Microscopics of disordered two-dimensional electron gases under high magnetic fields: Equilibrium properties and dissipation in the hydrodynamic regime

We develop in detail a new formalism [as a sequel to the work of T. Champel and S. Florens, Phys. Rev. B 75, 245326 (2007)] that is well-suited for treating quantum problems involving slowly-varying potentials at high magnetic fields in two-dimensional electron gases. For an arbitrary smooth potential we show that electronic Green's function is fully determined by closed recursive expressions that take the form of a high magnetic field expansion in powers of the magnetic length l_B. For illustration we determine entirely Green's function at order l_B^3, which is then used to obtain quantum expressions for the local charge and current electronic densities at equilibrium. Such results are valid at high but finite magnetic fields and for arbitrary temperatures, as they take into account Landau level mixing processes and wave function broadening. We also check the accuracy of our general functionals against the exact solution of a one-dimensional parabolic confining potential, demonstrating the controlled character of the theory to get equilibrium properties. Finally, we show that transport in high magnetic fields can be described hydrodynamically by a local equilibrium regime and that dissipation mechanisms and quantum tunneling processes are intrinsically included at the microscopic level in our high magnetic field theory. We calculate microscopic expressions for the local conductivity tensor, which possesses both transverse and longitudinal components, providing a microscopic basis for the understanding of dissipative features in quantum Hall systems.Comment: small typos corrected; published versio

Local pore size correlations determine flow distributions in porous media

The relationship between the microstructure of a porous medium and the observed flow distribution is still a puzzle. We resolve it with an analytical model, where the local correlations between adjacent pores, which determine the distribution of flows propagated from one pore downstream, predict the flow distribution. Numerical simulations of a two-dimensional porous medium verify the model and clearly show the transition of flow distributions from $\delta$-function-like via Gaussians to exponential with increasing disorder. Comparison to experimental data further verifies our numerical approach.Comment: 5 pages, 3 figures, supplemental materia

Enhanced surface acoustic wave cell sorting by 3D microfluidic chip design

We demonstrate an acoustic wave driven microfluidic cell sorter that combines advantages of multilayer device fabrication with planar surface acoustic wave excitation. We harness the strong vertical component of the refracted acoustic wave to enhance cell actuation by using an asymmetric flow field to increase cell deflection. Precise control of the 3-dimensional flow is realized by topographical structures implemented on the top of the microchannel. We experimentally quantify the effect of the structure dimensions and acoustic parameter. The design attains cell sorting rates and purities approaching those of state of the art fluorescence-activated cell sorters with all the advantages of microfluidic cell sorting

The scientific merit of Carpobrotus mellei L. based on antimicrobial activity and chemical profiling

Species of the genus Carpobrotus are similar in appearance and have been used for medicinal purposes over many generations. Carpobrotus mellei is endemic to the south-Western Cape of South Africa, and also used for various ailments. To date no scientific validation and information has been reported on C. mellei. This study investigated the antimicrobial potential of C. mellei against Staphylococcus aureus, Pseudomonas aeruginosa, Candida albicans and Mycobacterium smegmatis, and determined the minimum inhibitory concentrations (MIC) by two-fold serial dilution. C. mellei showed antimicrobial activity against S. aureus and M. smegmatis in the disc diffusion method. Eight chemical compounds showed clear zones of inhibition in the bioautograms, seven against S. aureus and three compounds were active against M. smegmatis. The ethyl acetate extracts have MIC values of 7.5 mg/ml and 15 mg/ml against S. aureus and M. smegmatis, respectively. Phytochemical tests indicated the presence of flavonoids, hydrolysable tannins, phytosterols and aromatic acids. High performance liquid chromatography (HPLC) showed a species-specific spectrum at a wavelength of 280 nm. The results confirm that C. mellei has scientific merit, and can substitute one of the other Carpobrotus species for antimicrobial usage within the south-Western Cape of South Africa

Activity driven fluctuations in living cells

We propose a model for the dynamics of a probe embedded in a living cell, where both thermal fluctuations and nonequilibrium activity coexist. The model is based on a confining harmonic potential describing the elastic cytoskeletal matrix, which undergoes random active hops as a result of the nonequilibrium rearrangements within the cell. We describe the probe's statistics and we bring forth quantities affected by the nonequilibrium activity. We find an excellent agreement between the predictions of our model and experimental results for tracers inside living cells. Finally, we exploit our model to arrive at quantitative predictions for the parameters characterizing nonequilibrium activity, such as the typical time scale of the activity and the amplitude of the active fluctuations.Comment: 6 pages, 4 figure

Monitoring spatially heterogeneous dynamics in a drying colloidal thin film

We report on a new type of experiment that enables us to monitor spatially and temporally heterogeneous dynamic properties in complex fluids. Our approach is based on the analysis of near-field speckles produced by light diffusely reflected from the superficial volume of a strongly scattering medium. By periodic modulation of an incident speckle beam we obtain pixel-wise ensemble averages of the structure function coefficient, a measure of the dynamic activity. To illustrate the application of our approach we follow the different stages in the drying process of a colloidal thin film. We show that we can access ensemble averaged dynamic properties on length scales as small as ten micrometers over the full field of view.Comment: To appear in Soft Material

Study of effects of fuel properties in turbine-powered business aircraft

Increased interest in research and technology concerning aviation turbine fuels and their properties was prompted by recent changes in the supply and demand situation of these fuels. The most obvious change is the rapid increase in fuel price. For commercial airplanes, fuel costs now approach 50 percent of the direct operating costs. In addition, there were occasional local supply disruptions and gradual shifts in delivered values of certain fuel properties. Dwindling petroleum reserves and the politically sensitive nature of the major world suppliers make the continuation of these trends likely. A summary of the principal findings, and conclusions are presented. Much of the material, especially the tables and graphs, is considered in greater detail later. The economic analysis and examination of operational considerations are described. Because some of the assumptions on which the economic analysis is founded are not easily verified, the sensitivity of the analysis to alternates for these assumptions is examined. The data base on which the analyses are founded is defined in a set of appendices

Skating on a Film of Air: Drops Impacting on a Surface

Drops impacting on a surface are ubiquitous in our everyday experience. This impact is understood within a commonly accepted hydrodynamic picture: it is initiated by a rapid shock and a subsequent ejection of a sheet leading to beautiful splashing patterns. However, this picture ignores the essential role of the air that is trapped between the impacting drop and the surface. Here we describe a new imaging modality that is sensitive to the behavior right at the surface. We show that a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the drop spreads. The thin film of air serves to lubricate the drop enabling the fluid to skate on the air film laterally outward at surprisingly high velocities, consistent with theoretical predictions. Eventually this thin film of air must break down as the fluid wets the surface. We suggest that this occurs in a spinodal-like fashion, and causes a very rapid spreading of a wetting front outwards; simultaneously the wetting fluid spreads inward much more slowly, trapping a bubble of air within the drop. Our results show that the dynamics of impacting drops are much more complex than previously thought and exhibit a rich array of unexpected phenomena that require rethinking classical paradigms.Comment: 4 pages, 4 figure