2,566 research outputs found
New stability results for long-wavelength convection patterns
We consider the transition from a spatially uniform state to a steady,
spatially-periodic pattern in a partial differential equation describing
long-wavelength convection. This both extends existing work on the study of
rolls, squares and hexagons and demonstrates how recent generic results for the
stability of spatially-periodic patterns may be applied in practice. We find
that squares, even if stable to roll perturbations, are often unstable when a
wider class of perturbations is considered. We also find scenarios where
transitions from hexagons to rectangles can occur. In some cases we find that,
near onset, more exotic spatially-periodic planforms are preferred over the
usual rolls, squares and hexagons.Comment: 25 pages, 8 figure
Quantum-degenerate mixture of fermionic lithium and bosonic rubidium gases
We report on the observation of sympathetic cooling of a cloud of fermionic
6-Li atoms which are thermally coupled to evaporatively cooled bosonic 87-Rb.
Using this technique we obtain a mixture of quantum-degenerate gases, where the
Rb cloud is colder than the critical temperature for Bose-Einstein condensation
and the Li cloud colder than the Fermi temperature. From measurements of the
thermalization velocity we estimate the interspecies s-wave triplet scattering
length |a_s|=20_{-6}^{+9} a_B. We found that the presence of residual rubidium
atoms in the |2,1> and the |1,-1> Zeeman substates gives rise to important
losses due to inelastic collisions.Comment: 4 pages, 3 figure
IMPACT: The Journal of the Center for Interdisciplinary Teaching and Learning. Volume 7, Issue 2, Summer 2018
IMPACT: The Journal of the Center for Interdisciplinary Teaching & Learning is a peer-reviewed, biannual online journal that publishes scholarly and creative non-fiction essays about the theory, practice and assessment of interdisciplinary education. Impact is produced by the Center for Interdisciplinary Teaching & Learning at the College of General Studies, Boston University (www.bu.edu/cgs/citl).In the weeks and months following August 12, 2017, members of the Boston University community
struggled
—
like Americans everywhere
—
to comprehend the series of troubling, and tragic, events which
would come, almost immediately, to be denoted in the national imagination by the metonym “Charlottesville.”
This special issue of Impact: The Journal of the Center for Interdisciplinary Teaching & Learning comprises a series of responses to these events and their aftermath, as well as the conditions which enabled them, by
faculty members from across the BU campus
Two-frequency forced Faraday waves: Weakly damped modes and pattern selection
Recent experiments (Kudrolli, Pier and Gollub, 1998) on two-frequency
parametrically excited surface waves exhibit an intriguing "superlattice" wave
pattern near a codimension-two bifurcation point where both subharmonic and
harmonic waves onset simultaneously, but with different spatial wavenumbers.
The superlattice pattern is synchronous with the forcing, spatially periodic on
a large hexagonal lattice, and exhibits small-scale triangular structure.
