Friedhelm Schönfeld

An optimised split-and-recombine micro-mixer with uniform ‘chaotic’ mixing

A second generation micro-mixer, being a further optimised version of a first prototype, relying on the consequent utilisation of the split-and-recombine principle is presented. We show that the mixing can be characterized by a positive finite-time Lyapunov exponent although being highly regular and uniform. Using computational fluid dynamics (CFD) we investigate the mixing performance for Reynolds numbers in the range of about 1 to about 100. In particular for low Reynolds numbers (Re < 15) the CFD results predict an almost ideal multi-lamination. Thus, the developed mixer is especially suited for efficient mixing of highly viscous fluids. Furthermore, the numerical results are experimentally validated by investigations of mixing of water-glycerol solutions. The experimental results are found to be in excellent agreement with the numerical data and prove the high mixing efficiency.

Microfluidic Technologies for Miniaturized Analysis Systems

Microfluidics: Fundamentals and Engineering Concepts.- Electrohydrodynamic and Magnetohydrodynamic Micropumps.- Mixing in Microscale.- Control of Liquids by Surface Energies.- Electrowetting: Thermodynamic Foundation and Application to Microdevices.- Magnetic Beads in Microfluidic Systems - Towards New Analytical Applications.- Manipulation of Microobjects by Optical Tweezers.- Dielectrophoretic Microfluidics.- Ultrasonic Particle Manipulation.- Electrophoresis in Microfluidic Systems.- Chromatography in Microstructures.- Microscale Field-Flow Fractionation: Theory and Practice.- Nucleic Acid Amplification in Microsystems.- Cytometry on Microfluidic Chips.

Integrated polymer chip for two-dimensional capillary gel electrophoresis.

Two-dimensional (2D) gel electrophoresis (GE) is one of the most powerful methods for nucleic acid and protein separation, but generally suffers from high laboratory efforts connected with high analysis costs. Therefore, we herein present the development of a miniaturized 2D capillary GE (CGE) device which allows for an efficient protein separation in analysis times of about 1.5 h. This integrated 2D-CGE chip comprises a first channel for isoelectric focussing (IEF), a second specially designed transfer channel, 300 parallel micro channels, each having a cross section of 50 microm x 50 microm, and buffer reservoirs. The present work discusses fabrication aspects, in particular the combination of different microfabrication technologies, experimental separation performances of isoelectric focussing (IEF) and CGE, and presents computer simulations and first experimental results of protein transfer from the first to the second dimension.

Sessile-drop-induced bending of atomic force microscope cantilevers: a model system for monitoring microdrop evaporation

To monitor the evaporation kinetics of drops from solid surfaces, and to investigate the interaction between liquids and solids, microscopic drops of liquids were deposited onto atomic force microscope cantilevers. Due to the surface tension of the liquid, the Laplace pressure inside the drop, and the change of the interfacial stress at the solid–liquid boundary, the cantilever bends and is deflected by typically a few hundred nanometers. We used liquids with different vapour pressures and surface tensions, in order to vary the evaporation time and also the magnitude of the surface forces exerted by the drops. For fast evaporating drops the cantilever bending along the longitudinal axis was measured versus time. In the case of non-evaporating drops the overall bending was recorded with optical methods. We developed a FEM model for cantilever bending as an improvement to a previously presented analytical model. FEM simulations are confirmed by experimental results.

Capillary forces between soft, elastic spheres

The adhesive capillary force between two elastic spheres or a sphere and a plane is usually described by Fadh = 4πR*, where γ is the surface tension of the liquid and R* is the effective radius of the sphere(s). With approximate analytical calculations we show that for soft materials and radii larger than 5r3E2/γ2 (r: radius of curvature of the liquid and E: Youngs modulus) the capillary force increases more steeply and scales with R*2. This result is confirmed by finite element simulations. It may be relevant for an understanding of bioadhesion of insects and tree frogs and for an understanding of film formation from dispersions of soft particles.

Protein Diffusion Across the Interface in Aqueous Two-Phase Systems

We present a detailed study of the diffusive transport of proteins across a fluid phase boundary within aqueous two-phase systems. The aim of the work is to investigate whether local effects at the phase boundary cause a retardation of the diffusive transport between the phases. Possible modifications of interfacial mass transfer could be due to protein adsorption at the phase boundary or local electric fields from electric double layers. Experiments with a microfluidic system have been performed in which protein diffusion (bovine serum albumin and ovalbumin) within a bilaminated configuration of two phases containing polyethylene glycol and dextran is analyzed. A one-dimensional model incorporating phase-specific diffusion constants and the difference in chemical potential between the phases has been formulated. A comparison of experimental and simulation data shows a good overall agreement and suggests that a potential local influence of the phase boundary on protein transport is insignificant for the systems under investigation.

FINITE TEMPERATURE DENSITY-MATRIX-RENORMALIZATION-GROUP INVESTIGATION OF THE SPIN-PEIERLS TRANSITION IN CUGEO3

We present a numerical study of thermodynamical properties of dimerized frustrated Heisenberg chains down to extremely low temperatures with applications to

Unified quantum mechanical picture for confined spinons in dimerized and frustrated spin \(\) chains

{\mathrm{CuGeO}}_{3}

Sinusoidal crossflow microfiltration device—experimental and computational flowfield analysis

. A variant of the finite temperature density matrix renormalization group (DMRG) allows the study of the dimerized phase previously unaccessible to ab initio calculations. We investigate static dimerized systems as well as the instability of the quantum chain towards lattice dimerization. The crossover from a quadratic response in the free energy to the distortion field at finite temperature to nonanalytic behavior at zero temperature is studied quantitatively. Various physical quantities are derived and compared with experimental data for

Transition zone dynamics in combined isotachophoretic and electro-osmotic transport

{\mathrm{CuGeO}}_{3}