9,258 research outputs found
A versatile dual spot laser scanning confocal microscopy system for advanced fluorescence correlation spectroscopy analysis in living cell
A fluorescence correlation spectroscopy (FCS) system based on two independent
measurement volumes is presented. The optical setup and data acquisition
hardware are detailed, as well as a complete protocol to control the location,
size and shape of the measurement volumes. A method that allows to monitor
independently the excitation and collection efficiency distribution is
proposed. Finally, a few examples of measurements that exploit the two spots in
static and/or scanning schemes, are reported.Comment: Accepted for publication in Review of Scientific Instrumen
Direct Observation of Controlled Coupling in an Individual Quantum Dot Molecule
We report the direct observation of quantum coupling in individual quantum
dot molecules and its manipulation using static electric fields. A pronounced
anti-crossing of different excitonic transitions is observed as the electric
field is tuned. Comparison of our experimental results with theory shows that
the observed anti-crossing occurs between excitons with predominant spatially
\emph{direct} and \emph{indirect} character. The electron component of the
exciton wavefunction is shown to have molecular character at the anti-crossing
and the quantum coupling strength is deduced optically. In addition, we
determine the dependence of the coupling strength on the inter-dot separation
and identify a field driven transition of the nature of the molecular ground
state.Comment: 11 pages, 4 figures submitted to Physical Review Letter
Local estimates for entropy densities in coupled map lattices
We present a method to derive an upper bound for the entropy density of
coupled map lattices with local interactions from local observations. To do
this, we use an embedding technique being a combination of time delay and
spatial embedding. This embedding allows us to identify the local character of
the equations of motion. Based on this method we present an approximate
estimate of the entropy density by the correlation integral.Comment: 4 pages, 5 figures include
Magnetohydrodynamic activity inside a sphere
We present a computational method to solve the magnetohydrodynamic equations
in spherical geometry. The technique is fully nonlinear and wholly spectral,
and uses an expansion basis that is adapted to the geometry:
Chandrasekhar-Kendall vector eigenfunctions of the curl. The resulting lower
spatial resolution is somewhat offset by being able to build all the boundary
conditions into each of the orthogonal expansion functions and by the
disappearance of any difficulties caused by singularities at the center of the
sphere. The results reported here are for mechanically and magnetically
isolated spheres, although different boundary conditions could be studied by
adapting the same method. The intent is to be able to study the nonlinear
dynamical evolution of those aspects that are peculiar to the spherical
geometry at only moderate Reynolds numbers. The code is parallelized, and will
preserve to high accuracy the ideal magnetohydrodynamic (MHD) invariants of the
system (global energy, magnetic helicity, cross helicity). Examples of results
for selective decay and mechanically-driven dynamo simulations are discussed.
In the dynamo cases, spontaneous flips of the dipole orientation are observed.Comment: 15 pages, 19 figures. Improved figures, in press in Physics of Fluid
Probing cell membrane mechanics by magnetic particle actuation and 3D rotational particle tracking
The mechanical properties of the cell membrane and the actin cortex determine a variety of cellular processes. An accurate description of their mechanics and dynamics necessitates a measurement technique that can capture the inherent anisotropy of the system. We combine magnetic particle actuation with rotational and translational particle tracking to simultaneously measure the mechanical stiffness of the membrane and the actin cortex in living cells in three rotational and two translational directions. We demonstrate the technique by targeting various types of membrane receptors. When using particles that bind via integrins, we measured an isotropic stiffness and a characteristic power-law dependence of the shear modulus on the applied frequency. When using particles functionalized with immunoglobulin G, we measured an anisotropic stiffness with a strongly reduced value in one dimension. We suggest that the observed reduced stiffness is caused by a local detachment of the membrane from the subjacent cytoskeletal cortex. Furthermore, we use functionalized particles as phagocytic targets for macrophages. Although phagocytosis is an inherently mechanical process, little is known about the forces and energies that a cell requires for internalization. We use our technique to measure the stiffnesses of the phagocytic cup as a function of time. The measured values and their time-dependence can be interpreted with a model of a pre-stressed membrane connected to an elastically deformable actin cortex. A comparison of model and data allows a determination of the speed at which the membrane advances around the particle. This approach is a novel way of measuring the progression of phagocytic cups and their mechanical properties in real-time. We expect that our technique will enable new insights into the mechanical properties of cells and will help to better understand numerous cellular processes
