Pattern formation in directional solidification under shear flow. I: Linear stability analysis and basic patterns

An asymptotic interface equation for directional solidification near the absolute stabiliy limit is extended by a nonlocal term describing a shear flow parallel to the interface. In the long-wave limit considered, the flow acts destabilizing on a planar interface. Moreover, linear stability analysis suggests that the morphology diagram is modified by the flow near the onset of the Mullins-Sekerka instability. Via numerical analysis, the bifurcation structure of the system is shown to change. Besides the known hexagonal cells, structures consisting of stripes arise. Due to its symmetry-breaking properties, the flow term induces a lateral drift of the whole pattern, once the instability has become active. The drift velocity is measured numerically and described analytically in the framework of a linear analysis. At large flow strength, the linear description breaks down, which is accompanied by a transition to flow-dominated morphologies, described in a companion paper. Small and intermediate flows lead to increased order in the lattice structure of the pattern, facilitating the elimination of defects. Locally oscillating structures appear closer to the instability threshold with flow than without.Comment: 20 pages, Latex, accepted for Physical Review

Extending the scope of microscopic solvability: Combination of the Kruskal-Segur method with Zauderer decomposition

Successful applications of the Kruskal-Segur approach to interfacial pattern formation have remained limited due to the necessity of an integral formulation of the problem. This excludes nonlinear bulk equations, rendering convection intractable. Combining the method with Zauderer's asymptotic decomposition scheme, we are able to strongly extend its scope of applicability and solve selection problems based on free boundary formulations in terms of partial differential equations alone. To demonstrate the technique, we give the first analytic solution of the problem of velocity selection for dendritic growth in a forced potential flow.Comment: Submitted to Europhys. Letters, No figures, 5 page

What are the interactions in quantum glasses?

The form of the low-temperature interactions between defects in neutral glasses is reconsidered. We analyse the case where the defects can be modelled either as simple 2-level tunneling systems, or tunneling rotational impurities. The coupling to strain fields is determined up to 2nd order in the displacement field. It is shown that the linear coupling generates not only the usual $1/r^3$ Ising-like interaction between the rotational tunneling defect modes, which cause them to freeze around a temperature $T_G$, but also a random field term. At lower temperatures the inversion symmetric tunneling modes are still active - however the coupling of these to the frozen rotational modes, now via the 2nd-order coupling to phonons, generates another random field term acting on the inversion symmetric modes (as well as shorter-range $1/r^5$ interactions between them). Detailed expressions for all these couplings are given.Comment: 12 pages, 2 figures. Minor modifications, published versio

Integrated atomic quantum technologies in demanding environments: Development and qualification of miniaturized optical setups and integration technologies for UHV and space operation

Employing compact quantum sensors in field or in space (e.g., small satellites) implies demanding requirements on components and integration technologies. Within our work on integrated sensors, we develop miniaturized, ultra-stable optical setups for optical cooling and trapping of cold atomic gases. Besides challenging demands on alignment precision, and thermo-mechanical durability, we specifically address ultra-high vacuum (UHV) compatibility of our integration technologies and optical components. A prototype design of an UHV-compatible, crossed beam optical dipole trap setup and its application within a cold atomic quantum sensor is described. First qualification efforts on adhesive micro-integration technologies are presented. These tests are conducted in application-relevant geometries and material combinations common for micro-integrated optical setups. Adhesive aging will be investigated by thermal cycling or gamma radiation exposure. For vacuum compatibility testing, a versatile UHV testing system is currently being set up, enabling residual gas analysis and measurement of total gas rates down to 5•10-10mbar l/s at a base pressure of 10-11 mbar, exceeding the common ASTM E595 test

