7,876 research outputs found
Thrifty swimming with shear-thinning
Microscale propulsion is integral to numerous biomedical systems, for example
biofilm formation and human reproduction, where the surrounding fluids comprise
suspensions of polymers. These polymers endow the fluid with non-Newtonian
rheological properties, such as shear-thinning and viscoelasticity. Thus, the
complex dynamics of non-Newtonian fluids presents numerous modelling
challenges, strongly motivating experimental study. Here, we demonstrate that
failing to account for "out-of-plane" effects when analysing experimental data
of undulatory swimming through a shear-thinning fluid results in a significant
overestimate of fluid viscosity around the model swimmer C. elegans. This
miscalculation of viscosity corresponds with an overestimate of the power the
swimmer expends, a key biophysical quantity important for understanding the
internal mechanics of the swimmer. As experimental flow tracking techniques
improve, accurate experimental estimates of power consumption using this
technique will arise in similar undulatory systems, such as the planar beating
of human sperm through cervical mucus, will be required to probe the
interaction between internal power generation, fluid rheology, and the
resulting waveform
Undulatory swimming in fluids with polymer networks
The motility behavior of the nematode Caenorhabditis elegans in polymeric
solutions of varying concentrations is systematically investigated in
experiments using tracking and velocimetry methods. As the polymer
concentration is increased, the solution undergoes a transition from the
semi-dilute to the concentrated regime, where these rod-like polymers entangle,
align, and form networks. Remarkably, we find an enhancement in the nematode's
swimming speed of approximately 65% in concentrated solutions compared to
semi-dilute solutions. Using velocimetry methods, we show that the undulatory
swimming motion of the nematode induces an anisotropic mechanical response in
the fluid. This anisotropy, which arises from the fluid micro-structure, is
responsible for the observed increase in swimming speed.Comment: Published 1 November 2013 in Europhysics Letter
X,Y,Z-Waves: Extended Structures in Nonlinear Lattices
Motivated by recent experimental and theoretical results on optical X-waves,
we propose a new type of waveforms in 2D and 3D discrete media -- multi-legged
extended nonlinear structures (ENS), built as arrays of lattice solitons (tiles
or stones, in the 2D and 3D cases, respectively). First, we study the stability
of the tiles and stones analytically, and then extend them numerically to
complete ENS forms for both 2D and 3D lattices. The predicted patterns are
relevant to a variety of physical settings, such as Bose-Einstein condensates
in deep optical lattices, lattices built of microresonators, photorefractive
crystals with optically induced lattices (in the 2D case) and others.Comment: 4 pages, 4 figure
Results for the response function determination of the Compact Neutron Spectrometer
The Compact Neutron Spectrometer (CNS) is a Joint European Torus (JET)
Enhancement Project, designed for fusion diagnostics in different plasma
scenarios. The CNS is based on a liquid scintillator (BC501A) which allows good
discrimination between neutron and gamma radiation. Neutron spectrometry with a
BC501A spectrometer requires the use of a reliable, fully characterized
detector. The determination of the response matrix was carried out at the Ion
Accelerator Facility (PIAF) of the Physikalisch-Technische Bundesanstalt (PTB).
This facility provides several monoenergetic beams (2.5, 8, 10, 12 and 14 MeV)
and a 'white field'(Emax ~17 MeV), which allows for a full characterization of
the spectrometer in the region of interest (from ~1.5 MeV to ~17 MeV. The
energy of the incoming neutrons was determined by the time of flight method
(TOF), with time resolution in the order of 1 ns. To check the response matrix,
the measured pulse height spectra were unfolded with the code MAXED and the
resulting energy distributions were compared with those obtained from TOF. The
CNS project required modification of the PTB BC501A spectrometer design, to
replace an analog data acquisition system (NIM modules) with a digital system
developed by the 'Ente per le Nuove tecnologie, l'Energia e l'Ambiente' (ENEA).
