5,545 research outputs found
Microrheology with optical tweezers: data analysis
We present a data analysis procedure that provides the solution to a long-standing issue in microrheology studies, i.e. the evaluation of the fluids' linear viscoelastic properties from the analysis of a finite set of experimental data, describing (for instance) the time-dependent mean-square displacement of suspended probe particles experiencing Brownian fluctuations. We report, for the first time in the literature, the linear viscoelastic response of an optically trapped bead suspended in a Newtonian fluid, over the entire range of experimentally accessible frequencies. The general validity of the proposed method makes it transferable to the majority of microrheology and rheology techniques
Introducing Quantum Ricci Curvature
Motivated by the search for geometric observables in nonperturbative quantum
gravity, we define a notion of coarse-grained Ricci curvature. It is based on a
particular way of extracting the local Ricci curvature of a smooth Riemannian
manifold by comparing the distance between pairs of spheres with that of their
centres. The quantum Ricci curvature is designed for use on non-smooth and
discrete metric spaces, and to satisfy the key criteria of scalability and
computability. We test the prescription on a variety of regular and random
piecewise flat spaces, mostly in two dimensions. This enables us to quantify
its behaviour for short lattices distances and compare its large-scale
behaviour with that of constantly curved model spaces. On the triangulated
spaces considered, the quantum Ricci curvature has good averaging properties
and reproduces classical characteristics on scales large compared to the
discretization scale.Comment: 43 pages, 27 figure
Center vortex model for the infrared sector of Yang-Mills theory - Quenched Dirac spectrum and chiral condensate
The Dirac operator describing the coupling of continuum quark fields to SU(2)
center vortex world-surfaces composed of elementary squares on a hypercubic
lattice is constructed. It is used to evaluate the quenched Dirac spectral
density in the random vortex world-surface model, which previously has been
shown to quantitatively reproduce both the confinement properties and the
topological susceptibility of SU(2) Yang-Mills theory. Under certain conditions
on the modeling of the vortex gauge field, a behavior of the quenched chiral
condensate as a function of temperature is obtained which is consistent with
measurements in SU(2) lattice Yang-Mills theory.Comment: 36 LaTeX pages, 13 ps figures included via epsf; minor reformulations
and added cross-referencing for the purpose of clarit
Motion magnification in coronal seismology
We introduce a new method for the investigation of low-amplitude transverse
oscillations of solar plasma non-uniformities, such as coronal loops,
individual strands in coronal arcades, jets, prominence fibrils, polar plumes,
and other contrast features, observed with imaging instruments. The method is
based on the two-dimensional dual tree complex wavelet transform
(DTWT). It allows us to magnify transverse, in the
plane-of-the-sky, quasi-periodic motions of contrast features in image
sequences. The tests performed on the artificial data cubes imitating
exponentially decaying, multi-periodic and frequency-modulated kink
oscillations of coronal loops showed the effectiveness, reliability and
robustness of this technique. The algorithm was found to give linear scaling of
the magnified amplitudes with the original amplitudes provided they are
sufficiently small. Also, the magnification is independent of the oscillation
period in a broad range of the periods. The application of this technique to
SDO/AIA EUV data cubes of a non-flaring active region allowed for the improved
detection of low-amplitude decay-less oscillations in the majority of loops.Comment: Accepted for publication in Solar Physic
SoFiA: a flexible source finder for 3D spectral line data
We introduce SoFiA, a flexible software application for the detection and
parameterization of sources in 3D spectral-line datasets. SoFiA combines for
the first time in a single piece of software a set of new source-finding and
parameterization algorithms developed on the way to future HI surveys with
ASKAP (WALLABY, DINGO) and APERTIF. It is designed to enable the general use of
these new algorithms by the community on a broad range of datasets. The key
advantages of SoFiA are the ability to: search for line emission on multiple
scales to detect 3D sources in a complete and reliable way, taking into account
noise level variations and the presence of artefacts in a data cube; estimate
the reliability of individual detections; look for signal in arbitrarily large
data cubes using a catalogue of 3D coordinates as a prior; provide a wide range
of source parameters and output products which facilitate further analysis by
the user. We highlight the modularity of SoFiA, which makes it a flexible
package allowing users to select and apply only the algorithms useful for their
data and science questions. This modularity makes it also possible to easily
expand SoFiA in order to include additional methods as they become available.
The full SoFiA distribution, including a dedicated graphical user interface, is
publicly available for download.Comment: MNRAS, accepted. SoFiA is registered at the Astrophysics Source Code
Library with ID ascl:1412.001. Download SoFiA at
https://github.com/SoFiA-Admin/SoFi
Automated calibration of multi-sensor optical shape measurement system
A multi-sensor optical shape measurement system (SMS) based on the fringe
projection method and temporal phase unwrapping has recently been commercialised
as a result of its easy implementation, computer control using a spatial light
modulator, and fast full-field measurement. The main advantage of a multi-sensor
SMS is the ability to make measurements for 360° coverage without the requirement
for mounting the measured component on translation and/or rotation stages. However,
for greater acceptance in industry, issues relating to a user-friendly calibration of the
multi-sensor SMS in an industrial environment for presentation of the measured data
in a single coordinate system need to be addressed.
The calibration of multi-sensor SMSs typically requires a calibration artefact, which
consequently leads to significant user input for the processing of calibration data, in
order to obtain the respective sensor's optimal imaging geometry parameters. The
imaging geometry parameters provide a mapping from the acquired shape data to real
world Cartesian coordinates. However, the process of obtaining optimal sensor
imaging geometry parameters (which involves a nonlinear numerical optimization
process known as bundle adjustment), requires labelling regions within each point
cloud as belonging to known features of the calibration artefact. This thesis describes
an automated calibration procedure which ensures that calibration data is processed
through automated feature detection of the calibration artefact, artefact pose
estimation, automated control point selection, and finally bundle adjustment itself. [Continues.
Electric-field-induced nematic-cholesteric transition and 3-D director structures in homeotropic cells
We study the phase diagram of director structures in cholesteric liquid
crystals of negative dielectric anisotropy in homeotropic cells of thickness d
which is smaller than the cholesteric pitch p. The basic control parameters are
the frustration ratio d/p and the applied voltage U. Fluorescence Confocal
Polarising Microscopy allows us to directly and unambiguously determine the 3-D
director structures. The results are of importance for potential applications
of the cholesteric structures, such as switchable gratings and eyewear with
tunable transparency based.Comment: Will be published in Physical Review
Perfect lattice action for asymptotically free theories
There exist lattice actions which give cut--off independent physical
predictions even on coarse grained lattices. Rotation symmetry is restored, the
spectrum becomes exact and, in addition, the classical equations have scale
invariant instanton solutions. This perfect action can be made short ranged. It
can be determined by combining analytical calculations with numerical
simulations on small lattices. We illustrate the method and the benefits on the
non--linear --model.Comment: 29 pages(part 1) + 9 postscript figures(part 2, compressed by
uufiles). No changes, replaced due to transmission error observed, BUTP-93/1
- …