11,332 research outputs found
Coupling between membrane tilt-difference and dilation: a new ``ripple'' instability and multiple crystalline inclusions phases
A continuum Landau theory for the micro-elasticity of membranes is discussed,
which incorporates a coupling between the bilayer thickness variation and the
difference in the two monolayers' tilts. This coupling stabilizes a new phase
with a rippled micro-structure. Interactions among membrane inclusions combine
a dilation-induced attraction and a tilt-difference-induced repulsion that
yield 2D crystal phases, with possible coexistence of different lattice
spacings for large couplings. Inclusions favoring crystals are those with
either a long-convex or a short-concave hydrophobic core.Comment: EURO LaTeX, 6 pages, 4 figures, to be published in Europhys. Let
Wormlike chain or tense string? A question of resolution
It is shown that a wormlike chain, i.e., a filament with a fixed
contour-length S and a bending elasticity kappa, attached to a frame of length
L, can be described--at low resolutions--by the same type of elastic
free-energy as a tense string. The corresponding tension is calculated as a
function of temperature, L, kappa and S.Comment: 13 pages, 3 figures. To appear in Continuum Mechanics and
Thermodynamic
Microscopic membrane elasticity and interactions among membrane inclusions: Interplay between the shape, dilation, tilt and tilt-difference modes
A phenomenological Landau elasticity for the shape, dilation, and lipid-tilt
of bilayer membranes is developed. The shape mode couples with the sum of the
monolayers' tilt, while the dilation mode couples with the difference of the
monolayers' tilts. Interactions among membrane inclusions within regular arrays
are discussed. Inclusions modifying the membrane thickness and/or inducing a
tilt-difference due to their convex or concave shape yield a dilation-induced
attraction and a tilt-difference-induced repulsion. The resulting interaction
can stabilize 2D crystal phases, with the possible coexistence of different
lattice spacings when the dilation-tilt-difference coupling is large.
Inclusions favoring crystals are those with either a long-convex or a
short-concave hydrophobic core. Inclusions inducing a local membrane curvature
due to their conical shape repel one another. At short inclusions separations,
a tilt comparable with the inclusion's cone angle develops: it relaxes the
membrane curvature and reduces the repulsion. At large separations the tilt
vanishes, whatever the value of the shape-tilt coupling.Comment: 13 pages, 19 figure
N-body Study of Anisotropic Membrane Inclusions: Membrane Mediated Interactions and Ordered Aggregation
We study the collective behavior of inclusions inducing local anisotropic
curvatures in a flexible fluid membrane. The N-body interaction energy for
general anisotropic inclusions is calculated explicitly, including multi-body
interactions. Long-range attractive interactions between inclusions are found
to be sufficiently strong to induce aggregation. Monte Carlo simulations show a
transition from compact clusters to aggregation on lines or circles. These
results might be relevant to proteins in biological membranes or colloidal
particles bound to surfactant membranes.Comment: 4 pages, 3 figs, LaTe
On the surface tension of fluctuating quasi-spherical vesicles
We calculate the stress tensor for a quasi-spherical vesicle and we thermally
average it in order to obtain the actual, mechanical, surface tension of
the vesicle. Both closed and poked vesicles are considered. We recover our
results for by differentiating the free-energy with respect to the
proper projected area. We show that may become negative well before the
transition to oblate shapes and that it may reach quite large negative values
in the case of small vesicles. This implies that spherical vesicles may have an
inner pressure lower than the outer one.Comment: To appear in Eur. Phys. J. E, revised versio
Effect of nose bluntness and afterbody shape on aerodynamic characteristics of a monoplanar missile concept with bodies of circular and elliptical cross sections at a Mach number of 2.50
The tests were performed at a Mach number of 2.50 and at angles of attack from about -4 deg to 32 deg. The results indicate that increasing nose bluntness increases zero lift drag and decreases both the maximum lift-drag ratio and the level of directional stability. The center of pressure generally moves forward with increasing nose size; however, small nose radii on the modified elliptical configurations move the center of pressure rearward. The circular bodied configurations exhibit the greatest longitudinal stability and the least directional stability. Concepts with the variable geometry afterbody contour display the most directional stability and the greatest zero lift drag
Aerodynamic characteristics of a supersonic cruise airplane configuration at Mach numbers of 2.30, 2.96, and 3.30
An investigation was made in the Langley Unitary Plan wind tunnel at Mach numbers of 2.30, 2.96, and 3.30 to determine the static longitudinal and lateral aerodynamic characteristics of a model of a supersonic cruise airplane. The configuration, with a design Mach number of 3.0, has a highly swept arrow wing with tip panels of lesser sweep, a fuselage chine, outboard vertical tails, and outboard engines mounted in nacelles beneath the wings. For wind tunnel test conditions, a trimmed value above 6.0 of the maximum lift-drag ratio was obtained at the design Mach number. The configuration was statically stable, both longitudinally and laterally. Data are presented for variations of vertical-tail roll-out and toe-in and for various combinations of components. Some roll control data are shown as are data for the various sand grit sizes used in fixing the boundary layer transition location
Dynamin recruitment by clathrin coats: a physical step?
Recent structural findings have shown that dynamin, a cytosol protein playing
a key-role in clathrin-mediated endocytosis, inserts partly within the lipid
bilayer and tends to self-assemble around lipid tubules. Taking into account
these observations, we make the hypothesis that individual membrane inserted
dynamins imprint a local cylindrical curvature to the membrane. This imprint
may give rise to long-range mechanical forces mediated by the elasticity of the
membrane. Calculating the resulting many-body interaction between a collection
of inserted dynamins and a membrane bud, we find a regime in which the dynamins
are elastically recruited by the bud to form a collar around its neck, which is
reminiscent of the actual process preempting vesicle scission. This physical
mechanism might therefore be implied in the recruitment of dynamins by clathrin
coats.Comment: 11 pages, 6 figures, to appear in C.R.A.S. ser II
Stability and control characteristics at Mach numbers from 0.20 to 4.63 of a cruciform air-to-air missile with triangular canard controls and a trapezoidal wing
Investigations have been conducted in the Langley 8-foot transonic pressure tunnel and the Langley Unitary Plan wind tunnel at Mach numbers from 0.20 to 4.63 to determine the stability and control characteristics of a cruciform air-to-air missile with triangular canard controls and a trapezoidal wing. The results indicate that canards are effective in producing pitching moment throughout most of the test angle-of-attack and Mach number range and that the variations of pitching moment with lift for trim conditions are relatively linear. There is a decrease in canard effectiveness with an increase in angle of attack up to about Mach 2.50 as evidenced by the beginning of coalescence of the pitching-moment curves. At a Mach number above 2.50, there is an increase in effectiveness at moderate to high angles of attack. Simulated launch straps have little effect on the lift and pitch characteristics but do cause an increase in drag, and this increase in drag induces a rolling moment at a zero roll attitude where the straps cause an asymmetric geometric shape. The canards are not suitable devices for roll control and, at some Mach numbers and roll attitudes, are not effective in producing pure yawing moments
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