Similar patterns have been shown to exist as primary solution branches of a
generic 12-dimensional -equivariant bifurcation problem, and may
be stable if the nonlinear coefficients of the bifurcation problem satisfy
certain inequalities (Silber and Proctor, 1998). Here we use the spatial and
temporal symmetries of the problem to argue that weakly damped harmonic waves
may be critical to understanding the stabilization of this pattern in the
Faraday system. We illustrate this mechanism by considering the equations
developed by Zhang and Vinals (1997, J. Fluid Mech. 336) for small amplitude,
weakly damped surface waves on a semi-infinite fluid layer. We compute the
relevant nonlinear coefficients in the bifurcation equations describing the
onset of patterns for excitation frequency ratios of 2/3 and 6/7. For the 2/3
case, we show that there is a fundamental difference in the pattern selection
problems for subharmonic and harmonic instabilities near the codimension-two
point. Also, we find that the 6/7 case is significantly different from the 2/3
case due to the presence of additional weakly damped harmonic modes. These
additional harmonic modes can result in a stabilization of the superpatterns.Comment: 26 pages, 8 figures; minor text revisions, corrected figure 8; this
version to appear in a special issue of Physica D in memory of John David
Crawfor
Stabilizing unstable periodic orbits in the Lorenz equations using time-delayed feedback control
For many years it was believed that an unstable periodic orbit with an odd
number of real Floquet multipliers greater than unity cannot be stabilized by
the time-delayed feedback control mechanism of Pyragus. A recent paper by
Fiedler et al uses the normal form of a subcritical Hopf bifurcation to give a
counterexample to this theorem. Using the Lorenz equations as an example, we
demonstrate that the stabilization mechanism identified by Fiedler et al for
the Hopf normal form can also apply to unstable periodic orbits created by
subcritical Hopf bifurcations in higher-dimensional dynamical systems. Our
analysis focuses on a particular codimension-two bifurcation that captures the
stabilization mechanism in the Hopf normal form example, and we show that the
same codimension-two bifurcation is present in the Lorenz equations with
appropriately chosen Pyragus-type time-delayed feedback. This example suggests
a possible strategy for choosing the feedback gain matrix in Pyragus control of
unstable periodic orbits that arise from a subcritical Hopf bifurcation of a
stable equilibrium. In particular, our choice of feedback gain matrix is
informed by the Fiedler et al example, and it works over a broad range of
parameters, despite the fact that a center-manifold reduction of the
higher-dimensional problem does not lead to their model problem.Comment: 21 pages, 8 figures, to appear in PR
Simultaneous IUE, EXOSAT and optical observations of the unusual AM Her type variable H058+608
Simultaneous observations of the AM Her type variable H0538+608 made with IUE, EXOSAT, and a 1.3 m ground based telescope, and subsequent optical spectrophotometry at high and low resolution are discussed. The X-ray and optical data show clear evidence of a 3.30 + or - 0.03 hr period. Three SWP spectra were taken outside of eclipse and during overlapping phase intervals. The UV spectra contain strong emission lines characteristic of this class of objects and a flat continuum which appears to be deficient, given the brightness of source at optical and X-ray wavelengths. There is evidence for intensity variations in emission lines, particularly C IV. The X-ray light curves for H0538+608 reveal behavior which may be related to irregularities in its accretion flow
Application of magnetically induced hyperthermia on the model protozoan Crithidia fasciculata as a potential therapy against parasitic infections
Magnetic hyperthermia is currently an EU-approved clinical therapy against
tumor cells that uses magnetic nanoparticles under a time varying magnetic
field (TVMF). The same basic principle seems promising against trypanosomatids
causing Chagas disease and sleeping sickness, since therapeutic drugs available
display severe side effects and drug-resistant strains. However, no
applications of this strategy against protozoan-induced diseases have been
reported so far. In the present study, Crithidia fasciculata, a widely used
model for therapeutic strategies against pathogenic trypanosomatids, was
targeted with Fe_{3}O_{4} magnetic nanoparticles (MNPs) in order to remotely
provoke cell death using TVMFs. The MNPs with average sizes of d approx. 30 nm
were synthesized using a precipitation of FeSO_{4}4 in basic medium. The MNPs
were added to Crithidia fasciculata choanomastigotes in exponential phase and
incubated overnight. The amount of uploaded MNPs per cell was determined by
magnetic measurements. Cell viability using the MTT colorimetric assay and flow
cytometry showed that the MNPs were incorporated by the cells with no
noticeable cell-toxicity effects. When a TVMF (f = 249 kHz, H = 13 kA/m) was
applied to MNP-bearing cells, massive cell death was induced via a
non-apoptotic mechanism. No effects were observed by applying a TVMF on control
(without loaded MNPs) cells. No macroscopic rise in temperature was observed in
the extracellular medium during the experiments. Scanning Electron Microscopy
showed morphological changes after TVMF experiments. These data indicate (as a
proof of principle) that intracellular hyperthermia is a suitable technology to
induce the specific death of protozoan parasites bearing MNPs. These findings
expand the possibilities for new therapeutic strategies that combat parasitic
infections.Comment: 9 pages, four supplementary video file
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