Norbert Riefler

Single-particle evanescent light scattering simulations for total internal reflection microscopy.

We simulate and measure light scattering of a micrometer-sized spherical particle suspended in solution close to a glass substrate. The model, based on the discrete sources method, is developed to describe the experimental situation of total internal reflection microscopy experiments; i.e., the particle is illuminated by an evanescent light field originating from the glass-solvent interface. In contrast to the well-established assumption of a simple exponential decay of the scattering intensity with distance, we demonstrate significant deviations for a certain range of penetration depths and polarization states of the incident light.

Investigation of a measurement technique to estimate concentration and size of inclusions in droplets

Inhomogeneous droplets including small spherical inclusions are characterized by the estimation of droplet diameter, inclusion concentration and inclusion diameter and a measure for the polydispersity of the inclusions. In most cases, it is reasonable to assume that the material parameters are known and therefore the index of refraction and the shape of inclusions. In this paper, the results from a measurement technique are investigated. The method will be evaluated on the basis of light scattering measurements for a range of scattering angles. These measurements have been taken with a fast CCD line scan camera and appropriate optics. An attempt is made to derive information from these measurements only. The continuous wavelet transform, speckle image analysis and turbidity measurement methods are used to estimate the concentration and the diameter of the monodisperse polystyrene particles within a droplet. The droplets, generated by a drop-on-demand droplet generator, are nearly monodisperse. The volume concentration of the inclusions within the suspensions varies between 0.01% and 9%. The inclusions are monodisperse. However, it seems to be possible that they coagulate due to the fast fluid flows at the droplet generation. As a result, the technique may be used only for the estimation of average values of size and concentration of inclusions from the measurements.

Intercomparison of Numerical Inversion Algorithms for Particle Size Determination of Polystyrene Suspensions Using Spectral Turbidimetry

The continuous monitoring of the particle size distribution in particulate processes with suspensions or emulsions requires measurement techniques that can be used as in situ devices in contrast to ex situ or laboratory methods. In this context, for the evaluation of turbidimetric spectral measurements, the application of different numerical inversion algorithms is investigated with respect to the particle size distribution determination of polystyrene suspensions. A modified regularization concept consisting of a Twomey-Phillips-Regularization with an integrated nonnegative constraint and a modified L-curve criterion for the selection of the regularization parameter is used. The particle size (i.e., particle diameter) of polystyrene suspensions in the range –3 µm was validated via dynamic light scattering and differential centrifugal sedimentation and compared to the retrieved particle size distribution from the inverted turbidimetry measurements.

Flexible Piezoelectric Drop-On-Demand Droplet Generation

We would like to thank the German Research Foundation (DFG) for financial support of this project.

T-matrix Simulation of Plasmon Resonances of Particles on or Near a Surface

We present the light scattering response of gold and silver particles on or near surfaces consisting of different materials. A comparison is made between a particle near a perfectly conducting surface and near a gold surface. The resulting scattering diagrams are found to be different. Beyond this, an approximation with a mirror particle shows little aggrement with a particle near a metal surface. Furthermore, we compare the spectral response of a combination of gold and silver materials for particles at different heights. Surface plasmons of small noble metal spheres can be detected as resonance peaks in the measured light scattering spectra. Transmission dark field microscopy is a technique where only the particles scatter light into the direction of the microscope objective. Such a measuring device can visualize very small particles as colored discs. The surface plasmon resonance frequency from a nonspherical particle or from a particle aggregate is different compared with a single spherical particle. With this effect, measuring techniques which use white light as illumination are capable to differ between aggregated particles and a single particle because of their different color. Even when a bio receptor molecule attached to a gold or silver sphere detects a biomolecular counterpart, the resonance frequency changes. In the following we first describe the underlying scattering theory. Then we give some simulation examples of particles on or near a surface. We compare these results to some approximations found in the literature. This leads us to statements about the applicability of these approximations. 2. Theory