title: Drop impact onto a thin film of another liquid
Creators: name: Stumpf, Bastian
Publisher: name: UNSPECIFIED
Year: 2024

description: The impact of drops onto solid substrates wetted by a liquid film of the same or a different liquid is relevant to numerous natural phenomena and various industrial or technical applications. Examples are inkjet printing, spray cooling or agricultural sprays, to name a few. Under certain conditions, the drop-wall-film interaction leads to the formation of secondary droplets (splash) that may influence the process parameters of systems like internal combustion engines or exhaust gas after-treatment systems. The high relevance of drop impact has motivated extensive research investigating the hydrodynamics of the drop-wall-film interaction, defining different regimes or characterising the drop impact outcome. These studies focus largely on impact conditions where the liquids of drop and film are the same (one-component). 
In many real-world applications, the fluid properties of the liquids in the film and the drop differ (two-component). In this case, additional influencing parameters related to the different fluid properties lead to a complex drop-wall-film interaction that is not entirely understood, thus making further research necessary. 

This work is dedicated to improving the understanding of the complex drop-wall-film interactions during drop impact by considering three main aspects.
First, the focus is on the flow dynamics in the wall near lamella formed during drop impact. Second, splashing mechanisms, particularly the phenomenon of corona detachment and the associated breakup mechanisms, are investigated. Finally, a new tool for the characterisation of secondary droplets is developed. 

 The impact of a drop onto a wetted substrate generates a radially expanding lamella flow adjacent to the wall. After impact, a growing viscous boundary layer increasingly decelerates the lamella flow until it eventually comes to a complete stop, while the thickness of the lamella reaches an asymptotic value. A theoretical model is developed that describes the dynamics of this lamella flow accounting for the viscous boundary layer in both the drop and the wall-film liquid. Experiments measuring the temporal evolution of the lamella thickness for a large range of impact parameters varying the Weber and Reynolds number as well as the ratio of film to drop viscosity provide physical evidence for the theoretical description. 

Further focus is placed on gaining a better understanding of the mechanism of corona detachment, where the crown completely and almost instantaneously detaches from the wall-film. A high-speed camera system is used to observe and characterise the drop impact under conditions that lead to corona detachment. Moreover, special experiments are conducted where the Taylor-Culick relation is utilised to gain film thickness information from evaluating the propagation velocity of artificially induced ruptures in the crown sheet. 
A hypothesis is formulated that attributes the detachment to small holes that are triggered by disturbances emerging from the early instances of impact. These holes spread, merge and thus undercut the crown sheet leading to its detachment. Building upon this hypothesis, the specific hole formation rate and the hole propagation velocity are modelled based on which the time of detachment is predicted. The theoretical scaling of time of detachment agrees well with the experiments. 

Finally, an imaging technique is developed to determine the volume fraction of two-component drops containing immiscible liquids by evaluating images of a drop from two perpendicular perspectives. This technique is supported by a machine learning algorithm that is taught using synthetically generated images. The approach is validated by evaluating images of single two-component droplets with known volume fractions that are placed in an acoustic levitator. The application of this technique is demonstrated by measuring the volume fraction of secondary droplets that emerge from a two-component splash.