On the direct employment of dipolar particle interaction in microfluidic systems

This review article will summarize recent developments in the employment of dipolar coupled magnetic particle structures. We will discuss the basics of magnetic dipolar particle interaction in static and rotating magnetic fields. In dependence on the magnetic fields employed, agglomerates of different dimensionality may form within the carrier liquid. The stability and formation dynamics of these particle structures will be presented. Furthermore, we will review recent microfluidic applications based on the interaction of magnetic particles and present methods for surface patterning with micron-sized and nano-sized particles which employ dipolar particle coupling.Alexander von Humboldt-StiftungBielefeld University. Research Group FOR 94

Lab-on-a-Chip Magneto-Immunoassays: How to Ensure Contact between Superparamagnetic Beads and the Sensor Surface

Lab-on-a-chip immuno assays utilizing superparamagnetic beads as labels suffer from the fact that the majority of beads pass the sensing area without contacting the sensor surface. Different solutions, employing magnetic forces, ultrasonic standing waves, or hydrodynamic effects have been found over the past decades. The first category uses magnetic forces, created by on-chip conducting lines to attract beads towards the sensor surface. Modifications of the magnetic landscape allow for additional transport and separation of different bead species. The hydrodynamic approach uses changes in the channel geometry to enhance the capture volume. In acoustofluidics, ultrasonic standing waves force µm-sized particles onto a surface through radiation forces. As these approaches have their disadvantages, a new sensor concept that circumvents these problems is suggested. This concept is based on the granular giant magnetoresistance (GMR) effect that can be found in gels containing magnetic nanoparticles. The proposed design could be realized in the shape of paper-based test strips printed with gel-based GMR sensors

Interaction of magnetic beads in microfluidic systems : fundamentals and applications

Wittbracht F. Interaction of magnetic beads in microfluidic systems : fundamentals and applications. Bielefeld: Universität Bielefeld; 2012

Towards a programmable microfluidic valve: Formation dynamics of two-dimensional magnetic bead arrays in transient magnetic fields

Wittbracht F, Eickenberg B, Weddemann A, Hütten A. Towards a programmable microfluidic valve: Formation dynamics of two-dimensional magnetic bead arrays in transient magnetic fields. Journal of Applied Physics. 2011;109(11): 114503.The induction of dipolar coupling has proven to allow for the initiation of self-assembled, reconfigurable particle clusters of superparamagnetic microbeads suspended in a carrier liquid. The adjustment of the interplay between magnetic and hydrodynamic forces opens various possibilities for guiding strategies of these superstructures within microfluidic devices. In this work, the formation dynamics of such particle clusters under the influence of a rotating magnetic field are studied. Different agglomeration regimes are characterized by the dimensionality of the confined objects. The growth dynamics of the obtained agglomerates are analyzed quantitatively in order to deduce the microscopic growth mechanisms. The growth of two-dimensional clusters is governed by the addition of bead chains to previously formed agglomerates. Time scales for the cluster growth are characterized by the chain dissociation rate. Based on the experimental findings, we may conclude to a linear dependence of the chain dissociation rate on the rotation frequency of the applied magnetic field. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3582133

The homogeneous ice nucleation rate of water droplets produced in a microfluidic device and the role of temperature uncertainty

Riechers B, Wittbracht F, Hütten A, Koop T. The homogeneous ice nucleation rate of water droplets produced in a microfluidic device and the role of temperature uncertainty. Physical Chemistry Chemical Physics. 2013;15(16):5873-5887.Ice nucleation was investigated experimentally in water droplets with diameters between 53 and 96 micrometres. The droplets were produced in a microfluidic device in which a flow of methyl-cyclohexane and water was combined at the T-junction of micro-channels yielding inverse (water-in-oil) emulsions consisting of water droplets with small standard deviations. In cryo-microscopic experiments we confirmed that upon cooling of such emulsion samples ice nucleation in individual droplets occurred independently of each other as required for the investigation of a stochastic process. The emulsion samples were then subjected to cooling at 1 Kelvin per minute in a differential scanning calorimeter with high temperature accuracy. From the latent heat released by freezing water droplets we inferred the volume-dependent homogeneous ice nucleation rate coefficient of water at temperatures between 236.5 and 237.9 Kelvin. A comparison of our newly derived values to existing rate coefficients from other studies suggests that the volume-dependent ice nucleation rate in supercooled water is slightly lower than previously thought. Moreover, a comprehensive error analysis suggests that absolute temperature accuracy is the single most important experimental parameter determining the uncertainty of the derived ice nucleation rates in our experiments, and presumably also in many previous experiments. Our analysis, thus, also provides a route for improving the accuracy of future ice nucleation rate measurements

