Ralph Lindken

The Endothelin-1 Pathway and the Development of Cardiovascular Defects in the Haemodynamically Challenged Chicken Embryo

Background/Aims: Ligating the right lateral vitelline vein of chicken embryos (venous clip) results in cardiovascular malformations. These abnormalities are similar to malformations observed in knockout mice studies of components of the endothelin-1 (ET-1)/endothelin-converting enzyme-1/endothelin-A receptor pathway. In previous studies we demonstrated that cardiac ET-1 expression is decreased 3 h after clipping, and ventricular diastolic filling is disturbed after 2 days. Therefore, we hypothesise that ET-1-related processes are involved in the development of functional and morphological cardiovascular defects after venous clip. Methods: In this study, ET-1 and endothelin receptor antagonists (BQ-123, BQ-788 and PD145065) were infused into the HH18 embryonic circulation. Immediate haemodynamic effects on the embryonic heart and extra-embryonic vitelline veins were examined by Doppler and micro-particle image velocimetry. Ventricular diastolic filling characteristics were studied at HH24, followed by cardiovascular morphologic investigation (HH35). Results: ET-1 and its receptor antagonists induced haemodynamic effects at HH18. At HH24, a reduced diastolic ventricular passive filling component was demonstrated, which was compensated by an increased active filling component. Thinner ventricular myocardium was shown in 42% of experimental embryos. Conclusion: We conclude that cardiovascular malformations after venous clipping arise from a combination of haemodynamic changes and altered gene expression patterns and levels, including those of the endothelin pathway.

Velocity Measurements in Multiphase Flow by Means of Particle Image Velocimetry

The modeling and scaling of an apparatus involving mass-transfer, e.g. a bubble column reactor, requires detailed knowledge of the fluid mechanical processes in the vicinity of the contact surfaces of the respective phases. The flow around bubbles rising in water is investigated by means of particle image velocimetry (PIV). The interaction of the gaseous and the liquid phase are of great interest. Optical, non-intrusive measuring methods are particularly appropriate if the gas content in the liquid phase is low. Particle image velocimetry as a whole-field method can freeze and record the state of the flow at a given instant of time and thus resolve large-scale structures in the flow, e.g. the wake behind rising bubbles. Particular turbulent structures or states can be attributed to a certain configuration of bubbles. This paper describes the principle of PIV and its application to measuring velocities in multiphase systems (liquid/ solid) and the flow around bubbles rising in water. The algorithm developed for evaluating PIV recordings is described. The aim of the measurements in the multiphase system is to simultaneously determine the velocity distributions of the individual phases. A digital mask technique is used for separating the information resulting from the different phases present in the system.

Velocity Measurements in Microscopic Two-Phase Flows by Means of Micro PIV

This article examines the velocity distributions of microscopic liquid-liquid two-phase flows by means of micro particle image velocimetry (micro-PIV). Aqueous droplets are dispersed into an oil bulk at the T-junction of a micro fluidic Polydimethylsiloxane (PDMS) device. The channel geometry is rectangular (H: 100μm, W: 100μm). The flow is pressure driven. Tracer particles (D: 0.5–1.2μm) are added to either phase, enabling simultaneous measurements in both phases. However, the use of immiscible liquids causes optical disturbances due to a difference in refractive indices of the two liquids and due to a curved interface geometry. Particle images are thus imaged in a distorted field of view. The results of a PIV analysis will be inaccurate in scaling as well as in location of the velocity vectors — depending on the mismatch of the refractive index. We present a basic analysis on the effect of mismatched refractive indices on the precision of the velocity measurements. The estimation is based on Snell’s law and the simplified geometry of a spherical droplet. Furthermore, we propose a method to match not only the index of refraction accurately but also to leave one additional degree of freedom to set an additional property of the liquid-liquid system, e.g. viscosity ratio or density ratio. The latter ensures that properties of the modified liquid-liquid system are close to those of the non-modified two-phase system. The findings of this study are part of the design of a Lab-on-a-Chip device. It performs a DNA analysis in an online quality control application. The miniaturization of a two-phase flow combines the benefits of confined sample compartments (i.e. droplets) with the easy-to-control process parameters of a miniaturized device (e.g. temperature, pressure). Thus band broadening of the sample by Taylor-Aris dispersion is avoided and the processes can be set accurately.Copyright

Numerical Study on the Flow Physics of a T-Shaped Micro Mixer

A T-shaped micro mixer has two characteristic flow regimes that dependent on the Reynolds number and the geometry in which the mixing of the fluids entering in the two channels is determined by diffusion or by convective transport. In one regime the flow in the T-shaped micro mixer is plane-symmetric with respect to one symmetry plane of the T mixer and mixing of fluids from the two inlet channels is determined by diffusion. This regime is referred to as the diffusion regime in the remainder of this paper. In the other regime the flow is symmetric with respect to the mixing channel’s centerline, and the mixing of the fluids from the two inlet channels is primarily determined by convection. The aim of this work is to study numerically the flow topology in the transition from the flow regime of diffusive to convective mixing. Therefore a systematic study was performed to evaluate the influence of the discretization scheme, the spatial resolution and the choice of channel inlet lengths on the flow topology. The systematic investigation showed that an improper choice of spatial resolution as well as an insufficient channel inlet length can lead to a complete elimination of the Reynolds number dependent onset of convective mixing in the flow. The T-shaped micro mixer flow is represented by vortex core regions which are defined by a λ 2-criterion. They show that the secondary vortical structures in the flow consist of two Dean vortex pairs, both in the flow regime of diffusive and convective mixing. The convective regime distinguishes itself from the diffusive one by an unequal swirling strength of the Dean vortices which leads to the characteristic co-rotating vortices in the mixing channel. We show that stationary flow states exist in a narrow Reynolds number range that link the flow regime of diffusive and convective mixing. From our investigation we conclude that the transition from a diffusion to a convection dominated T-mixer flow is continuous.

