240 research outputs found
On the stress and torque tensors in fluid membranes
We derive the membrane elastic stress and torque tensors using the standard
Helfrich model and a direct variational method in which the edges of a membrane
are infinitesimally translated and rotated. We give simple expressions of the
stress and torque tensors both in the local tangent frame and in projection
onto a fixed frame. We recover and extend the results of Capovilla and Guven
[J. Phys. A, 2002, \textbf{35}, 6233], which were obtained using covariant
geometry and Noether's theorem: we show that the Gaussian rigidity contributes
to the torque tensor and we include the effect of a surface potential in the
stress tensor. Many interesting situations may be investigated directly using
force and torque balances instead of full energy minimization. As examples, we
consider the force exerted at the end of a membrane tubule, membrane adhesion
and domain contact conditions.Comment: 7 pages, 5 figure
Elastic interaction between "hard'' or "soft" pointwise inclusions on biological membranes
We calculate the induced elastic-interaction between pointwise membrane
inclusions that locally interact up to quadratic order with the membrane
curvature tensor. For isotropic inclusions, we recover the usual interaction
proportional to the inverse fourth power of the separation, however with a
prefactor showing a non-trivial dependence on the rigidity of the
quadratic potential. In the large limit, corresponding to ``hard''
inclusions, we recover the standard prefactor first obtained by Goulian et al.
[Europhys. Lett. \textbf{22}, 145 (1993)]. In the small limit,
corresponding to "soft" inclusions, we recover the recent result of Marchenko
and Misbah [Eur. Phys. J. E \textbf{8}, 477 (2002)]. This shows that the latter
result bears no fundamental discrepancy with previous works, but simply
corresponds to the limit of soft inclusions. We discuss how the same inclusion
can be depicted as hard or soft according to the degree of coarse-graining of
the pointwise description. Finally, we argue that conical transmembrane
proteins should be fundamentally considered as hard inclusions.Comment: 6 page
Membrane properties revealed by spatiotemporal response to a local inhomogeneity
We study theoretically the spatiotemporal response of a lipid membrane
submitted to a local chemical change of its environment, taking into account
the time-dependent profile of the reagent concentration due to diffusion in the
solution above the membrane. We show that the effect of the evolution of the
reagent concentration profile becomes negligible after some time. It then
becomes possible to extract interesting properties of the membrane response to
the chemical modification. We find that a local density asymmetry between the
two monolayers relaxes by spreading diffusively in the whole membrane. This
behavior is driven by intermonolayer friction. Moreover, we show how the ratio
of the spontaneous curvature change to the equilibrium density change induced
by the chemical modification can be extracted from the dynamics of the local
membrane deformation. Such information cannot be obtained by analyzing the
equilibrium vesicle shapes that exist in different membrane environments in
light of the area-difference elasticity model.Comment: 11 pages, 4 figure
General up to next-nearest neighbour elasticity of triangular lattices in three dimensions
We establish the most general form of the discrete elasticity of a 2D
triangular lattice embedded in three dimensions, taking into account up to
next-nearest neighbour interactions. Besides crystalline system, this is
relevant to biological physics (e.g., red blood cell cytoskeleton) and soft
matter (e.g., percolating gels, etc.). In order to correctly impose the
rotational invariance of the bulk terms, it turns out to be necessary to take
into account explicitly the elasticity associated with the vertices located at
the edges of the lattice. We find that some terms that were suspected in the
litterature to violate rotational symmetry are in fact admissibl
Bilayer elasticity at the nanoscale: the need for new terms
Continuum elastic models that account for membrane thickness variations are
especially useful in the description of nanoscale deformations due to the
presence of membrane proteins with hydrophobic mismatch. We show that terms
involving the gradient and the Laplacian of the area per lipid are significant
and must be retained in the effective Hamiltonian of the membrane. We reanalyze
recent numerical data, as well as experimental data on gramicidin channels, in
light of our model. This analysis yields consistent results for the term
stemming from the gradient of the area per molecule. The order of magnitude we
find for the associated amplitude, namely 13-60 mN/m, is in good agreement with
the 25 mN/m contribution of the interfacial tension between water and the
hydrophobic part of the membrane. The presence of this term explains a
systematic variation in previously published numerical data.Comment: 34 pages, 9 figure
A Gaussian model for the membrane of red blood cells with cytoskeletal defects
We study a Gaussian model of the membrane of red blood cells: a ``phantom" triangular network of springs attached at its vertices to a fluid bilayer with curvature elasticity and tension. We calculate its fluctuation spectrum and we discuss the different regimes and non-monotonic features, including the precise crossover at the mesh size between the already known limits with two different tensions and the renormalisation of the bending rigidity at low wavevectors. We also show that the non-diagonal correlations reveal, in ``dark field", the cytoskeletal defects. As a first step toward a non-invasive defect spectroscopy, the specific case of lacking bonds is studied numerically and analytically
Universal amplitudes of the Casimir-like interactions between four types of rods in fluid membranes
The fluctuation-induced, Casimir-like interaction between two parallel rods
of length L adsorbed on a fluid membrane is calculated analytically at short
separations d<<L. The rods are modeled as constraints imposed on the membrane
curvature along a straight line. This allows to define four types of rods,
according to whether the membrane can twist along the rod and/or curve across
it. For stiff constraints, all the interaction potentials between the different
types of rods are attractive and proportional to L/d. Two of the four types of
rods are then equivalent, which yields six universal Casimir amplitudes.
Repulsion can occur between different rods for soft constraints. Numerical
results obtained for all ranges of d/L show that the attraction potential
reaches kT for d/L\simeq0.2. At separations smaller than d_c \approx
L(L/l_p)^(1/3), where l_p is the rod persistence length, two rods with fixed
ends will bend toward each other and finally come into contact because of the
Casimir interaction.Comment: 6 pages, 3 figure
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