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Doctor Thesis Doctoral theses at NTNU, 2020:87 Sigrid Rønneberg Sigrid Rønneberg Fundamental Mechanisms of Ice Adhesion ISBN 978-82-326-4527-8 (printed version) ISBN 978-82-326-4524-5 (electronic version) ISSN 1503-8181 Doctoral theses at NTNU, 2020:87 NTNU Faculty of Engineering Norwegian University of Science and Technology Department of Structural Engineering Sigrid Rønneberg Fundamental Mechanisms of Ice Adhesion Thesis for the degree of Philosophiae Doctor Trondheim, 05.03.2020 Norwegian University of Science and Technology Faculty of Engineering Department of Structural Engineering NTNU Norwegian University of Science and Technology Thesis for the degree of Philosophiae Doctor Faculty of Engineering Department of Structural Engineering © Sigrid Rønneberg ISBN 978-82-326-4527-8 (printed version) ISBN 978-82-326-4524-5 (electronic version) ISSN 1503-8181 Doctoral theses at NTNU, 2020:87 Printed by Skipnes Kommunikasjon as i Beautiful! Powerful! Dangerous! Cold! Ice has a magic, can’t be controlled This icy force both foul and fair Has a frozen heart worth mining! There’s beauty and there’s danger here Split the ice apart! Frozen Heart by Kristen Anderson-Lopez and Robert Lopez Now suppose that there were many possible ways in which water could crystallize, could freeze. Suppose that the sort of ice we skate upon and put into highballs - what we might call ice-one - is only one of several types of ice. And suppose that there were one form - which we will call ice-nine, with a melting point of 50◦C. (...) I tried to alarm them about ice-nine being a means to end- ing life on earth. (...) It was winter now, and forever. Cat’s Cradle by Kurt Vonnegut ii iii Abstract Unwanted icing cause daily problems, as well as dangerous situations and costly repairs. Both de-icing and anti-icing techniques are applied to mitigate unwanted ice formation, and the most promising technique is the development of low ice adhesion surfaces. At present, there exists several types of such low ice adhesion surfaces. Although research of low ice adhesion strength continuously increases, there is no unified definition of icephobicity and the fundamental mechanisms of ice adhesion and ice detachment are largely unknown. Furthermore, each research group creates its own experimental set-up to generate ice and test the ice adhesion strength of their developed surfaces. As a result, the reported ice adhesion strength values are not comparable. Ice is an inherently chaotic substance, and the properties and interaction of ice situated on a surface depends greatly on the environmental conditions and ice formation process. It is the forces and interactions on atomic level which determine the ice adhesion strength. Of these, the electrostatic forces are the most prominent in ice-solid adhesion processes. In order to lower the ice adhesion strength, the forces at the ice-solid interface must be minimised. However, different icing conditions result in different types of ice, which leads to different ice adhesion mechanisms. There are several different types of ice, which are neither agreed upon nor uniformly defined. As each ice type has very different properties, such a lack of agreement might result in misunderstandings and challenge the comparability of research performed at different facilities. In this thesis, the ice adhesion strength of three types of ice on bare aluminium surfaces were investigated. The three ice types were precipitation ice, in-cloud ice and bulk water ice, and the ice adhesion strength was measured with a centrifugal adhesion test. The results showed a significant difference when comparing the ice adhesion strength of the three ice types. Precipitation ice has a higher ice adhesion strength than the other two, and bulk water ice is the easiest to remove. Bulk water ice only displays 40% of the ice adhesion of precipitation ice under similar conditions, and the standard deviation is quite high for all three ice types. The difference is thought to result from differences in the density of the ice types, due to mechanical properties of the ice based on their formation process. As there is no available and recognised standard today, all quantitative ice adhesion strength results need to be adjusted in order to be directly comparable. In this thesis, it has been shown that the reported ice adhesion strengths are very sensitive to measurement set-up in addition to the ice type. Due to the many different applications of low ice adhesion surfaces and coatings, an ideal standard measurement technique for ice adhesion strength may never exist. However, with iv a common reference test, a specific comparison between both past and present reported values of ice adhesion strength may be compared to each other. We suggest such a reference test as a horizontal shear test with bulk water ice on a bare aluminium surface under given atmospheric conditions. By establishing a database of reported ice adhesion strengths from this reference set-up, and testing the experimental parameters