Nanoparticle image velocimetry at topologically structured surfaces

Nanoparticle image velocimetry (nano-PIV), based on total internal reflection fluorescent microscopy, is very useful to investigate fluid flows within approximately 100 nm from a surface; but so far it has only been applied to flow over smooth surfaces. Here we show that it can also be applied to flow over a topologically structured surface, provided that the surface structures can be carefully configured not to disrupt the evanescent-wave illumination. We apply nano-PIV to quantify the flow velocity distribution over a polydimethylsiloxane surface, with a periodic gratinglike structure (with 215 nm height and 2 mum period) fabricated using our customized multilevel lithography method. The measured tracer displacement data are in good agreement with the computed theoretical values. These results demonstrate new possibilities to study the interactions between fluid flow and topologically structured surfaces.

Single-Cell Level Measurement of Shape, Shear Stress Distribution and Gene Expression of Endothelial Cells in Microfluidic Chips

A measurement technique based on micro-PIV that allows a non-intrusive, non-tactile determination of the topography and shear stress distribution on endothelial cell cultures with a super-cell resolution is presented. The cells are grown in microfluidic perfusion chips where constant steady flow is applied. The expression of shear responsive gene KLF2 of individual cells is measured simultaneously with the shape and shear stress distribution of the same cell. Due to the super-cell resolution of the technique we can study the correlation of the measured quantities in different parts within the cell such as the nucleus or the cytoplasm. Results on human and chicken endothelial cells are shown.Copyright

Laser-optical methods for transport studies in low temperature fuel cells

Abstract: The flow distribution and structure in a polymer electrolyte membrane fuel cell (PEMFC) or a direct methanol fuel cell (DMFC) strongly affects the performance of the cell. The main functions of the micro-channel flow field are the homogeneous distribution of the reactants, an enhancement of an effective mixing and the expulsion of the reaction products such as liquid water. To properly design the flow field of a cell and the manifold of a stack, detailed experimental velocity measurements are essential. Non-intrusive Laser-optical measurement techniques such as particle-image velocimetry, micro-particle image velocimetry, laser Doppler velocimetry, and molecular-tagging velocimetry qualify for such investigations and are more and more used in fuel cell research. The basic measurement principles of these measurement techniques are explained and the relevant applications in fuel cell research are depicted and critically reviewed. A forecast to the future use of these measurement techniques in fuel cell research is risked and additional sources of information concerning the measurement techniques and fluid mechanical problems are presented.

Micro Particle-Image-Velocimetry für Gasströmungen in Mikrokanälen

Zusammenfassung Es wird ein Messverfahren vorgestellt, mit dem sich die Geschwindigkeitsverteilung einer Mikrokanalströmung in der Gas- oder Flüssigphase bestimmen lässt. Diese Micro Particle-Image-Velocimetry (μPIV) genannte Technik ist ein berührungsloses, laseroptisches Geschwindigkeitsmessverfahren, das auf der Detektion von kleinsten der Strömung zugegebenen Partikeln bzw. deren Verschiebung beruht. Die μPIV ist insbesondere beim Einsatz in Gasströmungen entscheidend von den Partikeleigenschaften wie Fluoreszenz, Partikelgrößenverteilung und Partikelkonzentration abhängig. Das Partikelfolgeverhalten stellt einen kritischen Parameter der μPIV in Gasströmung dar. Die Messtechnik wird hier exemplarisch in einem optisch transparenten Mikrokanal mit einer 90°-Umlenkung angewendet, um die Anwendbarkeit der μPIV sowohl in einer Wasser- als auch einer Gasströmung zu demonstrieren. Abstract We present a measurement technique that allows the determination of the velocity distribution of a micro-channel flow in the gas or liquid phase. Micro Particle-Image Velocimetry (μPIV) is a non-intrusive, laser-optical velocity measurement technique that is based on the determination of the displacement of small particles that are added to the flow. Especially in the gas phase μPIV strongly depends on particle characteristics like fluorescence, particle size distributions, and particle concentrations. The fidelity of the particles to adequately follow the flow is one of the key parameters of μPIV. The measurement technique is exemplarily applied in an optically transparent micro-channel with a 90° elbow to demonstrate the applicability of μPIV in a water flow as well as in a gaseous flow.

A Scaling Analysis Inquiring Into Two-Phase Flow Reversal

Physical processes limit the maximum achievable heat flux when miniaturising heat transfer equipment. In case of boiling heat transfer literature reports large pressure fluctuations, flow instabilities, and possible vapour backflow. The occurrence of the flow instabilities during boiling in small channels (defined by the Confinement Number, Co > 0.5) are explained by the formation of slug bubbles blocking the entire channel. These particular bubbles are likely to emerge during nucleate flow boiling in small diameter channels. Slug bubble blockage during flow boiling is investigated experimentally by creating a single hotspot in a small-diameter channel (Co∼5). For different liquid flow rates the detachment length of such a blocking slug bubble is determined. A scaling analysis offers to insight into the physical phenomena causing the flow instabilities. The position of the bubble caps as a function of time is identified as an important parameter.© 2008 ASME

Visualisation and quantitative analysis of the near nozzle formation and structure of a high pressure water jet in air and water.

This work is being funded through the “FH-Struktur2016” venue for universities of applied sciences by the ministry for innovation, science and research of the state of Nordrhein-Westfalen, Germany (AZ: 322-8.03.04.02).