Frank Wittbracht

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

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.

Magnetic ratchet for biotechnological applications

Transport and separation of magnetic beads are important in “lab on a chip” environments for biotechnological applications. One possible solution for this is the on-off ratchet concept. An asymmetric magnetic potential and Brownian motion of magnetic beads are required for such a ratchet. The asymmetric magnetic potential is achieved by combining an external magnetic field with a spatially periodic array of conducting lines. In this work finite element method simulations are carried out to design this asymmetric potential and to evaluate transport rates. Furthermore, experiments are carried out so as to compare to the simulation results.

How to design magneto-based total analysis systems for biomedical applications

This article reviews recent developments on magnetoresistive detection of magnetic beads or nanoparticles by nanoscale sized sensors. Sensors are analyzed from an experimental and a numerical point of view in respect to their capability to either localize the position of a single magnetic particle or to detect the number of particles in a certain range. Guidelines are shown up on how to extend single sensors to sensor arrays with very high spatial resolution and how to modify the sensor shape in order to provide long distance measurements. Further, sensors in biological lab-on-a-chip environments are discussed. The magnetic ratchet and a gravitation based microfluidic component are reviewed as important tools to position and, therefore, detect biological components in continuous-flow devices.

A hydrodynamic switch: Microfluidic separation system for magnetic beads

In this work a device for separating small magnetic particles in continuous flow is introduced, consisting of two microfluidic channels that are connected by a junction channel. Applying two different flow rates, particles can be separated combining hydrodynamic and magnetophoretic effects. The two different flow rates introduce an additional degree of freedom that enables the microfluidic geometry to act as a hydrodynamic switch that can overcome diffusive contributions making the device applicable for particles of the size scale below 100 nm. Theoretical predictions based on finite element methods are compared to experimental observations.

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

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.

Giant magnetoresistance effects in gel-like matrices

We present transport measurements with magnetoresistance effect amplitudes of up to 260% at room temperature obtained in granular systems consisting of Co nanoparticles embedded in conductive gels as a non-magnetic matrix. In order to gain a better understanding of the transport mechanism in gel during measurement, the granular system was simultaneously monitored by optical microscopy. Gel-like matrices with different conductivities and viscosities were tested and will allow us to realize a highly sensitive granular giant magnetoresistance sensor without the need for lithographic techniques.

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

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 dissociatio...

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

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 et al. [J. Coll. Interf. Sci. 318, 88 (2008)].

On the resolution limits of tunnel magnetoresistance sensors for particle detection

Elliptical magnetoresistance sensors are analyzed with respect to their capability of high spatial resolution of particle detection. The sensor response with respect to the particle position is investigated by solving the equation of static micromagnetism. We demonstrate how the shape of the sensor can be used to obtain information about the particle position. It is verified that the external fields applied to bring particles into saturation can be utilized to tune the measurement: increasing the external fields will increase the spatial resolution of the sensor but will also reduce the visibility field in which a particle can still be detected.

Toward the magnetoresistive detection of single magnetic nanoparticles: New strategies for particle detection by adjustment of sensor shape

In this work, different approaches in order to enhance the sensitivity of tunnel magnetoresistive sensors are discussed by means of finite element simulations. Several sensor layouts consisting of a free CoFeB sensing layer and a pinned bottom electrode are investigated. A decrease in the detection threshold is predicted by introducing magnetic areas at the sensor boundaries which can be easily switched due to a combined interaction of exchange contribution and stray field coupling of the layers.