71 research outputs found
Rectification of Confined Soft Vesicles Containing Active Particles
One of the most promising features of active systems is that they can extract
energy from their environment and convert it to mechanical work. Self propelled
particles enable rectification when in contact with rigid boundaries. They can
rectify their own motion when confined in asymmetric channels and that of
microgears. In this paper, we study the shape fluctuations of two dimensional
flexible vesicles containing active Brownian particles. We show how these
fluctuations not only are capable of easily squeezing a vesicle through narrow
openings, but are also responsible for its rectification when placed within
asymmetric confining channels (ratchetaxis). We detail the conditions under
which this process can be optimized, and sort out the complex interplay between
elastic and active forces responsible for the directed motion of the vesicle
across these channels.Comment: 8 pages, 8 figure
The crumpling transition of active tethered membranes
We perform numerical simulations of active ideal and self-avoiding tethered
membranes. Passive ideal membranes with bending interactions are known to
exhibit a continuous crumpling transition between a low temperature flat phase
and a high temperature crumpled phase. Conversely, self-avoiding membranes
remain in an extended (flat) phase for all temperatures even in the absence of
a bending energy. We find that the introduction of active fluctuations into the
system produces a phase behavior that is overall consistent with that observed
for passive membranes. The phases and the nature of the transition for ideal
membranes is unchanged and active fluctuations can be remarkably accounted for
by a simple rescaling of the temperature. For the self-avoiding membrane, we
find that the extended phase is preserved even in the presence of very large
active fluctuations.Comment: 9 pages, 7 figure
Rigidity transitions in zero-temperature polygons
We study geometrical clues of a rigidity transition due to the emergence of a
system-spanning state of self stress in under-constrained systems of individual
polygons and spring networks constructed from such polygons. When a polygon
with harmonic bond edges and an area spring constraint is subject to an
expansive strain, we observe that convexity of the polygon is a necessary
condition for such a self stress. We prove that the cyclic configuration of the
polygon is a sufficient condition for the self stress. This correspondence of
geometry and rigidity is akin to the straightening of a one dimensional chain
of springs to rigidify it. We predict the onset of the rigidity transition
using a purely geometrical method. We also estimate the transition strain for a
given initial configuration by approximating irregular polygons as regular
polygons. These findings help determine the rigidity of an area-preserving
polygon just by looking at it. Since two-dimensional spring networks can be
considered as a network of polygons, we look for similar geometric features in
under-constrained spring networks under isotropic expansive strain. In
particular, we observe that all polygons attain convexity at the rigidity
transition such that the fraction of convex, but not cyclic, polygons predicts
the onset of the rigidity transition. Interestingly, acyclic polygons in the
network correlate with larger tensions, thus, forming effective force chains.Comment: 12 pages, 10 figure
Spontaneous crumpling of active spherical shells
The existence of a crumpled phase for self-avoiding elastic surfaces was
postulated more than three decades ago using simple Flory-like scaling
arguments. Despite much effort, its stability in a microscopic environment has
been the subject of much debate. In this Letter we show how a crumpled phase
develops reliably and consistently upon subjecting a thin spherical shell to
active fluctuations. We find a master curve describing how the relative volume
of a shell changes with the strength of the active forces, that applies for
every shell independent of size and elastic constants. Furthermore, we extract
a general expression for the onset active force beyond which a shell begins to
crumple. Finally, we calculate how the size exponent varies along the crumpling
curve.Comment: 6 pages and 6 figures including the appendi
Loops versus lines and the compression stiffening of cells
Both animal and plant tissue exhibit a nonlinear rheological phenomenon known
as compression stiffening, or an increase in moduli with increasing uniaxial
compressive strain. Does such a phenomenon exist in single cells, which are the
building blocks of tissues? One expects an individual cell to compression
soften since the semiflexible biopolymer-based cytoskeletal network maintains
the mechanical integrity of the cell and in vitro semiflexible biopolymer
networks typically compression soften. To the contrary, we find that mouse
embryonic fibroblasts (mEFs) compression stiffen under uniaxial compression via
atomic force microscopy (AFM) studies. To understand this finding, we uncover
several potential mechanisms for compression stiffening. First, we study a