Dynamic simulations of the dipolar driven demagnetization process of magnetic multi-core nanoparticles

Weddemann A, Auge A, Kappe D, Wittbracht F, Hütten A. Dynamic simulations of the dipolar driven demagnetization process of magnetic multi-core nanoparticles. JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS. 2010;322(6):643-646.In this work, we investigate dynamically the dipolar driven demagnetization process of magnetic multicore particles by solving the Landau-Lifshitz equation for single-domain particles distributed on a three-dimensional sphere. We analyze the relaxation time in respect to different geometry and material parameters. Further we show that the demagnetization times differ from the behaviour of a single magnetic sphere in the case of low damping. To explain these dynamics nanoparticular systems of different dimensions are investigated. We show that deviations can be attributed to a confinement of the relaxation dynamics to a lower dimensional submanifold of the k-space. (C) 2009 Elsevier B. V. All rights reserved

Magnetic Field Induced Assembly of Highly Ordered Two-Dimensional Particle Arrays

Weddemann A, Wittbracht F, Eickenberg B, Hütten A. Magnetic Field Induced Assembly of Highly Ordered Two-Dimensional Particle Arrays. Langmuir. 2010;26(24):19225-19229.Suspended magnetic beads are exposed to an external homogeneous magnetic field which rotates around the axis perpendicular to the field direction. Because of dipolar interactions, magnetic beads assemble in highly ordered two-dimensional hexagonal arrays perpendicular to the rotation axis. By continuous provision of the particle concentration, the growth modes of two-dimensional particle clusters and monolayers are observed. The structure of the resulting assembled objects is analyzed for different field frequencies and particle concentrations. We identify dynamic processes which enhance stability and reduce lattice distortions and, thus, allow for the application of these particle agglomerations as dynamic components in lab-on-a-chip technologies

On the direct employment of dipolar particle interaction in microfluidic systems

Wittbracht F, Weddemann A, Eickenberg B, Hütten A. On the direct employment of dipolar particle interaction in microfluidic systems. Microfluidics And Nanofluidics. 2012;13(4):543-554.This review article will summarize recent developments in the employment of dipolar coupled magnetic particle structures. We will discuss the basics of magnetic dipolar particle interaction in static and rotating magnetic fields. In dependence on the magnetic fields employed, agglomerates of different dimensionality may form within the carrier liquid. The stability and formation dynamics of these particle structures will be presented. Furthermore, we will review recent microfluidic applications based on the interaction of magnetic particles and present methods for surface patterning with micron-sized and nano-sized particles which employ dipolar particle coupling

A level set based approach for modeling oxidation processes of ligand stabilized metallic nanoparticles

Auge A, Weddemann A, Vogel B, Wittbracht F, Hütten A. A level set based approach for modeling oxidation processes of ligand stabilized metallic nanoparticles. APPLIED PHYSICS LETTERS. 2010;96(9): 93111.The oxidation behavior of metallic nanoparticles is investigated in respect to ligand influences. The nanoparticle oxidation is modeled in a shell-core approach. The shell represents oxidation of surface atoms modeled by Johnson-Mehl-Avrami-Kolmogorov equations for isothermal growth. The oxidation of the nanoparticle core is described by a model introduced by Cabrera and Mott [Rep. Prog. Phys. 12, 163 (1949)]. In order to investigate the ligand influence one single parameter is introduced for both surface and bulk oxidation. The growth of the oxide layer is simulated in a level set framework via finite element methods. The theoretical results are compared to experimental findings of Kanninen [J. Coll. Interf. Sci. 318, 88 (2008)]

A combined reaction-separation lab-on-a-chip device for low Peclet number applications

Weddemann A, Eickenberg B, Wittbracht F, Auge A, Hütten A. A combined reaction-separation lab-on-a-chip device for low Peclet number applications. Journal of Applied Physics. 2009;106(2): 024510.A microfluidic continuous flow lab-on-a-chip structure is presented, for combined reaction and separation implementation. Thereby, the geometry is optimized with respect to a long duration time in the reaction area and a narrow preconditioning along the separation site to enhance the separation yield. We theoretically predict high stability of separation and transport properties of the device against particle diffusion whereas the particle dynamics in the reaction site maintain a diffusive character. We derive a formula setting the limitations of the structure, relating the desired duration of the particles in the reaction site to the particle size under the condition of a narrow spatial particle distribution along the separation site. We experimentally realize the low diffusion limit to prove the theoretically predicted properties of the device. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3176942