1,853 research outputs found
Self-assembled granular walkers
Mechanisms of locomotion in microscopic systems are of great interest not
only for technological applications, but also for the sake of understanding,
and potentially harnessing, processes far from thermal equilibrium.
Down-scaling is a particular challenge, and has led to a number of interesting
concepts including thermal ratchet systems and asymmetric swimmers. Here we
present a system which is particularly intriguing, as it is self-assembling and
uses a robust mechanism which can be implemented in various settings. It
consists of small spheres of different size which adhere to each other, and are
subject to an oscillating (zero average) external force eld. An inherent
nonlinearity in the mutual force network leads to force rectication and hence
to locomotion. We present a model that accounts for the observed behaviour and
demonstrates the wide applicability and potential scalability of the concept.Comment: 17 pages, 4 figure
Capillary focusing close to a topographic step: Shape and instability of confined liquid filaments
Step-emulsification is a microfluidic technique for droplet generation which
relies on the abrupt decrease of confinement of a liquid filament surrounded by
a continuous phase. A striking feature of this geometry is the transition
between two distinct droplet breakup regimes, the "step-regime" and
"jet-regime", at a critical capillary number. In the step-regime, small and
monodisperse droplets break off from the filament directly at a topographic
step, while in the jet-regime a jet protrudes into the larger channel region
and large plug-like droplets are produced. We characterize the breakup behavior
as a function of the filament geometry and the capillary number and present
experimental results on the shape and evolution of the filament for a wide
range of capillary numbers in the jet-regime. We compare the experimental
results with numerical simulations. Assumptions based on the smallness of the
depth of the microfluidic channel allow to reduce the governing equations to
the Hele-Shaw problem with surface tension. The full nonlinear equations are
then solved numerically using a volume-of-fluid based algorithm. The
computational framework also captures the transition between both regimes,
offering a deeper understanding of the underlying breakup mechanism
Spin-orbit induced longitudinal spin-polarized currents in non-magnetic solids
For certain non-magnetic solids with low symmetry the occurrence of
spin-polarized longitudinal currents is predicted. These arise due to an
interplay of spin-orbit interaction and the particular crystal symmetry. This
result is derived using a group-theoretical scheme that allows investigating
the symmetry properties of any linear response tensor relevant to the field of
spintronics. For the spin conductivity tensor it is shown that only the
magnetic Laue group has to be considered in this context. Within the introduced
general scheme also the spin Hall- and additional related transverse effects
emerge without making reference to the two-current model. Numerical studies
confirm these findings and demonstrate for (AuPt)Sc that
the longitudinal spin conductivity may be in the same order of magnitude as the
conventional transverse one. The presented formalism only relies on the
magnetic space group and therefore is universally applicable to any type of
magnetic order.Comment: 5 pages, 1 table, 2 figures (3 & 2 subfigures
Shape of a liquid front upon dewetting
We examine the profile of a liquid front of a film that is dewetting a solid
substrate. Since volume is conserved, the material that once covered the
substrate is accumulated in a rim close to the three phase contact line.
Theoretically, such a profile of a Newtonian liquid resembles an exponentially
decaying harmonic oscillation that relaxes into the prepared film thickness.
