734 research outputs found
Characterizing anomalous diffusion in crowded polymer solutions and gels over five decades in time with variable-lengthscale fluorescence correlation spectroscopy
The diffusion of macromolecules in cells and in complex fluids is often found
to deviate from simple Fickian diffusion. One explanation offered for this
behavior is that molecular crowding renders diffusion anomalous, where the
mean-squared displacement of the particles scales as with . Unfortunately, methods such as
fluorescence correlation spectroscopy (FCS) or fluorescence recovery after
photobleaching (FRAP) probe diffusion only over a narrow range of lengthscales
and cannot directly test the dependence of the mean-squared displacement (MSD)
on time. Here we show that variable-lengthscale FCS (VLS-FCS), where the volume
of observation is varied over several orders of magnitude, combined with a
numerical inversion procedure of the correlation data, allows retrieving the
MSD for up to five decades in time, bridging the gap between diffusion
experiments performed at different lengthscales. In addition, we show that
VLS-FCS provides a way to assess whether the propagator associated with the
diffusion is Gaussian or non-Gaussian. We used VLS-FCS to investigate two
systems where anomalous diffusion had been previously reported. In the case of
dense cross-linked agarose gels, the measured MSD confirmed that the diffusion
of small beads was anomalous at short lengthscales, with a cross-over to simple
diffusion around m, consistent with a caged diffusion process.
On the other hand, for solutions crowded with marginally entangled dextran
molecules, we uncovered an apparent discrepancy between the MSD, found to be
linear, and the propagators at short lengthscales, found to be non-Gaussian.
These contradicting features call to mind the "anomalous, yet Brownian"
diffusion observed in several biological systems, and the recently proposed
"diffusing diffusivity" model
Background Oriented Schlieren (BOS) of a Supersonic Aircraft In Flight
This article describes the development and use of Background Oriented Schlieren on a full-scale supersonic jet in flight. A series of flight tests was performed in October, 2014 and February 2015 using the flora of the desert floor in the Supersonic Flight Corridor on the Edwards Air Force Base as a background. Flight planning was designed based on the camera resolution, the mean size and color of the predominant plants, and the navigation and coordination of two aircraft. Software used to process the image data was improved with additional utilities. The planning proved to be effective and the vast majority of the passes of the target aircraft were successfully recorded. Results were obtained that are the most detailed schlieren imagery of an aircraft in flight to date
D-Terms from Generalized NS-NS Fluxes in Type II
Orientifolds of type II string theory admit a certain set of generalized
NS-NS fluxes, including not only the three-form field strength H, but also
metric and non-geometric fluxes, which are related to H by T-duality. We
describe in general how these fluxes appear as parameters of an effective N=1
supergravity theory in four dimensions, and in particular how certain
generalized NS-NS fluxes can act as charges for R-R axions, leading to D-term
contributions to the effective scalar potential. We illustrate these phenomena
in type IIB with the example of a certain orientifold of T^6/Z_4.Comment: 31+1 pages, uses utarticle.cls; v2: references adde
Gaugino Condensation in M-theory on S^1/Z_2
In the low energy limit of for M-theory on S^1/Z_2, we calculate the gaugino
condensate potential in four dimensions using the background solutions due to
Horava. We show that this potential is free of delta-function singularities and
has the same form as the potential in the weakly coupled heterotic string. A
general flux quantization rule for the three-form field of M-theory on S^1/Z_2
is given and checked in certain limiting cases. This rule is used to fix the
free parameter in the potential originating from a zero mode of the form field.
