53 research outputs found
Generalized Elastic Model: thermal vs non-thermal initial conditions. Universal scaling, roughening, ageing and ergodicity
We study correlation properties of the generalized elastic model which
accounts for the dynamics of polymers, membranes, surfaces and fluctuating
interfaces, among others. We develop a theoretical framework which leads to the
emergence of universal scaling laws for systems starting from thermal
(equilibrium) or non-thermal (non-equilibrium) initial conditions. Our analysis
incorporates and broadens previous results such as observables' double scaling
regimes, (super)roughening and anomalous diffusion, and furnishes a new scaling
behavior for correlation functions at small times (long distances). We discuss
ageing and ergodic properties of the generalized elastic model in
non-equilibrium conditions, providing a comparison with the situation occurring
in continuous time random walk. Our analysis also allows to assess which
observable is able to distinguish whether the system is in or far from
equilibrium conditions in an experimental set-up
General theory for plane extensible elastica with arbitrary undeformed shape
A general expression for the strain energy of a homogeneous, isotropic, plane extensible elastica with an arbitrary undeformed configuration is derived. This expression appears to be suitable for one-dimensional models of polymers or vesicles, the natural configuration of which is characterized by locally changing curvature. In a linear setting, we derive the macroscopic stress–strain relations, providing an universal criterion for the neutral curve location. In this respect, we further demonstrate that the neutral curve existence constitutes the fundamental requirement for the conformational dynamics of any inextensbile biological filament
Stationary growth and unique invariant harmonic measure of cylindrical DLA
We prove that the harmonic measure is stationary, unique and invariant on the
interface of DLA growing on a cylinder surface. We provide a detailed
theoretical analysis puzzling together multiscaling, multifractality and
conformal invariance, supported by extensive numerical simulations of clusters
built using conformal mappings and on lattice. The growth properties of the
active and frozen zones are clearly elucidated. We show that the unique scaling
exponent characterizing the stationary growth is the DLA fractal dimension
Volume changes during active shape fluctuations in cells
Cells modify their volume in response to changes in osmotic pressure but it
is usually assumed that other active shape variations do not involve
significant volume fluctuations. Here we report experiments demonstrating that
water transport in and out of the cell is needed for the formation of blebs,
commonly observed protrusions in the plasma membrane driven by cortex
contraction. We develop and simulate a model of fluid mediated membrane-cortex
deformations and show that a permeable membrane is necessary for bleb formation
which is otherwise impaired. Taken together our experimental and theoretical
results emphasize the subtle balance between hydrodynamics and elasticity in
actively driven cell morphological changes.Comment: Phys. Rev. Lett. in press. 13 pages 4 figures, 9 supplementary
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Mechanical Properties of Growing Melanocytic Nevi and the Progression to Melanoma
Melanocytic nevi are benign proliferations that sometimes turn into malignant
melanoma in a way that is still unclear from the biochemical and genetic point
of view. Diagnostic and prognostic tools are then mostly based on dermoscopic
examination and morphological analysis of histological tissues. To investigate
the role of mechanics and geometry in the morpholgical dynamics of melanocytic
nevi, we study a computation model for cell proliferation in a layered
non-linear elastic tissue. Numerical simulations suggest that the morphology of
the nevus is correlated to the initial location of the proliferating cell
starting the growth process and to the mechanical properties of the tissue. Our
results also support that melanocytes are subject to compressive stresses that
fluctuate widely in the nevus and depend on the growth stage. Numerical
simulations of cells in the epidermis releasing matrix metalloproteinases
display an accelerated invasion of the dermis by destroying the basal membrane.
Moreover, we suggest experimentally that osmotic stress and collagen inhibit
growth in primary melanoma cells while the effect is much weaker in metastatic
cells. Knowing that morphological features of nevi might also reflect geometry
and mechanics rather than malignancy could be relevant for diagnostic purpose
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