10,305 research outputs found
Front Propagation in the Pearling Instability of Tubular Vesicles
Recently Bar-Ziv and Moses discovered a dynamical shape transformation
induced in cylindrical lipid bilayer vesicles by the action of laser tweezers.
We develop a hydrodynamic theory of fluid bilayers in interaction with the
surrounding water and argue that the effect of the laser is to induce a sudden
tension in the membrane. We refine our previous analysis to account for the
fact that the shape transformation is not uniform but propagates outward from
the laser trap. Applying the marginal stability criterion to this situation
gives us an improved prediction for the selected initial wavelength and a new
prediction for the propagation velocity, both in rough agreement with the
experimental values. For example, a tubule of initial radius 0.7\micron\ has a
predicted initial sinusoidal perturbation in its diameter with wavelength
5.5\micron, as observed. The perturbation propagates as a front with the
qualitatively correct front velocity a bit less than 100\micron/sec. In
particular we show why this velocity is initially constant, as observed, and so
much smaller than the natural scale set by the tension. We also predict that
the front velocity should increase linearly with laser power. Finally we
introduce an approximate hydrodynamic model applicable to the fully nonlinear
regime. This model exhibits propagating fronts as well as fully-developed
``pearled" vesicles similar to those seen in the experiments.Comment: 42 pages, 6 eps figures included with text in uuencoded file, ps file
available from ftp://dept.physics.upenn.edu/pub/Nelson/pearl_propagation.ps
submitted to Journal de Physiqu
Sound radiation from a high speed axial flow fan due to the inlet turbulence quadrupole interaction
A formula is obtained for the total acoustic power spectra radiated out the front of the fan as a function of frequency. The formula involves the design parameters of the fan as well as the statistical properties of the incident turbulence. Numerical results are calculated for values of the parameters in the range of interest for quiet fans tested at the Lewis Research Center. As in the dipole analysis, when the turbulence correlation lengths become equal to the interblade spacing, the predicted spectra exhibit peaks around the blade passing frequency and its harmonics. There has recently been considerable conjecture about whether the stretching of turbulent eddies as they enter a stationary fan could result in the inlet turbulence being the dominant source of pure tones from nontranslating fans. The results of the current analysis show that, unless the turbulent eddies become quite elongated, this noise source contributes predominantly to the broadband spectrum
Real Time Evolution in Quantum Many-Body Systems With Unitary Perturbation Theory
We develop a new analytical method for solving real time evolution problems
of quantum many-body systems. Our approach is a direct generalization of the
well-known canonical perturbation theory for classical systems. Similar to
canonical perturbation theory, secular terms are avoided in a systematic
expansion and one obtains stable long-time behavior. These general ideas are
illustrated by applying them to the spin-boson model and studying its
non-equilibrium spin dynamics.Comment: Final version as accepted for publication in Phys. Rev. B (4 pages, 3
figures
Dynamical typicality of quantum expectation values
We show that the vast majority of all pure states featuring a common
expectation value of some generic observable at a given time will yield very
similar expectation values of the same observable at any later time. This is
meant to apply to Schroedinger type dynamics in high dimensional Hilbert
spaces. As a consequence individual dynamics of expectation values are then
typically well described by the ensemble average. Our approach is based on the
Hilbert space average method. We support the analytical investigations with
numerics obtained by exact diagonalization of the full time-dependent
Schroedinger equation for some pertinent, abstract Hamiltonian model.
Furthermore, we discuss the implications on the applicability of projection
operator methods with respect to initial states, as well as on irreversibility
in general.Comment: 4 pages, 1 figure, accepted for publication in Phys. Rev. Let
Dynamics of non-equilibrium membrane bud formation
The dynamical response of a lipid membrane to a local perturbation of its
molecular symmetry is investigated theoretically. A density asymmetry between
the two membrane leaflets is predominantly released by in-plane lipid diffusion
or membrane curvature, depending upon the spatial extent of the perturbation.