For the first time, we were able to observe this behavior experimentally. A
non-Newtonian liquid - a polymer melt - however, behaves differently. Here,
viscoelastic properties come into play. We will demonstrate that by analyzing
the shape of the rim profile. On a nm scale, we gain access to the rheology of
a non-Newtonian liquid.Comment: 4 pages, 4 figure
Generic morphologies of viscoelastic dewetting fronts
A simple model is put forward which accounts for the occurrence of certain
generic dewetting morphologies in thin liquid coatings. It demonstrates that by
taking into account the elastic properties of the coating, a morphological
phase diagram may be derived which describes the observed structures of
dewetting fronts. It is demonstrated that dewetting morphologies may also serve
to determine nanoscale rheological properties of liquids.Comment: 4 pages, 2 figure
Energy dissipation in sheared wet granular assemblies
Energy dissipation in sheared dry and wet granulates is considered in the presence of an externally applied confining pressure. Discrete element simulations reveal that for sufficiently small confining pressures, the energy dissipation is dominated by the effects related to the presence of cohesive forces between the particles. The residual resistance against shear can be quantitatively explained by a combination of two effects arising in a wet granulate: (i) enhanced friction at particle contacts in the presence of attractive capillary forces and (ii) energy dissipation due to the rupture and reformation of liquid bridges. Coulomb friction at grain contacts gives rise to an energy dissipation which grows linearly with increasing confining pressure for both dry and wet granulates. Because of a lower Coulomb friction coefficient in the case of wet grains, as the confining pressure increases the energy dissipation for dry systems is faster than for wet ones
Production of negative-emissions steel using a reducing gas derived from dfb gasification
A dual fluidized bed (DFB) gasification process is proposed to produce sustainable reducing gas for the direct reduction (DR) of iron ore. This novel steelmaking route is compared with the established process for DR, which is based on natural gas, and with the emerging DR technology using electrolysis-generated hydrogen as the reducing gas. The DFB-DR route is found to produce reducing gas that meets the requirement of the DR reactor, based on existing MIDREX plants, and which is produced with an energetic efficiency comparable with the natural gas route. The DFB-DR path is the only route considered that allows negative CO2 emissions, enabling a 145% decrease in emissions relative to the traditional blast furnaceâbasic oxygen furnace (BFâBOF) route. A reducing gas cost between 45â60 EUR/MWh is obtained, which makes it competitive with the hydrogen route, but not the natural gas route. The cost estimation for liquid steel production shows that, in Sweden, the DFB-DR route cannot compete with the natural gas and BFâBOF routes without a cost associated with carbon emissions and a revenue attributed to negative emissions. When the cost and revenue are set as equal, the DFB-DR route becomes the most competitive for a carbon price >60 EUR/tCO2
The PETfold and PETcofold web servers for intra- and intermolecular structures of multiple RNA sequences
The function of non-coding RNA genes largely depends on their secondary structure and the interaction with other molecules. Thus, an accurate prediction of secondary structure and RNAâRNA interaction is essential for the understanding of biological roles and pathways associated with a specific RNA gene. We present web servers to analyze multiple RNA sequences for common RNA structure and for RNA interaction sites. The web servers are based on the recent PET (Probabilistic Evolutionary and Thermodynamic) models PETfold and PETcofold, but add user friendly features ranging from a graphical layer to interactive usage of the predictors. Additionally, the web servers provide direct access to annotated RNA alignments, such as the Rfam 10.0 database and multiple alignments of 16 vertebrate genomes with human. The web servers are freely available at: http://rth.dk/resources/petfold
Dual Fluidized Bed Gasification Configurations for Carbon Recovery from Biomass
Techniques that produce chemicals and fuels from sustainable carbon sources will have to maximize the carbon recovery to support circularity. In dual fluidized bed (DFB) gasification, to facilitate carbon recovery, the CO2\ua0from the flue gas can be concentrated using pure oxygen as an oxidant. The heat required by the process can also be provided electrically or by oxidizing an oxygen-carrying bed material, rather than combusting part of the char, thereby concentrating all of the carbon in the syngas. In this work, the three configurations of oxyfuel, electrical, and chemical-looping gasification (CLG) are compared to each other, as well as to the standard or âairâ configuration, which corresponds to the combustion of char with air and the separation of CO2\ua0from both the flue gas and syngas. The configurations are compared based on their carbon distributions and energy demands for CO2\ua0separation. We show that the air and oxyfuel configurations lead to similar carbon distributions, whereas the CLG configuration gives the lowest carbon recovery in the form of an end product. The oxyfuel and CLG configurations show the lowest energy demands for CO2\ua0separation, while the air configuration exhibits the highest. The electrical configuration has the lowest potential to benefit from heat integration to cover this energy demand. An investigation into the optimal gasification temperature for the air and oxyfuel configurations shows that there is no driver for operation at high temperatures
- âŠ