3,070 research outputs found
Mean flow in hexagonal convection: stability and nonlinear dynamics
Weakly nonlinear hexagon convection patterns coupled to mean flow are
investigated within the framework of coupled Ginzburg-Landau equations. The
equations are in particular relevant for non-Boussinesq Rayleigh-B\'enard
convection at low Prandtl numbers. The mean flow is found to (1) affect only
one of the two long-wave phase modes of the hexagons and (2) suppress the
mixing between the two phase modes. As a consequence, for small Prandtl numbers
the transverse and the longitudinal phase instability occur in sufficiently
distinct parameter regimes that they can be studied separately. Through the
formation of penta-hepta defects, they lead to different types of transient
disordered states. The results for the dynamics of the penta-hepta defects shed
light on the persistence of grain boundaries in such disordered states.Comment: 33 pages, 20 figures. For better
figures:http://astro.uchicago.edu/~young/hexmeandi
Theoretical Model for Cellular Shapes Driven by Protrusive and Adhesive Forces
The forces that arise from the actin cytoskeleton play a crucial role in determining the cell shape. These include protrusive forces due to actin polymerization and adhesion to the external matrix. We present here a theoretical model for the cellular shapes resulting from the feedback between the membrane shape and the forces acting on the membrane, mediated by curvature-sensitive membrane complexes of a convex shape. In previous theoretical studies we have investigated the regimes of linear instability where spontaneous formation of cellular protrusions is initiated. Here we calculate the evolution of a two dimensional cell contour beyond the linear regime and determine the final steady-state shapes arising within the model. We find that shapes driven by adhesion or by actin polymerization (lamellipodia) have very different morphologies, as observed in cells. Furthermore, we find that as the strength of the protrusive forces diminish, the system approaches a stabilization of a periodic pattern of protrusions. This result can provide an explanation for a number of puzzling experimental observations regarding cellular shape dependence on the properties of the extra-cellular matrix
Stability of monolayers and bilayers in a copolymer-homopolymer blend model
We study the stability of layered structures in a variational model for
diblock copolymer-homopolymer blends. The main step consists of calculating the
first and second derivative of a sharp-interface Ohta-Kawasaki energy for
straight mono- and bilayers. By developing the interface perturbations in a
Fourier series we fully characterise the stability of the structures in terms
of the energy parameters.
In the course of our computations we also give the Green's function for the
Laplacian on a periodic strip and explain the heuristic method by which we
found it.Comment: 40 pages, 34 Postscript figures; second version has some minor
corrections; to appear in "Interfaces and Free Boundaries
Three basic issues concerning interface dynamics in nonequilibrium pattern formation
These are lecture notes of a course given at the 9th International Summer
School on Fundamental Problems in Statistical Mechanics, held in Altenberg,
Germany, in August 1997. In these notes, we discuss at an elementary level
three themes concerning interface dynamics that play a role in pattern forming
systems: (i) We briefly review three examples of systems in which the normal
growth velocity is proportional to the gradient of a bulk field which itself
obeys a Laplace or diffusion type of equation (solidification, viscous fingers
and streamers), and then discuss why the Mullins-Sekerka instability is common
to all such gradient systems. (ii) Secondly, we discuss how underlying an
effective interface description of systems with smooth fronts or transition
zones, is the assumption that the relaxation time of the appropriate order
parameter field(s) in the front region is much smaller than the time scale of
the evolution of interfacial patterns. Using standard arguments we illustrate
that this is generally so for fronts that separate two (meta)stable phases: in
such cases, the relaxation is typically exponential, and the relaxation time in
the usual models goes to zero in the limit in which the front width vanishes.
(iii) We finally summarize recent results that show that so-called ``pulled''
or ``linear marginal stability'' fronts which propagate into unstable states
have a very slow universal power law relaxation. This slow relaxation makes the
usual ``moving boundary'' or ``effective interface'' approximation for problems
with thin fronts, like streamers, impossible.Comment: 48 pages, TeX with elsart style file (included), 9 figure
Particles with selective wetting affect spinodal decomposition microstructures
We have used mesoscale simulations to study the effect of immobile particles
on microstructure formation during spinodal decomposition in ternary mixtures
such as polymer blends. Specifically, we have explored a regime of
interparticle spacings (which are a few times the characteristic spinodal
length scale) in which we might expect interesting new effects arising from
interactions among wetting, spinodal decomposition and coarsening. In this
paper, we report three new effects for systems in which the particle phase has
a strong preference for being wetted by one of the components (say, A). In the
presence of particles, microstructures are not bicontinuous in a symmetric
mixture. An asymmetric mixture, on the other hand, first forms a
non-bicontinuous microstructure which then evolves into a bicontinuous one at
intermediate times. Moreover, while wetting of the particle phase by the
preferred component (A) creates alternating A-rich and B-rich layers around the
particles, curvature-driven coarsening leads to shrinking and disappearance of
the first A-rich layer, leaving a layer of the non-preferred component in
contact with the particle. At late simulation times, domains of the matrix
components coarsen following the Lifshitz-Slyozov-Wagner law, .Comment: Accepted for publication in PCCP on 24th May 201
- …