Deformation modes of an oil-water interface under a local electric field: From Taylor cones to surface dimples

Fluidic interfaces disintegrate under sufficiently strong electric fields, leading to electrohydrodynamic (EHD) tip streaming. Taylor cones, which emit charged droplets from the tip of a conical cusp, are among the most prominent and well-studied examples of EHD instabilities. In liquid-liquid systems, more complex interface deformation modes than simple Taylor cones can be observed, with the interface being pushed away from the electrode, and additional cone structures emerging from the rim of the dimple. In this article, we investigate the mechanisms behind these deformation modes experimentally and numerically, and demonstrate that the presence of droplets triggers the dimple at the interface. In order to characterize the underlying processes, we replace the pin electrode by a hollow metallic needle with a prescribed electrolyte volume flow. The submerged electrospray introduces droplets of an aqueous KCl solution with varying ion concentrations into silicone oils with varying viscosities. By measuring the corresponding electric current and by optical investigation of the interface deformation, we study the system response to variations of the ionic concentration, viscosity, applied voltage as well as flow rate. In addition to the experiments, we use a finite element solver and compute the charge transport due to the droplets in the oil phase. Further, we compute the electric potential distribution, flow field and interface deformation. After calibration of our model with particle tracking velocimetry data of the flow inside the oil phase, we reproduce the experimentally observed dimple at the liquid-liquid interface. In summary, this work highlights the importance of charged droplets for the complex dynamic modes observed when a liquid-liquid interface is exposed to a local electric field.Comment: 20 pages, 10 figures; Revised version of the pape

Transport processes and instabilities induced by electric fields acting on fluidic interfaces

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

Deformation Modes of an Oil-Water Interface under a Local Electric Field: From Taylor Cones to Surface Dimples

This data set contains the relevant references to data and scripts used for the following publication: S. Dehe and S. Hardt, Deformation Modes of an Oil-Water Interface under a Local Electric Field: From Taylor Cones to Surface Dimples, Phys. Rev. Fluids 6, 123702 (2021)

Electrostatic Faraday instabilities

This data set contains the relevant references to data and scripts used for the following publications: S. Dehe, M. Hartmann, A. Bandopadhyay, and S. Hardt, The Spatial Structure of Electrostatically Forced Faraday Waves, J. Fluid Mech. 939, A6 (2022). S. Dehe, M. Hartmann, A. Bandopadhyay, and S. Hardt, Controlling the Electrostatic Faraday Instability Using Superposed Electric Fields, Phys. Rev. Fluids 7, L082002 (2022)

A Comb-Based Capacitive MEMS Microphone with High Signal-to-Noise Ratio: Modeling and Noise-Level Analysis

We present a physics-based system-level model for optimizing a novel comb-based capacitive MEMS microphone towards high signal-to-noise ratios. The model includes non-linear coupling effects between the electrodes as well as the physical dependencies on relevant design parameters, thus enabling predictive statements w.r.t. the device performance. It is calibrated and validated by finite element simulations and laser Doppler vibrometer measurements of first prototypes. Being formulated as a generalized Kirchhoffian network, it can be implemented in a standard circuit simulation tool. The predicted signal-to-noise ratio of this concept reaches up to 78 dB(A), which significantly exceeds state-of-the-art devices

The Spatial Structure of Electrostatically Forced Faraday Waves

The instability of the interface between a dielectric and a conducting liquid, excited by a spatially homogeneous interface-normal time-periodic electric field, is studied based on experiments and theory. Special attention is paid to the spatial structure of the excited Faraday waves. The dominant modes of the instability are extracted using high-speed imaging in combination with an algorithm evaluating light refraction at the liquid-liquid interface. The influence of the liquid viscosities on the critical voltage corresponding to the onset of instability and on the dominant wavelength is studied. Overall, good agreement with theoretical predictions that are based on viscous fluids in an infinite domain is demonstrated. Depending on the relative influence of the domain boundary, the patterns exhibit either discrete modes corresponding to surface harmonics or boundary-independent patterns. The agreement between experiments and theory confirms that the electrostatically forced Faraday instability is sufficiently well understood, which may pave the way to control electrostatically driven instabilities. Last but not least, the analogies to classical Faraday instabilities may enable new approaches to study effects that have so far only been observed for mechanical forcing.Comment: 28 pages, 10 figure

The SARS-CoV-2 Subgenome Landscape and its Novel Regulatory Features

International audienceCOVID-19, caused by Coronavirus SARS-CoV-2, is now in global pandemic. Coronaviruses are known to generate negative subgenomes through Transcription-Regulating Sequence (TRS)-dependent template switch, but the global dynamic landscapes of coronaviral subgenomes and regulatory rules remain unclear. Here, using NGS short-read and Nanopore long-read sequencing to profile poly(A) RNAs in two cell types at multiple time points post-infection of SARS-CoV-2, we identified hundreds of template switches and constructed the dynamic landscapes of SARS-CoV-2 subgenomes. Interestingly, template switch could occur in bidirectional manner, with diverse SARS-CoV-2 subgenomes generated from successive template switching events. Majority of template switches result from RNA-RNA interactions, including seed and compensatory modes, with terminal pairing status as a key determinant. Moreover, two TRS-independent template switch modes are also responsible for subgenome biogenesis. Collectively, our findings reveal the subgenome landscape of SARS-CoV-2 and its regulatory features, providing a molecular basis for the organization and regulation of coronaviral subgenomes