7,814 research outputs found
Investigation of a hopping transporter concept for lunar exploration
Performance and dynamic characteristics determined for hopping transporter for lunar exploratio
Gravity-Induced Shape Transformations of Vesicles
We theoretically study the behavior of vesicles filled with a liquid of
higher density than the surrounding medium, a technique frequently used in
experiments. In the presence of gravity, these vesicles sink to the bottom of
the container, and eventually adhere even on non - attractive substrates. The
strong size-dependence of the gravitational energy makes large parts of the
phase diagram accessible to experiments even for small density differences. For
relatively large volume, non-axisymmetric bound shapes are explicitly
calculated and shown to be stable. Osmotic deflation of such a vesicle leads
back to axisymmetric shapes, and, finally, to a collapsed state of the vesicle.Comment: 11 pages, RevTeX, 3 Postscript figures uuencode
Mapping vesicle shapes into the phase diagram: A comparison of experiment and theory
Phase-contrast microscopy is used to monitor the shapes of micron-scale
fluid-phase phospholipid-bilayer vesicles in aqueous solution. At fixed
temperature, each vesicle undergoes thermal shape fluctuations. We are able
experimentally to characterize the thermal shape ensemble by digitizing the
vesicle outline in real time and storing the time-sequence of images. Analysis
of this ensemble using the area-difference-elasticity (ADE) model of vesicle
shapes allows us to associate (map) each time-sequence to a point in the
zero-temperature (shape) phase diagram. Changing the laboratory temperature
modifies the control parameters (area, volume, etc.) of each vesicle, so it
sweeps out a trajectory across the theoretical phase diagram. It is a
nontrivial test of the ADE model to check that these trajectories remain
confined to regions of the phase diagram where the corresponding shapes are
locally stable. In particular, we study the thermal trajectories of three
prolate vesicles which, upon heating, experienced a mechanical instability
leading to budding. We verify that the position of the observed instability and
the geometry of the budded shape are in reasonable accord with the theoretical
predictions. The inability of previous experiments to detect the ``hidden''
control parameters (relaxed area difference and spontaneous curvature) make
this the first direct quantitative confrontation between vesicle-shape theory
and experiment.Comment: submitted to PRE, LaTeX, 26 pages, 11 ps-fi
Diffusing proteins on a fluctuating membrane: Analytical theory and simulations
Using analytical calculations and computer simulations we consider both the
lateral diffusion of a membrane protein and the fluctuation spectrum of the
membrane in which the protein is embedded. The membrane protein interacts with
the membrane shape through its spontaneous curvature and bending rigidity. The
lateral motion of the protein may be viewed as diffusion in an effective
potential, hence, the effective mobility is always reduced compared to the case
of free diffusion. Using a rigorous path-integral approach we derive an
analytical expression for the effective diffusion coefficient for small ratios
of temperature and bending rigidity, which is the biologically relevant limit.
Simulations show very good quantitative agreement with our analytical result.
The analysis of the correlation functions contributing to the diffusion
coefficient shows that the correlations between the stochastic force of the
protein and the response in the membrane shape are responsible for the
reduction.
Our quantitative analysis of the membrane height correlation spectrum shows
an influence of the protein-membrane interaction causing a distinctly altered
wave-vector dependence compared to a free membrane. Furthermore, the time
correlations exhibit the two relevant timescales of the system: that of
membrane fluctuations and that of lateral protein diffusion with the latter
typically much longer than the former. We argue that the analysis of the
long-time decay of membrane height correlations can thus provide a new means to
determine the effective diffusion coefficient of proteins in the membrane.Comment: 12 pages, 8 figure
Giant vesicles at the prolate-oblate transition: A macroscopic bistable system
Giant phospholipid vesicles are shown to exhibit thermally activated
transitions between a prolate and an oblate shape on a time scale of several
seconds. From the fluctuating contour of such a vesicle we extract ellipticity
as an effective reaction coordinate whose temporal probability distribution is
bimodal. We then reconstruct the effective potential from which we derive an
activation energy of the order of in agreement with theoretical
calculations. The dynamics of this transition is well described within a
Kramers model of overdamped diffusion in a bistable potential. Thus, this
system can serve as a model for macroscopic bistability.Comment: 10 pages, LaTeX, epsfig, 4 eps figures included, to appear in
Europhys. Let
Arbitrarily slow, non-quasistatic, isothermal transformations
For an overdamped colloidal particle diffusing in a fluid in a controllable,
virtual potential, we show that arbitrarily slow transformations, produced by
smooth deformations of a double-well potential, need not be reversible. The
arbitrarily slow transformations do need to be fast compared to the barrier
crossing time, but that time can be extremely long. We consider two types of
cyclic, isothermal transformations of a double-well potential. Both start and
end in the same equilibrium state, and both use the same basic operations---but
in different order. By measuring the work for finite cycle times and
extrapolating to infinite times, we found that one transformation required no
work, while the other required a finite amount of work, no matter how slowly it
was carried out. The difference traces back to the observation that when time
is reversed, the two protocols have different outcomes, when carried out
arbitrarily slowly. A recently derived formula relating work production to the
relative entropy of forward and backward path probabilities predicts the
observed work average.Comment: 6 pages, 6 figure
Spheres and Prolate and Oblate Ellipsoids from an Analytical Solution of Spontaneous Curvature Fluid Membrane Model
An analytic solution for Helfrich spontaneous curvature membrane model (H.
