Dynamics of Precursor Films: Experiment and Theory

Abstract

Despite the fact that moving droplets are very common, a moving contact line is a longstanding fundamental challenge in the field of fluid mechanics. This fundamental challenge is the main topic of this thesis in the context of ASML immersion lithography equipment. In Chapter 2 an overview of existing theoretical and experimental work is given, that focuses on the mechanism of singularity removal in case of a moving contact line. In order to get more insight in this mechanism, it was proposed to study the nanoscale region of a contact line while it is moving. In this region van der Waals forces come into play and the contact line is referred to as a precursor film. Two types of precursor films exist, namely adiabatic and diffusive, which are present in case of a moving or non-moving contact line respectively. The current experimental results demonstrate the existence of a precursor film, but remain qualitative only in case of a moving precursor film. Hence, there is no adequate comparison of experimental results with the complete theory. Furthermore, conflicting results exist regarding the existence of a precursor film in a partial wetting situation. Nevertheless, a good understanding of the behavior of precursor films in the case of complete wetting is present, and the experimental results for the diffusive precursor film are in good agreement with theory. However, there is no experimental data regarding (the film profile of) adiabatic precursor films. This is related to the inherent difficulty of probing the region of a precursor film due to the great disparity of length and time scales involved. Moreover, understanding the behavior of adiabatic films is the key to understanding moving contact lines. Therefore, a measurement technique with sufficient spatial as well as temporal resolution is developed to probe this region. Chapter 3 describes Total Internal Reflection Fluorescence Microscopy (TIRFM), which formed the basis for the developed measurement technique. TIRFM is an advanced microscopy technique that limits the illumination to a very thin layer at the substrate by using an evanescent wave. This offers an unprecedented signal-to-noise ratio since only the fluorescent particles or fluorophores within the penetration depth of the evanescent wave are producing signal towards the detector. Furthermore, the experimental setup is designed and built to accurately measure the precursor film profile while it is moving.