238 research outputs found
Spontaneous Expulsion of Giant Lipid Vesicles Induced by Laser Tweezers
Irradiation of a giant unilamellar lipid bilayer vesicle with a focused laser
spot leads to a tense pressurized state which persists indefinitely after laser
shutoff. If the vesicle contains another object it can then be gently and
continuously expelled from the tense outer vesicle. Remarkably, the inner
object can be almost as large as the parent vesicle; its volume is replaced
during the exit process. We offer a qualitative theoretical model to explain
these and related phenomena. The main hypothesis is that the laser trap pulls
in lipid and ejects it in the form of submicron objects, whose osmotic activity
then drives the expulsion.Comment: Plain TeX file; uses harvmac and epsf; .ps available at
http://dept.physics.upenn.edu/~nelson/expulsion.p
Theory of Myelin Coiling
A new model is proposed to explain coiling of myelins composed of fluid
bilayers. This model allows the constituent bilayer cylinders of a myelin to be
non-coaxial and the bilayer lateral tension to vary from bilayer to bilayer.
The calculations show that a myelin would bend or coil to lower its free energy
when the bilayer lateral tension is sufficiently large. From a mechanical point
of view, the proposed coiling mechanism is analogous to the classical Euler
buckling of a thin elastic rod under axial compression. The analysis of a
simple two-bilayer case suggests that a bilayer lateral tension of about 1
dyne/cm can easily induce coiling of myelins of typical lipid bilayers. This
model signifies the importance of bilayer lateral tension in determining the
morphology of myelinic structures.Comment: 17 pages, 8 figures, submitted to Eur. Phys. J.
Numerical Observation of a Tubular Phase in Anisotropic Membranes
We provide the first numerical evidence for the existence of a tubular phase,
predicted by Radzihovsky and Toner (RT), for anisotropic tethered membranes
without self-avoidance. Incorporating anisotropy into the bending rigidity of a
simple model of a tethered membrane with free boundary conditions, we show that
the model indeed has two phase transitions corresponding to the flat-to-tubular
and tubular-to-crumpled transitions. For the tubular phase we measure the Flory
exponent and the roughness exponent . We find
and , which are in reasonable agreement with the theoretical
predictions of RT --- and .Comment: 8 pages, LaTeX, REVTEX, final published versio
Dynamic Fluctuation Phenomena in Double Membrane Films
Dynamics of double membrane films is investigated in the long-wavelength
limit including the overdamped squeezing mode. We demonstrate that thermal
fluctuations essentially modify the character of the mode due to its nonlinear
coupling to the transversal shear hydrodynamic mode. The corresponding Green
function acquires as a function of the frequency a cut along the imaginary
semi-axis. Fluctuations lead to increasing the attenuation of the squeezing
mode it becomes larger than the `bare' value.Comment: 7 pages, Revte
Cell adhesion and cortex contractility determine cell patterning in the Drosophila retina
Hayashi and Carthew (Nature 431 [2004], 647) have shown that the packing of
cone cells in the Drosophila retina resembles soap bubble packing, and that
changing E- and N-cadherin expression can change this packing, as well as cell
shape.
The analogy with bubbles suggests that cell packing is driven by surface
minimization. We find that this assumption is insufficient to model the
experimentally observed shapes and packing of the cells based on their cadherin
expression. We then consider a model in which adhesion leads to a surface
increase, balanced by cell cortex contraction. Using the experimentally
observed distributions of E- and N-cadherin, we simulate the packing and cell
shapes in the wildtype eye. Furthermore, by changing only the corresponding
parameters, this model can describe the mutants with different numbers of
cells, or changes in cadherin expression.Comment: revised manuscript; 8 pages, 6 figures; supplementary information not
include
Universal Algebraic Relaxation of Velocity and Phase in Pulled Fronts generating Periodic or Chaotic States
We investigate the asymptotic relaxation of so-called pulled fronts
propagating into an unstable state. The ``leading edge representation'' of the
equation of motion reveals the universal nature of their propagation mechanism
and allows us to generalize the universal algebraic velocity relaxation of
uniformly translating fronts to fronts, that generate periodic or even chaotic
states. Such fronts in addition exhibit a universal algebraic phase relaxation.
We numerically verify our analytical predictions for the Swift-Hohenberg and
the Complex Ginzburg Landau equation.Comment: 4 pages Revtex, 2 figures, submitted to Phys. Rev. Let
Instability and `Sausage-String' Appearance in Blood Vessels during High Blood Pressure
A new Rayleigh-type instability is proposed to explain the `sausage-string'
pattern of alternating constrictions and dilatations formed in blood vessels
under influence of a vasoconstricting agent. Our theory involves the nonlinear
elasticity characteristics of the vessel wall, and provides predictions for the
conditions under which the cylindrical form of a blood vessel becomes unstable.Comment: 4 pages, 4 figures submitted to Physical Review Letter
Straightening of Thermal Fluctuations in Semi-Flexible Polymers by Applied Tension
We investigate the propagation of a suddenly applied tension along a
thermally excited semi-flexible polymer using analytical approximations,
scaling arguments and numerical simulation. This problem is inherently
non-linear. We find sub-diffusive propagation with a dynamical exponent of 1/4.
By generalizing the internal elasticity, we show that tense strings exhibit
qualitatively different tension profiles and propagation with an exponent of
1/2.Comment: Latex file; with three postscript figures; .ps available at
http://dept.physics.upenn.edu/~nelson/pull.p
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