single semiflexible polymer loop modeling the actomyosin cortex enclosing a
viscous medium modeled as an incompressible fluid. Second, we study a
two-dimensional semiflexible polymer/fiber network interspersed with
area-conserving loops, which are a proxy for vesicles and fluid-based
organelles. Third, we study two-dimensional fiber networks with
angular-constraining crosslinks, i.e. semiflexible loops on the mesh scale. In
the latter two cases, the loops act as geometric constraints on the fiber
network to help stiffen it via increased angular interactions. We find that the
single semiflexible polymer loop model agrees well with our AFM experiments
until approximately 35% compressive strain. We also find for the fiber network
with area-conserving loops model that the stress-strain curves are sensitive to
the packing fraction and size distribution of the area-conserving loops,
thereby creating a mechanical fingerprint across different cell types. Finally,
we make comparisons between this model and experiments on fibrin networks
interlaced with beads as well as discuss the tissue-scale implications of
cellular compression stiffening.Comment: 19 pages, 17 figure
Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field
We report on the microwave surface resistance of two polycrystalline Mg11B2
samples; one consists of pristine material, the other has been irradiated at
very high neutron fluence. It has already been reported that in the strongly
irradiated sample the two gaps merge into a single value. The mw surface
resistance has been measured in the linear regime as a function of the
temperature and the DC magnetic field, at increasing and decreasing fields. The
results obtained in the strongly irradiated sample are quite well justified in
the framework of a generalized Coffey and Clem model, in which we take into
account the field distribution inside the sample due to the critical state. The
results obtained in the pristine sample show several anomalies, especially at
low temperatures, which cannot be justified in the framework of standard models
for the fluxon dynamics. Only at temperatures near Tc and for magnetic fields
greater than 0.5Hc2(T) the experimental data can quantitatively be accounted
for by the Coffey and Clem model, provided that the upper-critical-field
anisotropy is taken into due account.Comment: RevTeX, 13 pages with 10 eps figures, in press on EPJ
Effect of oxygen incorporation on normal and superconducting properties of MgB2 films
Oxygen was systematically incorporated in MBE grown MgB2 films using in-situ
post-growth anneals in an oxygen environment. Connectivity analysis in
combination with measurements of the critical temperature and resistivity
indicate that oxygen is distributed both within and between the grains. High
values of critical current densities in field (~4x10^5 A/cm^2 at 8 T and 4.2
K), extrabolated critical fields (>45 T) and slopes of critical field versus
temperature (1.4 T/K) are observed. Our results suggest that low growth
temperatures (300oC) and oxygen doping (>0.65%) can produce MgB2 with high Jc
values in field and Hc2 for high-field magnet applications.Comment: 10 pages, 3 figure
High-field superconductivity in alloyed MgB2 thin films
We investigated the effect of alloying on the upper critical field
in 12 films, in which disorder was introduced by growth, carbon doping
or He-ion irradiation, finding a significant enhancement in C-alloyed
films, and an anomalous upward curvature of . Record high values of
and were observed
perpendicular and parallel to the ab plane, respectively. The temperature
dependence of is described well by a theory of dirty two-gap
superconductivity. Extrapolation of the experimental data to T=0 suggests that
approaches the paramagnetic limit of
The microRNA regulated SBP-box genes SPL9 and SPL15 control shoot maturation in Arabidopsis
Throughout development the Arabidopsis shoot apical meristem successively undergoes several major phase transitions such as the juvenile-to-adult and floral transitions until, finally, it will produce flowers instead of leaves and shoots. Members of the Arabidopsis SBP-box gene family of transcription factors have been implicated in promoting the floral transition in dependence of miR156 and, accordingly, transgenics constitutively over-expressing this microRNA are delayed in flowering. To elaborate their roles in Arabidopsis shoot development, we analysed two of the 11 miR156 regulated Arabidopsis SBP-box genes, i.e. the likely paralogous genes SPL9 and SPL15. Single and double mutant phenotype analysis showed these genes to act redundantly in controlling the juvenile-to-adult phase transition. In addition, their loss-of-function results in a shortened plastochron during vegetative growth, altered inflorescence architecture and enhanced branching. In these aspects, the double mutant partly phenocopies constitutive MIR156b over-expressing transgenic plants and thus a major contribution to the phenotype of these transgenics as a result of the repression of SPL9 and SPL15 is strongly suggested
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