelmy plate method was found to be suitable for studying liquid penetration, sorption, and dimensional stability of swelling materials. The results demonstrate that the wetting properties of wood are highly affected by surface chemistry and microstructure. It was shown that using both swelling and non-swelling liquids in wetting measurements allow to distinguish between capillary liquid uptake and swelling. Based on this, for chemically modified samples, it was demonstrated that acetylation mostly reduces swelling, while furfurylation reduces both swelling and capillary uptake. This is in line with the microstructural study with X-ray computed tomography where a significant change in the porosity was found as a result of furfurylation, conversely acetylation left the total porosity values unchanged. Wetting results for hydrophobised wood samples demonstrate that the multi-scale roughness obtained by combination of nanoparticle coating and plasma polymerization increased both the hydrophobicity and the forced wetting durability compared to the micro-scale roughness found on wood modified with plasma polymerisation alone.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. 89 p.
TRITA-CHE-Report, ISSN 1654-1081 ; 2015:56
Wood, dimensional stability, dynamic wettability, surface chemistry, microstructure, swelling, multicycle Wilhelmy plate method, contact angle, sorption, acetylation, furfurylation, surface modification
National Category
Materials Chemistry
Research subject
urn:nbn:se:kth:diva-175875 (URN)978-91-7595-707-4 (ISBN)
Public defence
2015-11-20, Konferencerummet nr 3, SP AB, Drottning Kristinasväg 45, Stockholm, 10:00 (English)
Sustainable wood modification

QC 20151029

Available from: 2015-10-29 Created: 2015-10-23 Last updated: 2015-11-18Bibliographically approved

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