individually, the effect of changing these parameters can be determined. To further explore the effect of changing ice adhesion strength test methods and experimental set-ups, an interlaboratory study between two facilities researching ice adhesion strength has been performed as part of this thesis. This study found that there are significant differences between the ice adhesion strength obtained from different ice adhesion tests, but that the same trends are apparent. From this study, it can be concluded that the most important property of a test well suited to measure ice adhesion strength is repeatability, due to the large effect of experimental outliers where the ice adhesion strength differ from normal values. Through atomistic simulations, it is possible to visualise and investigate the ice- solid detachment mechanisms at nanoscale and investigate fundamental relations governing ice-solid adhesion. Through such simulations, this thesis has validated the thermodynamic theory concerning the relation between water wettability and ice adhesion strength for ideal systems. This theory is not agreed upon from experiments, but the simulations indicate that this disagreement is due to surface material deformations and other experimental factors which may be controlled, and not the properties of water and ice. The reproduction of the nanoscale theory is interesting and important due to the gap in understanding between experimental observations and theoretical models. The results represents a step towards a more thorough understanding of the fundamental mechanisms of ice adhesion, and its relation to water wettability. With improved communication of experimental conditions, and thus enhanced comparability, the multitude of developed low ice adhesion surfaces may be dis- cussed and compared with a common framework and demands. By further inves- tigating the fundamental mechanisms of ice adhesion for different ice types, and determining their detachment process from a solid surface, we will be a large step closer to understanding the theory behind ice-solid adhesion, and thus to design surfaces that truly minimises the ice adhesion. v Summary for the general public Unwanted icing cause daily problems, as well as dangerous situations and costly repairs. To avoid such icing, several types of anti-ice surface coating has been developed where ice does not stick to the surface, but rather slides right off. These surfaces are denoted as low ice adhesion surfaces. However, although several such low ice adhesion surfaces have been developed over the past decades, the mechanisms governing the ice removal are still unknown. As a result, the development of new surfaces is performed by a trial-and-error approach. In this thesis, the focus has been to examine the underlying mechanisms of detachment of ice from a surface. It has been discovered that the type of ice to be removed, for instance clear ice or frost, has a great impact on the difficulty of removing the ice. Furthermore, the method of ice removal during the testing of the developed surfaces impacts how difficult the ice is to remove. When experimental equipment is custom-made at each laboratory, these differences lead to a profound lack of comparability between developed low ice adhesion surfaces. This thesis proposes a common reference that all future and past low ice adhesion surfaces may be compared to. The key point of such a reference is the repeatability of the experiments, and it will include both the type of ice tested, and the ice removal method. By computational simulations, it is also possible to investigate what actually happens to the molecules of ice and water when situated on the surface. Through such simulations, it has been shown that the shape of water droplets on a surface might predict the difficulty of ice removal of the same surface, as stated by thermo- dynamics. As experimental studies disagree upon the existence of the relationship, the simulations indicate that the disagreement is not due to the properties and inter- actions of ice and water, but rather experimental factors such as surface properties and material deformations. The only feasible strategy to develop fully automated ice removal through low ice adhesion surfaces is to understand what is happening during ice removal. This thesis represents a large step towards this fundamental understanding. vi vii Generelt sammendrag Uønsket ising skaper daglig problemer og situasjoner som kan være både farlige og kostbare. For å unngå slik ising har det blitt utviklet flere typer anti-is overflater som hindrer is i å feste seg på overflaten og heller sklir rett av. Disse overflatene har dermed lav isvedheft. Dessverre er mekanismene for hvordan disse overflatene virker i stor grad ukjente, selv om det er blitt forsket på dem i flere tiår. Dette gjør at utviklingen av nye overflater ofte gjennomføres med prøve- og feilemetoden. I denne avhandlingen har fokuset vært på å undersøke de underliggende mekanismene for hvordan is løsner fra en overflate.
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