1,721,030 research outputs found
A generic model for the thermodynamics and structure of lipid membranes containing integral and peripheral proteins.
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A generic model for the thermodynamics and structure of lipid membranes containing integral and peripheral proteins.
No full text
Amoeboid propulsion of active solid bodies, vesicles and droplets: a comparison
No full textRace between micro swimmers: droplets (blue), vesicles (orange) and deformable solids (green), driven by the same swim stroke, exhibit very different velocities and efficiencies.We present a unified discussion of three types of near-spherical amoeboid microswimmers, driven by periodic, axially symmetric, achiral deformations (swim strokes): a solid deformable body, a vesicle with incompressible fluid membrane, and a droplet. Minimal models are used, which characterize the swimmer type only by boundary conditions. We calculate the swimming velocities, the dissipated power and the Lighthill efficiencies within a second order perturbation expansion in the small deformation amplitudes. For solid bodies, we reproduce older results by Lighthill and Blake, for vesicles and for droplets we add new results. The unified approach allows for a detailed comparison between the three types of microswimmers. We present such comparisons for swim strokes made up of spherical harmonics of adjacent orders l and l + 1, as well as for a manifold of swim strokes, made up of spherical harmonics up to order l = 4, which respect volume- and surface-incompressibility. This manifold is two-dimensional, which allows to present swimming velocities and efficiencies in compact graphical form. In a race in which each swimmer can choose the stroke that maximizes its speed, the droplet always comes in first, the vesicle comes in second, while the particle finishes third. However, if the three swimmers perform the same stroke, other order of rankings become possible. The maximum of the total efficiency of a droplet is greater than that of a vesicle if the internal dissipation is small. The efficiency of the solid body turns out to be typically two orders of magnitude smaller than that of vesicles and droplets. Optimizing the Lighthill efficiency and optimizing the swimming velocity result in different optimal swim strokes.Race between micro swimmers: droplets (blue), vesicles (orange) and deformable solids (green), driven by the same swim stroke, exhibit very different velocities and efficiencies.We present a unified discussion of three types of near-spherical amoeboid microswimmers, driven by periodic, axially symmetric, achiral deformations (swim strokes): a solid deformable body, a vesicle with incompressible fluid membrane, and a droplet. Minimal models are used, which characterize the swimmer type only by boundary conditions. We calculate the swimming velocities, the dissipated power and the Lighthill efficiencies within a second order perturbation expansion in the small deformation amplitudes. For solid bodies, we reproduce older results by Lighthill and Blake, for vesicles and for droplets we add new results. The unified approach allows for a detailed comparison between the three types of microswimmers. We present such comparisons for swim strokes made up of spherical harmonics of adjacent orders l and l + 1, as well as for a manifold of swim strokes, made up of spherical harmonics up to order l = 4, which respect volume- and surface-incompressibility. This manifold is two-dimensional, which allows to present swimming velocities and efficiencies in compact graphical form. In a race in which each swimmer can choose the stroke that maximizes its speed, the droplet always comes in first, the vesicle comes in second, while the particle finishes third. However, if the three swimmers perform the same stroke, other order of rankings become possible. The maximum of the total efficiency of a droplet is greater than that of a vesicle if the internal dissipation is small. The efficiency of the solid body turns out to be typically two orders of magnitude smaller than that of vesicles and droplets. Optimizing the Lighthill efficiency and optimizing the swimming velocity result in different optimal swim strokes
A Monte Carlo study of surface sputtering by dual and rotated ion beams
Full text linkSeveral, recently proposed methods of surface manufacturing based on ion beam
sputtering, which involve dual beam setups, sequential application of ion beams
from different directions, or sample rotation, are studied with the method of
kinetic Monte Carlo simulation of ion beam erosion and surface diffusion. In
this work, we only consider erosion dominated situations. The results are
discussed by comparing them to a number of theoretical propositions and to
experimental findings. Two ion-beams aligned opposite to each other produce
stationary, symmetric ripples. Two ion beams crossing at right angle will
produce square patterns only, if they are exactly balanced. In all other cases
of crossed beams, ripple patterns are created, and their orientations are shown
to be predictable from linear continuum theory. In sequential ion beam
sputtering we find a very rapid destruction of structures created from the
previous beam direction after a rotation step, which leads to a transient
decrease of overall roughness. Superpositions of patterns from several rotation
steps are difficult to obtain, as they exist only in very short time windows.
In setups with a single beam directed towards a rotating sample, we find a
non-monotonic dependence of roughness on rotation frequency, with a very
pronounced minimum appearing at the frequency scale set by the relaxation of
prestructures observed in sequential ion beam setups. Furthermore we find that
the logarithm of the height of structures decreases proportional to the inverse
frequency
Pattern formation of ion channels with state-dependent charges and diffusion constants in fluid membranes
No full textA model of mobile, charged ion channels in a fluid membrane is studied. The channels may switch between an open and a closed state according to a simple two-state kinetics with constant rates. The effective electrophoretic charge and the diffusion constant of the channels may be different in the closed and in the open state. The system is modeled by densities of channel species, obeying simple equations of electrodiffusion. The lateral transmembrane voltage profile is determined from a cable-type equation. Bifurcations from the homegeneous, stationary state appear as hard-mode, soft-mode. or hard-mode oscillatory transitions within physiologically reasonable ranges of model parameters. We Study the dynamics beyond linear stability analysis and derive nonlinear evolution equations near the transitions to stationary patterns
The influence of beam divergence on ion-beam induced surface patterns
No full textWe present a continuum theory and a Monte Carlo model of self-organized surface pattern formation by ion-beam sputtering including effects of beam profiles. Recently, it has turned out that such secondary ion-beam parameters may have a strong influence on the types of emerging patterns. We first discuss several cases, for which beam profiles lead to random parameters in the theory of pattern formation. Subsequently we study the evolution of the averaged height profile in continuum theory and find that the typical Bradley-Harper scenario of dependence of ripple patterns on the angle of incidence can be changed qualitatively. Beam profiles are implemented in Monte Carlo simulations, where we find generic effects on pattern formation. Finally, we demonstrate that realistic beam profiles, taken from experiments, may lead to qualitative changes of surface patterns. (C) 2009 Elsevier B.V. All rights reserved
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