2,549 research outputs found
Decrumpling membranes by quantum effects
The phase diagram of an incompressible fluid membrane subject to quantum and
thermal fluctuations is calculated exactly in a large number of dimensions of
configuration space. At zero temperature, a crumpling transition is found at a
critical bending rigidity . For membranes of fixed lateral
size, a crumpling transition occurs at nonzero temperatures in an auxiliary
mean field approximation. As the lateral size L of the membrane becomes large,
the flat regime shrinks with .Comment: 9 pages, 4 figure
Interaction of Conical Membrane Inclusions: Effect of Lateral Tension
Considering two rigid conical inclusions embedded in a membrane subject to
lateral tension, we study the membrane-mediated interaction between these
inclusions that originates from the hat-shaped membrane deformations associated
with the cones. At non-vanishing lateral tensions, the interaction is found to
depend on the orientation of the cones with respect to the membrane plane. The
interaction of inclusions of equal orientation is repulsive at all distances
between them, while the inclusions of opposite orientation repel each other at
small separations, but attract each other at larger ones. Both the repulsive
and attractive forces become stronger with increasing lateral tension. This is
different from what has been predicted on the basis of the same static model
for the case of vanishing lateral tension. Without tension, the inclusions
repel each other at all distances independently of their relative orientation.
We conclude that lateral tension may induce the aggregation of conical membrane
inclusions.Comment: 10 pages (revtech), 5 figures (postscript
Thermal Casimir drag in fluctuating classical fields
A uniformly moving inclusion which locally suppresses the fluctuations of a
classical thermally excited field is shown to experience a drag force which
depends on the dynamics of the field. It is shown that in a number of cases the
linear friction coefficient is dominated by short distance fluctuations and
takes a very simple form. Examples where this drag can occur are for stiff
objects, such as proteins, nonspecifically bound to more flexible ones such as
polymers and membranes.Comment: 4 pages RevTex, 2 figure
Cycle Performance of a Pulse Detonation Engine with Supercritical Fuel Injection
Pulse detonation engines (PDE) rely on rapid ignition and formation of detonation waves. Because hydrocarbon fuels are composed typically of long carbon chains that must be reduced in the combustion process, it would be beneficial to create such reduction prior to injection of fuel into the engine. This study focused on PDE operation enhancements using dual detonation tube, concentric-counter-flow heat exchangers to elevate the fuel temperature up to supercritical temperatures. Variation of several operating parameters included fuel type (JP-8, JP-7, JP-10, RP-1, JP-900, and S-8), ignition delay, frequency, internal spiral length, and purge fraction. To quantify the performance, four key parameters examined were ignition time, deflagration to detonation transition time, detonation distance, and the percent of ignitions resulting in a detonation. In general, for all fuels except JP-10, increasing the fuel injection temperature decreased deflagration to detonation transition time and detonation distance, increased the percent of ignitions resulting in detonations (detonation percentage), and had no impact on ignition time. JP-10 was difficult to detonate, resulting in extremely poor performance. A minimum spiral length of 0.915 m (36 in) and a minimum purge fraction of 0.3 were determined. An increase in cycle frequency resulted in a decrease in deflagration to detonation transition time, but had little effect on ignition time and detonation distance. Analysis of ignition delay showed that 4 msec is the best ignition delay at high fuel injection temperatures, based on fire phase time and detonation percentage
Fluctuation induced interactions between domains in membranes
We study a model lipid bilayer composed of a mixture of two incompatible
lipid types which have a natural tendency to segregate in the absence of
membrane fluctuations. The membrane is mechanically characterized by a local
bending rigidity which varies with the average local lipid
composition . We show, in the case where varies weakly with
, that the effective interaction between lipids of the same type can
either be everywhere attractive or can have a repulsive component at
intermediate distances greater than the typical lipid size. When this
interaction has a repulsive component, it can prevent macro-phase separation
and lead to separation in mesophases with a finite domain size. This effect
could be relevant to certain experimental and numerical observations of
mesoscopic domains in such systems.Comment: 9 pages RevTex, 1 eps figur
A novel method for measuring the bending rigidity of model lipid membranes by simulating tethers
The tensile force along a cylindrical lipid bilayer tube is proportional to
the membrane's bending modulus and inversely proportional to the tube radius.
