In this thesis, the focus is on substrate properties influencing the splashing behaviour of impacting drops.
‘The PhD project was a collaboration between NWO and ASML,’ says Marise Gielen. ‘In the extreme ultraviolet (EUV) source chamber, liquid tin debris impact is causing a lot of contamination. To control the contamination many (substrate) properties can be tuned to suppress drop splashing after impact. We focused on three different substrate properties to suppress splashing’
First the influence of substrate orientation of a deep liquid pool was measured and modeled. ‘This changes the outcome of a drop impact event,’ says Marise. ‘Both the splashing threshold and the direction at which the splashed droplets are ejected can be controlled by tuning the substrate orientation, the drop size and its velocity.’
Also it was found that the temperature of a solid substrate influences drop impact behavior. On cold substrates drop solidification limits its spreading diameter. By measuring the impact dynamics, using bottom view recordings, the solidification is visualized during impact.
Finally, it was investigated how substrate elasticity can be used to suppress drop splashing. ‘We studied the impact of drops onto an elastic membrane,’ Marise says. ‘The splashing threshold was found to increase with decreasing membrane tension.’ The membrane deformation, measured using a profilometry technique, is used in a model to explain the experimentally measured increased splashing threshold with decreasing membrane tension.
‘During the PhD project, I have learned to pay detailed attention to substantiate the experimental findings,’ Marise says. ‘By careful modelling and scaling analysis, more depth of understanding is generated.’
A vast part of her working time Marise spent at ASML in Veldhoven. ‘The experts there were very helpful, sharing their actual and practical knowledge in the field,’ she says. ‘In industry processes are more dynamic than in academics, employees switching more frequently to new projects. However, there were also employees being involved in the project during the entire project.’
‘I had regular meetings with dr. ir. Hanneke Gelderblom at ASML, but a few days a month, on average, I went to Twente to meet with Professors Jacco Snoeijer or Detlef Lohse, to discuss with other PhD students or to perform some measurements.’
The initial research questions originated from practical experience by ASML, and during the project some new questions were suggested.
‘Especially a more fundamental understanding of the influence of the drop’s impact angle on the splashing dynamics was desired,’ Marise says. ‘We choose a fundamental approach to study a single drop impact experiment in a controlled way. Some of my findings are already published, but at the moment I am still writing some articles. These concern successful measurement we performed only last December.’
Marise would like to be employed at a company after her PhD Defense. ‘I like the dynamic working atmosphere, and that the results of my work will be more readily visible in products and work processes,’ she says.
Collaborating with colleagues will be an important part of her future work, Marise is sure of. ‘This is one of the main personal contributions of the PhD project. When I started, I worked more as an individual on the subject, trying to find my way in literature on my own. Now I have learned it is better to talk things through with colleagues and experts in the field. I now know better in an early stage when issues are too complex to be solved on your own.'