Anisotropic Electron Spin Lifetime in (In,Ga)As/GaAs (110) Quantum Wells

Anisotropic electron spin lifetimes in strained undoped (In,Ga)As/GaAs (110) quantum wells of different width and height are investigated by time-resolved Faraday rotation and time-resolved transmission and are compared to the (001)-orientation. From the suppression of spin precession, the ratio of in-plane to out-of-plane spin lifetimes is calculated. Whereas the ratio increases with In concentration in agreement with theory, a surprisingly high anisotropy of 480 is observed for the broadest quantum well, when expressed in terms of spin relaxation times.Comment: 4 pages, 4 figures, revise

Discrete element modeling and fibre optical measurements for fluidized bed spray granulation

Spout fluidized beds are frequently used for the production of granules or\ud particles through granulation. The products find application in a large variety of\ud applications, for example detergents, fertilizers, pharmaceuticals and food. Spout fluidized\ud beds have a number of advantageous properties, such as a high mobility of the particles,\ud which prevents undesired agglomeration and yields excellent heat transfer properties. The\ud particle growth mechanism in a spout fluidized bed as function of particle-droplet\ud interaction has a profound influence on the particle morphology and thus on the product\ud quality. Nevertheless, little is known about the details of the granulation process. This is\ud mainly due to the fact that the granulation process is not visually accessible. In this work\ud we use fundamental, deterministic models to enable the detailed investigation of\ud granulation behaviour in a spout fluidized bed. A discrete element model is used\ud describing the dynamics of the continuous gas-phase and the discrete droplets and\ud particles. For each element momentum balances are solved. The momentum transfer\ud among each of the three phases is described in detail at the level of individual elements.\ud The results from the discrete element model simulations are compared with local\ud measurements of particle volume fractions as well as particle velocities by using a novel\ud fibre optical probe in a fluidized bed of 400 mm I.D. Simulations and experiments were\ud carried out for two different cases using Geldart B type aluminium oxide particles: a\ud freely bubbling fluidized bed and a spout fluidized bed with the presence of droplets. It is\ud demonstrated how the discrete element model can be used to obtain information about the\ud interaction of the discrete phases, i.e. the growth zone in a spout fluidized bed. Eventually\ud this kind of information can be used to obtain closure information required in more coarse\ud grained model

Hard rod gas with long-range interactions: Exact predictions for hydrodynamic properties of continuum systems from discrete models

One-dimensional hard rod gases are explicitly constructed as the limits of discrete systems: exclusion processes involving particles of arbitrary length. Those continuum many-body systems in general do not exhibit the same hydrodynamic properties as the underlying discrete models. Considering as examples a hard rod gas with additional long-range interaction and the generalized asymmetric exclusion process for extended particles ($\ell$-ASEP), it is shown how a correspondence between continuous and discrete systems must be established instead. This opens up a new possibility to exactly predict the hydrodynamic behaviour of this continuum system under Eulerian scaling by solving its discrete counterpart with analytical or numerical tools. As an illustration, simulations of the totally asymmetric exclusion process ($\ell$-TASEP) are compared to analytical solutions of the model and applied to the corresponding hard rod gas. The case of short-range interaction is treated separately.Comment: 19 pages, 8 figure

Comparison of fibre optical measurements and discrete element simulations for the study of granulation in a spout fluidized bed

Spout fluidized beds are frequently used for the production of granules or particles through granulation. The products find application in a large variety of applications, for example detergents, fertilizers, pharmaceuticals and food. Spout fluidized beds have a number of advantageous properties, such as a high mobility of the particles, which prevents undesired agglomeration and yields excellent heat transfer properties. The particle growth mechanism in a spout fluidized bed as function of particle-droplet interaction has a profound influence on the particle morphology and thus on the product quality. Nevertheless, little is known about the details of the granulation process. This is mainly due to the fact that the granulation process is not visually accessible. In this work we use fundamental, deterministic models to enable the detailed investigation of granulation behaviour in a spout fluidized bed. A discrete element model is used describing the dynamics of the continuous gas-phase and the discrete droplets and particles. For each element momentum balances are solved. The momentum transfer among each of the three phases is described in detail at the level of individual elements. The results from the discrete element model simulations are compared with local measurements of particle volume fractions as well as particle velocities by using a novel fibre optical probe in a fluidized bed of 400 mm I.D. Simulations and experiments were carried out for three different cases using Geldart B type aluminium oxide particles: a freely bubbling fluidized bed; a spout fluidized bed without the presence of droplets and a spout fluidized bed with the presence of droplets. It is demonstrated how the discrete element model can be used to obtain information about the interaction of the discrete phases, i.e. the growth zone in a spout fluidized bed. Eventually this kind of information can be used to obtain closure information required in more coarse grained models

Detergency and its implications for oil emulsion sieving and separation

Separating petroleum hydrocarbons from water is an important problem to address in order to mitigate the disastrous effects of hydrocarbons on aquatic ecosystems. A rational approach to address the problem of marine oil water separation is to disperse the oil with the aid of surfactants in order to minimize the formation of large slicks at the water surface and to maximize the oil-water interfacial area. Here we investigate the fundamental wetting and transport behavior of such surfactant-stabilized droplets and the flow conditions necessary to perform sieving and separation of these stabilized emulsions. We show that, for water soluble surfactants, such droplets are completely repelled by a range of materials (intrinsically underwater superoleophobic) due to the detergency effect; therefore, there is no need for surface micro/nanotexturing or chemical treatment to repel the oil and prevent fouling of the filter. We then simulate and experimentally investigate the effect of emulsion flow rate on the transport and impact behavior of such droplets on rigid meshes to identify the minimum pore opening (w) necessary to filter a droplet with a given diameter (d) in order to minimize the pressure drop across the mesh and therefore maximize the filtering efficiency, which is strongly dependent on w. We define a range of flow conditions and droplet sizes where minimum droplet deformation is to be expected and therefore find that the condition of is sufficient for efficient separation. With this new understanding, we demonstrate the use of a commercially available filter--without any additional surface engineering or functionalization--to separate oil droplets from a surfactant stabilized emulsion with a flux of 11,000 L m$^{-2}$ hr$^{-1}$ bar$^{-1}$. We believe these findings can inform the design of future oil separation materials