Finally, we calculate soft supersymmetry breaking terms. We find that
corrections to the Kahler potential and the gauge kinetic function, which can
be large in the strongly coupled region, contribute significantly to certain
soft terms. In particular, for supersymmetry breaking in the T-modulus
direction, the small values of gaugino masses and trilinear couplings that
occur in the weakly coupled, large radius regime are enhanced to order m_3/2 in
M-theory. The scalar soft masses remain small even, in the strong coupling
M-theory limit.Comment: 20 pages, LATE
Cosmological Challenges in Theories with Extra Dimensions and Remarks on the Horizon Problem
We consider the cosmology that results if our observable universe is a
3-brane in a higher dimensional universe. In particular, we focus on the case
where our 3-brane is located at the symmetry fixed plane of a
symmetric five-dimensional spacetime, as in the Ho\v{r}ava-Witten model
compactified on a Calabi-Yau manifold. As our first result, we find that there
can be substantial modifications to the standard Friedmann-Robertson-Walker
(FRW) cosmology; as a consequence, a large class of such models is
observationally inconsistent. In particular, any relationship between the
Hubble constant and the energy density on our brane is possible, including (but
not only) FRW. Generically, due to the existence of the bulk and the boundary
conditions on the orbifold fixed plane, the relationship is not FRW, and hence
cosmological constraints coming from big bang nucleosynthesis, structure
formation, and the age of the universe difficult to satisfy. We do wish to
point out, however, that some specific choices for the bulk stress-energy
tensor components do reproduce normal FRW cosmology on our brane, and we have
constructed an explicit example. As our second result, for a broad class of
models, we find a somewhat surprising fact: the stabilization of the radius of
the extra dimension and hence the four dimensional Planck mass requires
unrealistic fine-tuning of the equation of state on our 3-brane. In the last
third of the paper, we make remarks about causality and the horizon problem
that apply to {\it any} theory in which the volume of the extra dimension
determines the four-dimensional gravitational coupling. We point out that some
of the assumptions that lead to the usual inflationary requirements are
modified.Comment: 15 page REVTeX file; to appear in Phys. Rev. D; clarified the
statement of being able to obtain any power dependence of the Hubble
expansion rate on the energy density; added reference
Scalar Three-point Functions in a CDL Background
Motivated by the FRW-CFT proposal by Freivogel, Sekino, Susskind and Yeh, we
compute the three-point function of a scalar field in a Coleman-De Luccia
instanton background. We first compute the three-point function of the scalar
field making only very mild assumptions about the scalar potential and the
instanton background. We obtain the three-point function for points in the FRW
patch of the CDL instanton and take two interesting limits; the limit where the
three points are near the boundary of the hyperbolic slices of the FRW patch,
and the limit where the three points lie on the past lightcone of the FRW
patch. We expand the past lightcone three-point function in spherical
harmonics. We show that the near boundary limit expansion of the three-point
function of a massless scalar field exhibits conformal structure compatible
with FRW-CFT when the FRW patch is flat. We also compute the three-point
function when the scalar is massive, and explain the obstacles to generalizing
the conjectured field-operator correspondence of massless fields to massive
fields.Comment: 42 pages + appendices, 10 figures; v2, v3: minor correction
Toward the End of Time
The null-brane space-time provides a simple model of a big crunch/big bang
singularity. A non-perturbative definition of M-theory on this space-time was
recently provided using matrix theory. We derive the fermion couplings for this
matrix model and study the leading quantum effects. These effects include
particle production and a time-dependent potential. Our results suggest that as
the null-brane develops a big crunch singularity, the usual notion of
space-time is replaced by an interacting gluon phase. This gluon phase appears
to constitute the end of our conventional picture of space and time.Comment: 31 pages, reference adde
Nonperturbative studies of supersymmetric matrix quantum mechanics with 4 and 8 supercharges at finite temperature
We investigate thermodynamic properties of one-dimensional U(N)
supersymmetric gauge theories with 4 and 8 supercharges in the planar large-N
limit by Monte Carlo calculations. Unlike the 16 supercharge case, the
threshold bound state with zero energy is widely believed not to exist in these
models. This led A.V. Smilga to conjecture that the internal energy decreases
exponentially at low temperature instead of decreasing with a power law. In the
16 supercharge case, the latter behavior was predicted from the dual black
0-brane geometry and confirmed recently by Monte Carlo calculations. Our
results for the models with 4 and 8 supercharges indeed support the exponential
behavior, revealing a qualitative difference from the 16 supercharge case.Comment: 16 pages, 7 figures, LaTeX2e, minor corrections in section 3, final
version accepted in JHE
Fluorescence correlation spectroscopy with sub-diffraction-limited resolution using near-field optical probes
We report fluorescence correlation spectroscopy (FCS) measurements using near-field scanning optical microscopy (NSOM) probes to produce a sub-diffraction-limited observation area. An order of magnitude reduction in the area compared to confocal FCS has been achieved. We also demonstrate a simple means to model the autocorrelation decay due to diffusion within the excitation profile at the NSOM probe aperture. The use of probes with smaller apertures is expected to provide an additional order of magnitude reduction in the observation area, thus enabling the study of cellular membranes with higher concentrations of fluorophores than is currently possible with diffraction-limited techniques
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