1,748 research outputs found
The unbinding transition of mixed fluid membranes
A phenomenological model for the unbinding transition of multi-component
fluid membranes is proposed, where the unbinding transition is described using
a theory analogous to Flory-Huggins theory for polymers. The coupling between
the lateral phase separation of inclusion molecules and the membrane-substrate
distance explains the phase coexistence between two unbound phases as observed
in recent experiments by Marx et al. [Phys. Rev. Lett. 88, 138102 (2002)].
Bellow a critical end-point temperature, we find that the unbinding transition
becomes first-order for multi-component membranes.Comment: 7 pages, 3 eps figure
Are stress-free membranes really 'tensionless'?
In recent years it has been argued that the tension parameter driving the
fluctuations of fluid membranes, differs from the imposed lateral stress, the
'frame tension'. In particular, stress-free membranes were predicted to have a
residual fluctuation tension. In the present paper, this argument is
reconsidered and shown to be inherently inconsistent -- in the sense that a
linearized theory, the Monge model, is used to predict a nonlinear effect.
Furthermore, numerical simulations of one-dimensional stiff membranes are
presented which clearly demonstrate, first, that the internal 'intrinsic'
stress in membranes indeed differs from the frame tension as conjectured, but
second, that the fluctuations are nevertheless driven by the frame tension.
With this assumption, the predictions of the Monge model agree excellently with
the simulation data for stiffness and tension values spanning several orders of
magnitude
Compression modulus of macroscopic fiber bundles
We study dense, disordered stacks of elastic macroscopic fibers. These stacks
often exhibit non-linear elasticity, due to the coupling between the applied
stress and the internal distribution of fiber contacts. We propose a
theoretical model for the compression modulus of such systems, and illustrate
our method by studying the conical shapes frequently observed at the
extremities of ropes and other fiber structures. studying the conical shapes
frequently observed at theextremities of ropes and other fiber structures
Rigid Chiral Membranes
Statistical ensembles of flexible two-dimensional fluid membranes arise
naturally in the description of many physical systems. Typically one encounters
such systems in a regime of low tension but high stiffness against bending,
which is just the opposite of the regime described by the Polyakov string. We
study a class of couplings between membrane shape and in-plane order which
break 3-space parity invariance. Remarkably there is only {\it one} such
allowed coupling (up to boundary terms); this term will be present for any
lipid bilayer composed of tilted chiral molecules. We calculate the
renormalization-group behavior of this relevant coupling in a simplified model
and show how thermal fluctuations effectively reduce it in the infrared.Comment: 11 pages, UPR-518T (This replaced version has fonts not used
removed.
Swelling of particle-encapsulating random manifolds
We study the statistical mechanics of a closed random manifold of fixed area
and fluctuating volume, encapsulating a fixed number of noninteracting
particles. Scaling analysis yields a unified description of such swollen
manifolds, according to which the mean volume gradually increases with particle
number, following a single scaling law. This is markedly different from the
swelling under fixed pressure difference, where certain models exhibit
criticality. We thereby indicate when the swelling due to encapsulated
particles is thermodynamically inequivalent to that caused by fixed pressure.
The general predictions are supported by Monte Carlo simulations of two
particle-encapsulating model systems -- a two-dimensional self-avoiding ring
and a three-dimensional self-avoiding fluid vesicle. In the former the
particle-induced swelling is thermodynamically equivalent to the
pressure-induced one whereas in the latter it is not.Comment: 8 pages, 6 figure
Possible effects of tilt order on phase transitions of a fixed connectivity surface model
We study the phase structure of a phantom tethered surface model shedding
light on the internal degrees of freedom (IDOF), which correspond to the
three-dimensional rod like structure of the lipid molecules. The so-called tilt
order is assumed as IDOF on the surface model. The model is defined by
combining the conventional spherical surface model and the XY model, which
describes not only the interaction between lipids but also the interaction
between the lipids and the surface. The interaction strength between IDOF and
the surface varies depending on the interaction strength between the variables
of IDOF. We know that the model without IDOF undergoes a first-order transition
of surface fluctuations and a first-order collapsing transition. We observe in
this paper that the order of the surface fluctuation transition changes from
first-order to second-order and to higher-order with increasing strength of the
interaction between IDOF variables. On the contrary, the order of collapsing
transition remains first-order and is not influenced by the presence of IDOF.Comment: 20 pages, 14 figure
Collapsing transition of spherical tethered surfaces with many holes
We investigate a tethered (i.e. fixed connectivity) surface model on
spherical surfaces with many holes by using the canonical Monte Carlo
simulations. Our result in this paper reveals that the model has only a
collapsing transition at finite bending rigidity, where no surface fluctuation
transition can be seen. The first-order collapsing transition separates the
smooth phase from the collapsed phase. Both smooth and collapsed phases are
characterized by Hausdorff dimension H\simeq 2, consequently, the surface
becomes smooth in both phases. The difference between these two phases can be
seen only in the size of surface. This is consistent with the fact that we can
see no surface fluctuation transition at the collapsing transition point. These
two types of transitions are well known to occur at the same transition point
in the conventional surface models defined on the fixed connectivity surfaces
without holes.Comment: 7 pages with 11 figure
Fluctuations of the Casimir-like force between two membrane inclusions
Although Casimir forces are inseparable from their fluctuations, little is
known about these fluctuations in soft matter systems. We use the membrane
stress tensor to study the fluctuations of the membrane-mediated Casimir-like
force. This method enables us to recover the Casimir force between two
inclusions and to calculate its variance. We show that the Casimir force is
dominated by its fluctuations. Furthermore, when the distance d between the
inclusions is decreased from infinity, the variance of the Casimir force
decreases as -1/d^2. This distance dependence shares a common physical origin
with the Casimir force itself.Comment: 5 pages, 3 figure
Hydrodynamic lift of vesicles under shear flow in microgravity
The dynamics of a vesicle suspension in a shear flow between parallel plates
has been investigated under microgravity conditions, where vesicles are only
submitted to hydrodynamic effects such as lift forces due to the presence of
walls and drag forces. The temporal evolution of the spatial distribution of
the vesicles has been recorded thanks to digital holographic microscopy, during
parabolic flights and under normal gravity conditions. The collected data
demonstrates that vesicles are pushed away from the walls with a lift velocity
proportional to where is the shear rate,
the vesicle radius and its distance from the wall. This scaling as well
as the dependence of the lift velocity upon vesicle aspect ratio are consistent
with theoretical predictions by Olla [J. Phys. II France {\bf 7}, 1533--1540
(1997)].Comment: 6 pages, 8 figure
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