Development of an experimental setup to study droplet dynamics at micro- and millimeter scale

Abstract
The deformation of droplets after laser impact has recently gained more interest because of its
application in EUV nanolithography machines. In the source of these machines, a laser pulse is fired
at liquid tin droplets to deform the droplet into a suitable target for plasma generation. The droplet
deformation process is studied extensively, by using a combination of shadowgraphy and stroboscopic
imaging techniques. However, the high refractive index of tin does not allow for the visualization of
pressure fields inside the droplet, something a transparent medium like water does allow for. Due
to dynamic similarity in fluid flows, the droplet deformation of water can be compared to tin if non-
dimensional parameters like the Weber number are the same. This thesis describes the development
of two experimental setups, which can be used to visualize droplet deformation at the micro- and
millimeter length scale. First an experimental setup at millimeter scale is designed, to evaluate
methods for studying water droplet deformation, where water droplets are deformed by impact on a
similar sized pillar. The setup is characterized in terms of droplet size and droplet velocity, giving a
Weber number range of 50 – 550. Then a setup at micrometer scale is designed, where microdroplets
are generated using a piezo-actuated droplet-on-demand system, operated in negative pulse voltage
mode. Characterization results show the system can produce droplets in a diameter range of d = 83
– 100 μm, with velocities of vd = 0.9 – 2.2 m/s, depending on the liquid properties and generator
settings. Due to the low Weber number of droplet impact on a pillar, its radial expansion is limited,
resulting in oscillation of the droplet. In an outlook, a vacuum setup is briefly introduced, which can
allow for future studies of droplet deformation at micrometer scale in a low pressure environment.