title: Fluid Interfaces with and without Adsorbed Species under Homogeneous Electric Fields
Creators: name: Hartmann, Johannes Jürgen
Publisher: name: UNSPECIFIED
Year: 2022

description: The SARS-CoV-2 pandemic, which began in 2020, has demonstrated the importance of fast and inexpensive techniques for analyzing and preparing liquid sample quantities in the milliliter and microliter range. In this framework, microfluidic systems, whose size and ease of use allow them to be operated outside of large laboratories without the need for specially trained personnel, have shown the potential to accelerate the medical, biological, and chemical analysis of small liquid samples. In particular, sample preparation using electric fields has proven helpful. A well-known example of this is the analysis of deoxyribonucleic acid (DNA) by electrophoresis. The influence of electric fields on liquid interfaces has led to further far-reaching technical advances in analyzing minute liquid sample quantities. In addition to using electrowetting in droplet microfluidics, probably the most important example for using the influence of electric fields on liquid interfaces is applying the electrospray technology in conjunction with mass spectroscopy. The mentioned examples show the potential of developing microfluidic systems which use electric fields to manipulate fluid interfaces for a technical benefit. Therefore, the behavior of fluid interfaces with and without adsorbed species under the influence of homogeneous electric fields will be investigated experimentally in the present Ph.D. thesis. For this purpose, three experiments based on microfluidics are presented. The first experiment is used to study concentration patterns of DNA molecules forming under an external homogeneous electric field at a liquid-liquid interface of two immiscible aqueous phases. The DNA is initially dissolved in one phase and is moved and attached to the liquid-liquid interface by the electric field. It is shown that hydrodynamic interactions between the molecules form the concentration pattern. These hydrodynamic interactions are due to the external electric field triggering an electro-osmotic flow in conjunction with the Debye layer around the DNA molecules. The findings from an experimental parameter study are compared to a nonlinear integrodifferential equation describing the time evolution of the DNA concentration field. Based on the presented experiments, an application-related example demonstrates how the studied concentration patterns can be utilized to enrich biomolecules in a microfluidic channel. In addition, it is revealed that the same experiment can separate different proteins at the liquid-liquid interface. In the next part of the thesis, a method is presented to suppress the coalescence of droplets on a liquid-infused surface (LIS) by using homogeneous electric fields. Experiments are used to quantify the electrostatic repulsive force leading to the suppression of droplet coalescence. It is shown that the repulsive forces are based on electric dipoles induced in the droplets by the applied electric field. By comparing the experimental results with numeric calculations and a semi-analytical model, this theory is confirmed. Finally, the possibility of taking tiny samples from the droplets using the same experimental setup with which the coalescence of the droplets can be suppressed is demonstrated. Due to the electrostatic deformation of the liquid-gaseous interface of the droplets and the resulting tip streaming, it is possible to take parallel samples from an array of sessile droplets. The last experiment presented in this thesis investigates the formation and development of a liquid jet that emerges from a sessile droplet and moves along a surface. The liquid jet develops analogously to the jet emerging in electrowetting or electrospinning due to an applied electric field between the sessile droplet and the counter electrode. High-speed time-synchronous imaging and electrical current measurements are used to investigate whether a distinct nanometer-thick precursor film forms in front of the liquid jet.