Naito, M.Okuda and Ou-Yang Zhong-Can, Phys. Rev. E {\bf 48}, 2304 (1993); {\bf
54}, 2816 (1996)), which has a conspicuous feature of representing the circular
biconcave shape, is studied. Results show that the solution in fact describes a
family of shapes, which can be classified as: i) the flat plane (trivial case),
ii) the sphere, iii) the prolate ellipsoid, iv) the capped cylinder, v) the
oblate ellipsoid, vi) the circular biconcave shape, vii) the self-intersecting
inverted circular biconcave shape, and viii) the self-intersecting nodoidlike
cylinder. Among the closed shapes (ii)-(vii), a circular biconcave shape is the
one with the minimum of local curvature energy.Comment: 11 pages, 11 figures. Phys. Rev. E (to appear in Sept. 1999
Bilayer Membrane in Confined Geometry: Interlayer Slide and Steric Repulsion
We derived free energy functional of a bilayer lipid membrane from the first
principles of elasticity theory. The model explicitly includes
position-dependent mutual slide of monolayers and bending deformation. Our free
energy functional of liquid-crystalline membrane allows for incompressibility
of the membrane and vanishing of the in-plane shear modulus and obeys
reflectional and rotational symmetries of the flat bilayer. Interlayer slide at
the mid-plane of the membrane results in local difference of surface densities
of the monolayers. The slide amplitude directly enters free energy via the
strain tensor. For small bending deformations the ratio between bending modulus
and area compression coefficient, Kb/KA, is proportional to the square of
monolayer thickness, h. Using the functional we performed self-consistent
calculation of steric potential acting on bilayer between parallel confining
walls separated by distance 2d. We found that temperature-dependent curvature
at the minimum of confining potential is enhanced four times for a bilayer with
slide as compared with a unit bilayer. We also calculate viscous modes of
bilayer membrane between confining walls. Pure bending of the membrane is
investigated, which is decoupled from area dilation at small amplitudes. Three
sources of viscous dissipation are considered: water and membrane viscosities
and interlayer drag. Dispersion has two branches. Confinement between the walls
modifies the bending mode with respect to membrane in bulk solution.
Simultaneously, inter-layer slipping mode, damped by viscous drag, remains
unchanged by confinement.Comment: 23 pages,3 figures, pd
Elastic deformation of a fluid membrane upon colloid binding
When a colloidal particle adheres to a fluid membrane, it induces elastic
deformations in the membrane which oppose its own binding. The structural and
energetic aspects of this balance are theoretically studied within the
framework of a Helfrich Hamiltonian. Based on the full nonlinear shape
equations for the membrane profile, a line of continuous binding transitions
and a second line of discontinuous envelopment transitions are found, which
meet at an unusual triple point. The regime of low tension is studied
analytically using a small gradient expansion, while in the limit of large
tension scaling arguments are derived which quantify the asymptotic behavior of
phase boundary, degree of wrapping, and energy barrier. The maturation of
animal viruses by budding is discussed as a biological example of such
colloid-membrane interaction events.Comment: 14 pages, 9 figures, REVTeX style, follow-up on cond-mat/021242
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