Participation and satisfaction after spinal cord injury: results of a vocational and leisure outcome study

Study design: Survey. Objectives: Insight in (1) the changes in participation in vocational and leisure activities and (2) satisfaction with the current participation level of people with spinal cord injuries (SCIs) after reintegration in society. Design: Descriptive analysis of data from a questionnaire. Setting: Rehabilitation centre with special department for patients with SCIs, Groningen, The Netherlands. Subjects: A total of 57 patients with traumatic SCI living in the community, who were admitted to the rehabilitation centre two to 12 years before the current assessment. Main outcome measures: Changes in participation in activities; current life satisfaction; support and unmet needs. Results: Participation expressed in terms of hours spent on vocational and leisure activities changed to a great extent after the SCI. This was mainly determined by a large reduction of hours spent on paid work. While 60% of the respondents successfully reintegrated in work, many changes took place in the type and extent of the job. Loss of work was partially compensated with domestic and leisure activities. Sports activities were reduced substantially. The change in participation level and compensation for the lost working hours was not significantly associated with the level of SCI-specific health problems and disabilities. As was found in other studies, most respondents were satisfied with their lives. Determinants of a negative life satisfaction several years following SCI were not easily indicated. Reduced quality of life was particularly related to an unsatisfactory work and leisure situation. Conclusions: Most people with SCI in this study group were able to resume work and were satisfied with their work and leisure situation

Use of Liquid Propellants in Pulsed Plasma Thrusters for Small Satellites

Abstract: There is increasing interest in small satellites weighing in the range of 100 kg or less. In particular, satellites weighing 1 -10 kg currently lack options for a suitable propulsion system for performing large-velocity-change (∆V) maneuvers. Pulsed plasma thrusters are a simple form of electric propulsion that have been operated in orbit many times since the beginning of space exploration. Due to their inherently simple design, they are very attractive for use in small satellites. However, issues such as charring of the solid propellant and late-time ablation can contribute to lowering of the overall thruster lifetime and the propellant utilization efficiency. The use of liquid propellants in pulsed plasma thrusters can potentially resolve some of these problems. We are currently investigating two very different approaches to the implementation of liquid delivery in pulsed plasma thrusters: passive ablative liquid-feeding and active electrospray-assisted droplet delivery. The former uses a non-volatile liquid propellant that is passively delivered using capillary forces, while the latter employs the electrospray process with a volatile liquid propellant to deliver very small liquid droplets to the thruster. Some preliminary data is presented here along with recommendations for future designs. The two different approaches have the potential to increase the efficiency and versatility of liquid-fed pulsed plasma thrusters

Exclusion process for particles of arbitrary extension: Hydrodynamic limit and algebraic properties

The behaviour of extended particles with exclusion interaction on a one-dimensional lattice is investigated. The basic model is called $\ell$-ASEP as a generalization of the asymmetric exclusion process (ASEP) to particles of arbitrary length $\ell$. Stationary and dynamical properties of the $\ell$-ASEP with periodic boundary conditions are derived in the hydrodynamic limit from microscopic properties of the underlying stochastic many-body system. In particular, the hydrodynamic equation for the local density evolution and the time-dependent diffusion constant of a tracer particle are calculated. As a fundamental algebraic property of the symmetric exclusion process (SEP) the SU(2)-symmetry is generalized to the case of extended particles

Exactly solvable interacting vertex models

We introduce and solvev a special family of integrable interacting vertex models that generalizes the well known six-vertex model. In addition to the usual nearest-neighbor interactions among the vertices, there exist extra hard-core interactions among pair of vertices at larger distances.The associated row-to-row transfer matrices are diagonalized by using the recently introduced matrix product {\it ansatz}. Similarly as the relation of the six-vertex model with the XXZ quantum chain, the row-to-row transfer matrices of these new models are also the generating functions of an infinite set of commuting conserved charges. Among these charges we identify the integrable generalization of the XXZ chain that contains hard-core exclusion interactions among the spins. These quantum chains already appeared in the literature. The present paper explains their integrability.Comment: 20 pages, 3 figure

4U 1909+07: a well-hidden pearl

We present the first detailed spectral and timing analysis of the High Mass X-ray Binary (HMXB) 4U 1909+07 with INTEGRAL and RXTE. 4U 1909+07 is detected in the ISGRI 20-40 keV energy band with an average countrate of 2.6 cps. The pulse period of ~604 sec is not stable, but changing erratically on timescales of years. The pulse profile is strongly energy dependent: it shows a double peaked structure at low energies, the secondary pulse decreases rapidly with increasing energy and above 20 keV only the primary pulse is visible. This evolution is consistent between PCA, HEXTE, and ISGRI. The phase averaged spectrum can be well described by the sum of a photoabsorbed power law with a cutoff at high energies and a blackbody component. To investigate the pulse profile, we performed phase resolved spectral analysis. We find that the changing spectrum can be best described with a variation of the folding energy. We rule out a correlation between the black body component and the continuum variation and discuss possible accretion geometries.Comment: 9 pages, 11 figures, accepted for publication in A&A Sect.