We show that this relation, which is experimentally exploited to measure
bending rigidities, can be applied with even greater ease in computer
simulations. Using a coarse-grained bilayer model we efficiently obtain bending
rigidities that compare very well with complementary measurements based on an
analysis of thermal undulation modes. We furthermore illustrate that no
deviations from simple quadratic continuum theory occur up to a radius of
curvature comparable to the bilayer thickness.Comment: 7 pages, 5 figures, 1 tabl
Monitoring cortical excitability during repetitive transcranial magnetic stimulation in children with ADHD: a single-blind, sham-controlled TMS-EEG study
Background: Repetitive transcranial magnetic stimulation (rTMS) allows non-invasive stimulation of the human brain. However, no suitable marker has yet been established to monitor the immediate rTMS effects on cortical areas in children.
Objective: TMS-evoked EEG potentials (TEPs) could present a well-suited marker for real-time monitoring. Monitoring is particularly important in children where only few data about rTMS effects and safety are currently available.
Methods: In a single-blind sham-controlled study, twenty-five school-aged children with ADHD received subthreshold 1 Hz-rTMS to the primary motor cortex. The TMS-evoked N100 was measured by 64-channel-EEG pre, during and post rTMS, and compared to sham stimulation as an intraindividual control condition.
Results: TMS-evoked N100 amplitude decreased during 1 Hz-rTMS and, at the group level, reached a stable plateau after approximately 500 pulses. N100 amplitude to supra-threshold single pulses post rTMS confirmed the amplitude reduction in comparison to the pre-rTMS level while sham stimulation had no influence. EEG source analysis indicated that the TMS-evoked N100 change reflected rTMS effects in the stimulated motor cortex. Amplitude changes in TMS-evoked N100 and MEPs (pre versus post 1 Hz-rTMS) correlated significantly, but this correlation was also found for pre versus post sham stimulation.
Conclusion: The TMS-evoked N100 represents a promising candidate marker to monitor rTMS effects on cortical excitability in children with ADHD. TMS-evoked N100 can be employed to monitor real-time effects of TMS for subthreshold intensities. Though TMS-evoked N100 was a more sensitive parameter for rTMS-specific changes than MEPs in our sample, further studies are necessary to demonstrate whether clinical rTMS effects can be predicted from rTMS-induced changes in TMS-evoked N100 amplitude and to clarify the relationship between rTMS-induced changes in TMS-evoked N100 and MEP amplitudes. The TMS-evoked N100 amplitude reduction after 1 Hz-rTMS could either reflect a globally decreased cortical response to the TMS pulse or a specific decrease in inhibition
New Approach on the General Shape Equation of Axisymmetric Vesicles
The general Helfrich shape equation determined by minimizing the curvature
free energy describes the equilibrium shapes of the axisymmetric lipid bilayer
vesicles in different conditions. It is a non-linear differential equation with
variable coefficients. In this letter, by analyzing the unique property of the
solution, we change this shape equation into a system of the two differential
equations. One of them is a linear differential equation. This equation system
contains all of the known rigorous solutions of the general shape equation. And
the more general constraint conditions are found for the solution of the
general shape equation.Comment: 8 pages, LaTex, submit to Mod. Phys. Lett.
Asymptotic Freedom of Elastic Strings and Barriers
We study the problem of a quantized elastic string in the presence of an
impenetrable wall. This is a two-dimensional field theory of an N-component
real scalar field which becomes interacting through the restriction that
the magnitude of is less than , for a spherical wall of
radius . The N=1 case is a string vibrating in a plane between
two straight walls. We review a simple nonperturbative argument that there is a
gap in the spectrum, with asymptotically-free behavior in the coupling (which
is the reciprocal of ) for N greater than or equal to one. This
scaling behavior of the mass gap has been disputed in some of the recent
literature. We find, however, that perturbation theory and the 1/N expansion
each confirms that these theories are asymptotically free. The large N limit
coincides with that of the O(N) nonlinear sigma model. A theta parameter exists
for the N=2 model, which describes a string confined to the interior of a
cylinder of radius .Comment: Text slightly improved, bibilography corrected, more typos corrected,
still Latex 7 page
Effective free energy for pinned membranes
We consider membranes adhered through specific receptor-ligand bonds. Thermal
undulations of the membrane induce effective interactions between adhesion
sites. We derive an upper bound to the free energy that is independent of
interaction details. To lowest order in a systematic expansion we obtain
two-body interactions which allow to map the free energy onto a lattice gas
with constant density. The induced interactions alone are not strong enough to
lead to a condensation of individual adhesion sites. A measure of the thermal
roughness is shown to depend on the inverse square root of the density of
adhesion sites, which is in good agreement with previous computer simulations.Comment: to appear as a Rapid Communication in Phys. Rev.
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