Harmonic lattice behavior of two-dimensional colloidal crystals
Using positional data from video-microscopy and applying the equipartition
theorem for harmonic Hamiltonians, we determine the wave-vector-dependent
normal mode spring constants of a two-dimensional colloidal model crystal and
compare the measured band-structure to predictions of the harmonic lattice
theory. We find good agreement for both the transversal and the longitudinal
mode. For , the measured spring constants are consistent with the
elastic moduli of the crystal.Comment: 4 pages, 3 figures, submitte
The laboratory telerobotic manipulator program
New opportunities for the application of telerobotic systems to enhance human intelligence and dexterity in the hazardous environment of space are presented by the NASA Space Station Program. Because of the need for significant increases in extravehicular activity and the potential increase in hazards associated with space programs, emphasis is being heightened on telerobotic systems research and development. The Laboratory Telerobotic Manipulator (LTM) program is performed to develop and demonstrate ground-based telerobotic manipulator system hardware for research and demonstrations aimed at future NASA applications. The LTM incorporates traction drives, modularity, redundant kinematics, and state-of-the-art hierarchical control techniques to form a basis for merging the diverse technological domains of robust, high-dexterity teleoperations and autonomous robotic operation into common hardware to further NASA's research
Hydrodynamic and magnetohydrodynamic computations inside a rotating sphere
Numerical solutions of the incompressible magnetohydrodynamic (MHD) equations
are reported for the interior of a rotating, perfectly-conducting, rigid
spherical shell that is insulator-coated on the inside. A previously-reported
spectral method is used which relies on a Galerkin expansion in
Chandrasekhar-Kendall vector eigenfunctions of the curl. The new ingredient in
this set of computations is the rigid rotation of the sphere. After a few
purely hydrodynamic examples are sampled (spin down, Ekman pumping, inertial
waves), attention is focused on selective decay and the MHD dynamo problem. In
dynamo runs, prescribed mechanical forcing excites a persistent velocity field,
usually turbulent at modest Reynolds numbers, which in turn amplifies a small
seed magnetic field that is introduced. A wide variety of dynamo activity is
observed, all at unit magnetic Prandtl number. The code lacks the resolution to
probe high Reynolds numbers, but nevertheless interesting dynamo regimes turn
out to be plentiful in those parts of parameter space in which the code is
accurate. The key control parameters seem to be mechanical and magnetic
Reynolds numbers, the Rossby and Ekman numbers (which in our computations are
varied mostly by varying the rate of rotation of the sphere) and the amount of
mechanical helicity injected. Magnetic energy levels and magnetic dipole
behavior are exhibited which fluctuate strongly on a time scale of a few eddy
turnover times. These seem to stabilize as the rotation rate is increased until
the limit of the code resolution is reached.Comment: 26 pages, 17 figures, submitted to New Journal of Physic
Probe method and a Carleman function
A Carleman function is a special fundamental solution with a large parameter
for the Laplace operator and gives a formula to calculate the value of the
solution of the Cauchy problem in a domain for the Laplace equation. The probe
method applied to an inverse boundary value problem for the Laplace equation in
a bounded domain is based on the existence of a special sequence of harmonic
functions which is called a {\it needle sequence}. The needle sequence blows up
on a special curve which connects a given point inside the domain with a point
on the boundary of the domain and is convergent locally outside the curve. The
sequence yields a reconstruction formula of unknown discontinuity, such as
cavity, inclusion in a given medium from the Dirichlet-to-Neumann map. In this
paper, an explicit needle sequence in {\it three dimensions} is given in a
closed form. It is an application of a Carleman function introduced by
Yarmukhamedov. Furthermore, an explicit needle sequence in the probe method
applied to the reduction of inverse obstacle scattering problems with an {\it
arbitrary} fixed wave number to inverse boundary value problems for the
Helmholtz equation is also given.Comment: 2 figures, final versio
Scale Invariance in the Nonstationarity of Physiological Signals
We introduce a segmentation algorithm to probe temporal organization of
heterogeneities in human heartbeat interval time series. We find that the
lengths of segments with different local values of heart rates follow a
power-law distribution. This scale-invariant structure is not a simple
consequence of the long-range correlations present in the data. We also find
that the differences in mean heart rates between consecutive segments display a
common functional form, but with different parameters for healthy individuals
and for patients with heart failure. This finding may provide information into
the way heart rate variability is reduced in cardiac disease.Comment: 13 pages, 5 figures, corrected typo
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