Markus Solberg Wahl successfully defends his PhD thesis entitled “Detection and analysis of liquid-solid phase transitions with fiber-optic sensors”, 19th of May, 2020.
The thesis addresses the very interesting topic of using fiber optic sensors to detect phase transitions, and is a collection of three papers. In the first paper, we use experiments and modelling to study how the interference spectrum and reproducibility depends on how the fiber is manufactured. In particular, the cleave-angle is discussed in detail. It is concluded that the spliced region, which is more difficult to measure, plays a significant role in the produced spectrum. The thesis proceeds to demonstrate how multi-mode fiber interferometers can be used in conjunction with a fiber-Bragg grating (FBG) to analyze phase transitions in binary mixtures of ethanol and water. The temperature and strain sensitivity of the FBG is used to decouple these parameters from the multi-mode interferometer (MMI) response through a unique procedure. The remaining RI sensitivity of the MMI is used to measure the increased ethanol concentration caused by pure ice forming in the mixture. The measured melting points show excellent agreement with tabulated values. The sensor system developed is then eventually used to study ice formation in supercooled water-alcohol mixtures. The results are compared to theoretical predictions from heterogeneous nucleation theory. The effect on the nucleation barrier from solute type and concentration is studied, as well as the reduction in this barrier as a function of container material and pre-experiment rinsing procedures. Because of the dependency of the nucleation rate on the self-diffusivity of water, ice growth rates are measured in different ethanol concentrations to estimate the diffusivity at the liquid-solid interface.
I have had the privilege to be the co-supervisor of Markus, and Prof. Dag Roar Wahl from NTNU has been his main-supervisor. The first opponent of the defense was Professor Yuliya Semenova, TU Dublin in Ireland. The second opponent was Senior Research Scientist Sigurd Weidemann Løvseth, my colleague from SINTEF Energy Research. The administrator of the committee was Adjunct Professor Peter James Thomas from the Department of Electronic Systems, NTNU. Due to the ongoing virus crisis, it was impossible for the opponents to come to Norway. Moreover, since most of us are staying at home, the PhD defense was successfully executed interactively. I feel very grateful for the chance to work with Markus these years. We have had both scientifically interesting and fun discussions. Markus has created a new, very interesting branch in the research portfolio of the group that includes unique experimental investigations of solid-fluid phase transitions.