title: Transport processes and instabilities induced by electric fields acting on fluidic interfaces
Creators: name: Dehe, Sebastian
Publisher: name: UNSPECIFIED
Year: 2022

description: Electrohydrodynamics (EHD) describes the area of research, which studies the interactions of fluid motion and electric fields. In liquids with non-negligible conductivity, charged regions are confined to thin layers closest to boundaries, where EHD effects are most pronounced. In the present work, different phenomena that involve the actuation of fluidic interfaces by electric fields are studied. Electro-osmosis describes the fluid flow due to electric fields acting on charged regions close to the interface of a fluidic domain. When a liquid is deposited above a microstructured superhydrophobic surface, additional charges can be brought to the enclosed gas-liquid interface by placing a gate electrode below the surface. In this work, the production of a superhydrophobic surface with both micro- and nano-scales is described. In addition to inducing charges, a gate electrode exerts a force on the gas-liquid interface, pulling it in between the structures. Experimentally, the wetting state stability is characterized using reflection microscopy, revealing a continuous range of wetting states at dual-scale surfaces. By using non-constant electro-osmotic flow, complex height-averaged flow fields can be induced in a Hele-Shaw cell, which is characterized by a small distance between the parallel bounding walls compared to a characteristic lateral length scale. The governing equations for of the flow field are derived, accounting both for stationary and oscillatory electric fields. The electro-osmotic flow field is characterized above a single disc-shaped gate electrode in a microfluidic channel, using particle tracking velocimetry. In addition, using proof-of-principle experiments, the ability to create complex flow patterns is demonstrated. In order to use flow shaping in biochemical applications, a height-averaged transport model for a passive species is derived using a perturbation method, accounting for advection, diffusion and sample dispersion. The effects of sample dispersion are represented by a non-isotropic dispersion tensor. The reduced-order model shows good agreement to three-dimensional simulations, and potential applications are discussed. Electric fields lead to forces on fluidic interfaces, and in this work, two different EHD instabilities at an interface between a dielectric and a conducting liquid are investigated. Upon application of a spatially homogeneous, harmonically oscillating electric field, a resonant response of the interface can be observed above a critical amplitude. An experimental setup with a circular domain is used to observe the spatial structure of the instability, which is extracted from light-refraction at the liquid-liquid interface. The resulting dominant wavelengths and instability modes show good agreement to an analytical model. Furthermore, the role of the domain boundary is investigated. Upon applying a spatially inhomogeneous, but time-constant electric field, the interface exhibits EHD tip streaming above a critical voltage, emitting droplets into the dielectric phase. The presence of conducting droplets alters the spatial structure from a Taylor cone located centric below the pin electrode to a surface depression, where the interface moves away from the electrode and cones emerge from the rim. By experimentally characterizing a submerged electrospray and using additional numerical modeling, it is shown that the droplets induce a flow in the dielectric liquid, which is responsible for the change of the spatial structure of the instability.