Kang K.,Forschungszentrum Jlich
Review of Scientific Instruments | Year: 2011
In this paper, a detailed description of equipment is given, specially designed to characterize the response of non-spherical colloids to external fields. To characterize slow structural changes on a large length scale we developed an image correlation method, fast dynamics on the colloidal-particle level is probed by means of a vertically mounted, small angle dynamic light scattering setup, while the orientational order (induced by the external field) is measured with a birefringence setup with off-normal incidence. The performance of this in situ set of instruments is illustrated by experiments on concentrated dispersions of very long and thin, charged colloidal rods (fd-virus particles) in external electric fields. Here, the frequency of the field is sufficiently low to polarize electrical double layers, leading to additional inter-colloidal interactions which are found to give rise to phase/state transitions and dynamical states (K. Kang and J. K. G. Dhont, Soft Matter 6, 273, 2010). © 2011 American Institute of Physics.
Meles G.A.,ETH Zurich |
Van Der Kruk J.,Forschungszentrum Jlich |
Greenhalgh S.A.,ETH Zurich |
Greenhalgh S.A.,University of Adelaide |
And 3 more authors.
IEEE Transactions on Geoscience and Remote Sensing | Year: 2010
We have developed a new full-waveform groundpenetrating radar (GPR) multicomponent inversion scheme for imaging the shallow subsurface using arbitrary recording configurations. It yields significantly higher resolution images than conventional tomographic techniques based on first-arrival times and pulse amplitudes. The inversion is formulated as a nonlinear least squares problem in which the misfit between observed and modeled data is minimized. The full-waveform modeling is implemented by means of a finite-difference time-domain solution of Maxwell's equations. We derive here an iterative gradient method in which the steepest descent direction, used to update iteratively the permittivity and conductivity distributions in an optimal way, is found by cross-correlating the forward vector wavefield and the backward-propagated vectorial residual wavefield. The formulation of the solution is given in a very general, albeit compact and elegant, fashion. Each iteration step of our inversion scheme requires several calculations of propagating wavefields. Novel features of the scheme compared to previous full-waveform GPR inversions are as follows: 1) The permittivity and conductivity distributions are updated simultaneously (rather than consecutively) at each iterative step using improved gradient and step length formulations; 2) the scheme is able to exploit the full vector wavefield; and 3) various data sets/survey types (e.g., crosshole and borehole-to-surface) can be individually or jointly inverted. Several synthetic examples involving both homogeneous and layered stochastic background models with embedded anomalous inclusions demonstrate the superiority of the new scheme over previous approaches. © 2006 IEEE.
Briels W.J.,University of Twente |
Vlassopoulos D.,University of Crete |
Kang K.,Forschungszentrum Jlich |
Dhont J.K.G.,Forschungszentrum Jlich
Journal of Chemical Physics | Year: 2011
A semimicroscopic derivation is presented of equations of motion for the density and the flow velocity of concentrated systems of entangled polymers. The essential ingredient is the transient force that results from perturbations of overlapping polymers due to flow. A Smoluchowski equation is derived that includes these transient forces. From this, an equation of motion for the polymer number density is obtained, in which body forces couple the evolution of the polymer density to the local velocity field. Using a semimicroscopic Ansatz for the dynamics of the number of entanglements between overlapping polymers, and for the perturbations of the pair-correlation function due to flow, body forces are calculated for nonuniform systems where the density as well as the shear rate varies with position. Explicit expressions are derived for the shear viscosity and normal forces, as well as for nonlocal contributions to the body force, such as the shear-curvature viscosity. A contribution to the equation of motion for the density is found that describes mass transport due to spatial variation of the shear rate. The two coupled equations of motion for the density and flow velocity predict flow instabilities that will be discussed in more detail in a forthcoming publication. © 2011 American Institute of Physics.
Thomsen K.,Paul Scherrer Institute |
Butzek M.,Forschungszentrum Jlich |
Gallmeier F.,Oak Ridge National Laboratory |
Wolters J.,Forschungszentrum Jlich
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2011
In the following short article preliminary results concerning the high-power suitability of a solid-state target cooled by water as in SINQ are reported as available by the summer of 2010. The assessment warrants further and more detailed studies into an approach that appears to combine in an ideal way design maturity, ample positive operational experience, reliability, upgradability and minimum hazards and risks with acceptable neutron production. © 2010 Elsevier B.V. All rights reserved.
Divin Y.,Forschungszentrum Jlich |
Lyatti M.,Forschungszentrum Jlich |
Lyatti M.,RAS Institute of Radio Engineering and Electronics |
Poppe U.,Forschungszentrum Jlich |
And 2 more authors.
IEEE Transactions on Applied Superconductivity | Year: 2011
One of future public security techniques will be related with non-invasive, fast and reliable detection of liquids. To distinguish between liquids under concern, we have suggested a concept using our Hilbert spectroscopy, based on high- Yc Josephson junctions. This spectroscopy is the only technique, which covers a frequency range of main dispersions of liquids from a few GHz to a few THz. Several demonstration setups of liquid identifiers, consisting of Hilbert spectrometers integrated on Stirling coolers and polychromatic radiation sources, have been developed. A critical consideration of the main sources of fluctuations in these measurements was carried out and a signal accuracy of around 0.3% has been reached with a total measurement time of a few seconds. Identification of samples of benign and threat liquids was demonstrated. © 2011 IEEE.