Results for the new digital system were evaluated using new software developed
specifically for this project.Comment: Proceedings of FNDA 201
Group classification of (1+1)-Dimensional Schr\"odinger Equations with Potentials and Power Nonlinearities
We perform the complete group classification in the class of nonlinear
Schr\"odinger equations of the form
where is an arbitrary
complex-valued potential depending on and is a real non-zero
constant. We construct all the possible inequivalent potentials for which these
equations have non-trivial Lie symmetries using a combination of algebraic and
compatibility methods. The proposed approach can be applied to solving group
classification problems for a number of important classes of differential
equations arising in mathematical physics.Comment: 10 page
Baryon Asymmetry of the Universe without Boltzmann or Kadanoff-Baym
We present a formalism that allows the computation of the baryon asymmetry of
the universe from first principles of statistical physics and quantum field
theory that is applicable to certain types of beyond the Standard Model physics
(such as the neutrino Minimal Standard Model -- MSM) and does not require
the solution of Boltzmann or Kadanoff-Baym equations. The formalism works if a
thermal bath of Standard Model particles is very weakly coupled to a new sector
(sterile neutrinos in the MSM case) that is out-of-equilibrium. The key
point that allows a computation without kinetic equations is that the number of
sterile neutrinos produced during the relevant cosmological period remains
small. In such a case, it is possible to expand the formal solution of the von
Neumann equation perturbatively and obtain a master formula for the lepton
asymmetry expressed in terms of non-equilibrium Wightman functions. The master
formula neatly separates CP-violating contributions from finite temperature
correlation functions and satisfies all three Sakharov conditions. These
correlation functions can then be evaluated perturbatively; the validity of the
perturbative expansion depends on the parameters of the model considered. Here
we choose a toy model (containing only two active and two sterile neutrinos) to
illustrate the use of the formalism, but it could be applied to other models.Comment: 26 pages, 10 figure
Limits on Lorentz Violation from the Highest Energy Cosmic Rays
We place several new limits on Lorentz violating effects, which can modify
particles' dispersion relations, by considering the highest energy cosmic rays
observed. Since these are hadrons, this involves considering the partonic
content of such cosmic rays. We get a number of bounds on differences in
maximum propagation speeds, which are typically bounded at the 10^{-21} level,
and on momentum dependent dispersion corrections of the form v = 1 +-
p^2/Lambda^2, which typically bound Lambda > 10^{21} GeV, well above the Planck
scale. For (CPT violating) dispersion correction of the form v = 1 + p/Lambda,
the bounds are up to 15 orders of magnitude beyond the Planck scale.Comment: 24 pages, no figures. Added references, very slight changes. Version
published in Physical Review
Universal Heat Conduction in YBa_2Cu_3O_6.9
The thermal conductivity of YBa_2Cu_3O_6.9 was measured at low temperatures
in untwinned single crystals with concentrations of Zn impurities from 0 to 3%
of Cu. A linear term kappa_0/T = 0.19 mW/K^2.cm is clearly resolved as T -> 0,
and found to be virtually independent of Zn concentration. The existence of
this residual normal fluid strongly validates the basic theory of transport in
unconventional superconductors. Moreover, the observed universal behavior is in
quantitative agreement with calculations for a gap function of d-wave symmetry.Comment: Latex file, 4 pages, 3 EPS figures, to appear in Physical Review
Letter
Exact Soliton-like Solutions of the Radial Gross-Pitaevskii Equation
We construct exact ring soliton-like solutions of the cylindrically symmetric
(i.e., radial) Gross- Pitaevskii equation with a potential, using the
similarity transformation method. Depending on the choice of the allowed free
functions, the solutions can take the form of stationary dark or bright rings
whose time dependence is in the phase dynamics only, or oscillating and
bouncing solutions, related to the second Painlev\'e transcendent. In each case
the potential can be chosen to be time-independent.Comment: 8 pages, 7 figures. Version 2: stability analysis of the dark
solutio
- …