Influence of external flows on crystal growth: numerical investigation

We use a combined phase-field/lattice-Boltzmann scheme [D. Medvedev, K. Kassner, Phys. Rev. E {\bf 72}, 056703 (2005)] to simulate non-facetted crystal growth from an undercooled melt in external flows. Selected growth parameters are determined numerically. For growth patterns at moderate to high undercooling and relatively large anisotropy, the values of the tip radius and selection parameter plotted as a function of the Peclet number fall approximately on single curves. Hence, it may be argued that a parallel flow changes the selected tip radius and growth velocity solely by modifying (increasing) the Peclet number. This has interesting implications for the availability of current selection theories as predictors of growth characteristics under flow. At smaller anisotropy, a modification of the morphology diagram in the plane undercooling versus anisotropy is observed. The transition line from dendrites to doublons is shifted in favour of dendritic patterns, which become faster than doublons as the flow speed is increased, thus rendering the basin of attraction of dendritic structures larger. For small anisotropy and Prandtl number, we find oscillations of the tip velocity in the presence of flow. On increasing the fluid viscosity or decreasing the flow velocity, we observe a reduction in the amplitude of these oscillations.Comment: 10 pages, 7 figures, accepted for Physical Review E; size of some images had to be substantially reduced in comparison to original, resulting in low qualit

High temperature properties of alloys being considered for design of a concentric canister launcher

This report describes a study to determine the high temperature mechanical properties of several titanium alloys and to compare them with properties of AISI 316L stainless steel and ASTM A 387 structural steel. The steel materials are less costly to procure but exhibit good resistance to corrosion in seawater environments. Six titanium alloys were evaluated as candidate materials for use in a c Concentric Canister Launcher (CCL). Each titanium alloy was tested at three temperatures (68°, 2000°F, and 2400°F). Strain-rate changes tests were used to determine the strain rate sensitivity of the alloys at each test temperature. Optical metallography was performed on two of the alloys to determine the relationship between test temperature and microstructure (presence of second phase precipitates, grain size). Complete test results are includes, a long with figures and tables of test data

Immunoadsorption of agonistic autoantibodies against α1-adrenergic receptors in patients with mild to moderate dementia

Dementia has been shown to be associated with agonistic autoantibodies. The deleterious action of autoantibodies on the {alpha}1-adrenergic receptor for brain vasculature has been demonstrated in animal studies. In the current study, 169 patients with dementia were screened for the presence of agonistic autoantibodies. 47% of patients suffering from mild to moderate Alzheimer's disease and/or vascular dementia carried these autoantibodies. Eight patients positive for autoantibodies underwent immunoadsorption. Patients treated on four consecutive days were subsequently negative for autoantibodies and displayed stabilization of cognitive and mental condition during 12-18 months' follow-up. In patients treated for 2-3 days, autoantibodies were reduced by only 78%. They suffered a rebound of autoantibodies during follow-up, benefited from immunoadsorption too, but their mental parameters worsened. We provide first data on the clinical relevance of agonistic autoantibodies in dementia and show that immunoadsorption is safe and efficient in removing autoantibodies with overall benefits for patients

Design and development of a low-cost mask-type eye tracker to collect quality fixation measurements in the sport domain

The aim of the study was to build a low-cost mask-type eye tracker with accuracy and precision levels similar to those reported for commercial eye tracking devices. To this end, head-mounted hardware was designed and developed, while open-source software was modified for digital image capture, manipulation, and fixation analysis. An image recognition application was also included with different lighting scenarios. Moreover, parallax and viewing perspective errors were controlled to ensure the quality of data collection. The device was wireless and lightweight (99 g) to allow for natural movement and avoid participant discomfort. After calibration of a 9-target monocular grid, spatial accuracy and precision of the eye tracker was evaluated by 30 participants, at four different lighting setups, both before and after a climbing task. Validity tests showed high levels of accuracy in all conditions as evidenced by a systematic error for a 13-target grid of <0.5°. The reliability tests also showed consistent measurements with no differences in accuracy recorded between participants, lighting conditions, and visual behaviors for the pre- versus post-climbing task. These results suggest that the present eye tracker reports spatial accuracy similar to other commercial systems with levels of high quality. Altogether, this innovative user interface is suitable for research purposes and/or performance analysis in physical activity and sport-related activities. Also, features of this mask-type eye tracking system make it a suitable perceptual user interface to investigate human–computer interactions in a large number of other research fields including psychology, education, marketing, transportation, and medicine