It may result in the formation of non-equilibrium structures (buds), for which
a dynamical size selection is observed. A preferred size in the micrometer
range is predicted, as a signature of the crossover between membrane and
solvent dominated dynamical membrane response.Comment: 7 pages 3 figure
Possible potentials responsible for stable circular relativistic orbits
Bertrand's theorem in classical mechanics of the central force fields
attracts us because of its predictive power. It categorically proves that there
can only be two types of forces which can produce stable, circular orbits. In
the present article an attempt has been made to generalize Bertrand's theorem
to the central force problem of relativistic systems. The stability criterion
for potentials which can produce stable, circular orbits in the relativistic
central force problem has been deduced and a general solution of it is
presented in the article. It is seen that the inverse square law passes the
relativistic test but the kind of force required for simple harmonic motion
does not. Special relativistic effects do not allow stable, circular orbits in
presence of a force which is proportional to the negative of the displacement
of the particle from the potential center.Comment: 11 pages, Latex fil
Environment Assisted Metrology with Spin Qubit
We investigate the sensitivity of a recently proposed method for precision
measurement [Phys. Rev. Lett. 106, 140502 (2011)], focusing on an
implementation based on solid-state spin systems. The scheme amplifies a
quantum sensor response to weak external fields by exploiting its coupling to
spin impurities in the environment. We analyze the limits to the sensitivity
due to decoherence and propose dynamical decoupling schemes to increase the
spin coherence time. The sensitivity is also limited by the environment spin
polarization; therefore we discuss strategies to polarize the environment spins
and present a method to extend the scheme to the case of zero polarization. The
coherence time and polarization determine a figure of merit for the
environment's ability to enhance the sensitivity compared to echo-based sensing
schemes. This figure of merit can be used to engineer optimized samples for
high-sensitivity nanoscale magnetic sensing, such as diamond nanocrystals with
controlled impurity density.Comment: 9 pages, 6 figure
A Preliminary Study of Three-dimensional Sonographic Measurements of the Fetus
OBJECTIVES: This study was aimed at establishing an ideal method for performing three-dimensional measurements of the fetus in order to improve the estimation of fetal weight. METHODS: The study consisted of two phases. Phase I was a prospective cross-sectional study performed between 28 and 40 weeks\u27 gestation. The study population (n=110) comprised low-risk singleton pregnancies who underwent a routine third-trimester sonographic estimation of fetal weight. The purpose of this phase was to establish normal values for the fetal abdominal and head volumes throughout the third trimester. Phase II was a prospective study that included patients admitted for an elective cesarean section or for induction of labor between 38 and 41 weeks\u27 gestation (n=91). This phase of the study compared the actual birth weight to two- (2D) and three-dimensional (3D) measurements of the fetus. Conventional 2D ultrasound fetal biometry was performed measuring the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur diaphysis length (FL). Volume estimates were computed utilizing Virtual Organ Computer-aided AnaLysis (VOCAL), and the correlation between measured volumes and actual neonatal weight was calculated. RESULTS: Overall, this longitudinal study consisted of 110 patients between 28 and 41 weeks\u27 gestation. Normal values were computed for the fetal abdomen and head volume throughout the third trimester. Ultrasound examination was performed within three days prior to delivery on 91 patients. A good correlation was found between birth weight and abdominal volume (r=0.77) and between birth weight and head volume (r=0.5). Correlation between bidimensional measurements and actual fetal weights was found to be comparable with previously published correlations. CONCLUSION: Volume measurements of the fetus may improve the accuracy of estimating fetal size. Additional studies using different volume measurement of the fetus are necessary
Environment Assisted Precision Measurement
We describe a method to enhance the sensitivity of precision measurements
that takes advantage of a quantum sensor's environment to amplify its response
to weak external perturbations. An individual qubit is used to sense the
dynamics of surrounding ancillary qubits, which are in turn affected by the
external field to be measured. The resulting sensitivity enhancement is
determined by the number of ancillas that are coupled strongly to the sensor
qubit; it does not depend on the exact values of the coupling strengths and is
resilient to many forms of decoherence. The method achieves nearly
Heisenberg-limited precision measurement, using a novel class of entangled
states. We discuss specific applications to improve clock sensitivity using
trapped ions and magnetic sensing based on electronic spins in diamond.Comment: 4 pages, 3 figure
Quasispecies evolution in general mean-field landscapes
I consider a class of fitness landscapes, in which the fitness is a function
of a finite number of phenotypic "traits", which are themselves linear
functions of the genotype. I show that the stationary trait distribution in
such a landscape can be explicitly evaluated in a suitably defined
"thermodynamic limit", which is a combination of infinite-genome and strong
selection limits. These considerations can be applied in particular to identify
relevant features of the evolution of promoter binding sites, in spite of the
shortness of the corresponding sequences.Comment: 6 pages, 2 figures, Europhysics Letters style (included) Finite-size
scaling analysis sketched. To appear in Europhysics Letter
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