title: Wetting hysteresis induced by temperature changes: Supercooled water on hydrophobic surfaces.
abstract: The state and stability of supercooled water on (super)hydrophobic surfaces is crucial for low temperature applications and it will affect 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 titanium dioxide 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 substrates 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. Condensation and subsequent frost formation at sub-zero temperatures induce the hysteresis. The freezing delay data show that the flat surface is 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.
authors.affiliation: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden. Electronic address: ghh@kth.se.
authors.forename: Golrokh
authors.initials: G
authors.lastname: Heydari
authors.affiliation: SP Technical Research Institute of Sweden, Chemistry, Materials and Surfaces, Box 5607, SE-114 86 Stockholm, Sweden; KTH Royal Institute of Technology, School of Architecture and the Built Environment, Department of Civil and Architectural Engineering, Building Materials, SE-100 44 Stockholm, Sweden.
authors.forename: Maziar
authors.initials: M
authors.lastname: Sedighi Moghaddam
authors.affiliation: SP Technical Research Institute of Sweden, Chemistry, Materials and Surfaces, Box 5607, SE-114 86 Stockholm, Sweden.
authors.forename: Mikko
authors.initials: M
authors.lastname: Tuominen
authors.affiliation: KTH Royal Institute of Technology, School of Engineering Sciences, Department of Applied Physics, Nanostructure Physics, SE-106 91 Stockholm, Sweden.
authors.forename: Matthew
authors.initials: M
authors.lastname: Fielden
authors.affiliation: TUT Tampere University of Technology, Aerosol Physics Laboratory, Department of Physics, P.O. Box 692, 33101 Tampere, Finland.
authors.forename: Janne
authors.initials: J
authors.lastname: Haapanen
authors.affiliation: TUT Tampere University of Technology, Aerosol Physics Laboratory, Department of Physics, P.O. Box 692, 33101 Tampere, Finland.
authors.forename: Jyrki M
authors.initials: JM
authors.lastname: Mäkelä
authors.affiliation: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden; SP Technical Research Institute of Sweden, Chemistry, Materials and Surfaces, Box 5607, SE-114 86 Stockholm, Sweden.
authors.forename: Per M
authors.initials: PM
authors.lastname: Claesson
keywords: Contact angle; Hydrophobization; Liquid flame spray (LFS); Morphology; Multi-scale roughness; Plasma polymerization; Supercooled water; Superhydrophobicity; Wetting hysteresis; Wood