2,699 research outputs found
A three-sphere swimmer for flagellar synchronization
In a recent letter (Friedrich et al., Phys. Rev. Lett. 109:138102, 2012), a
minimal model swimmer was proposed that propels itself at low Reynolds numbers
by a revolving motion of a pair of spheres. The motion of the two spheres can
synchronize by virtue of a hydrodynamic coupling that depends on the motion of
the swimmer, but is rather independent of direct hydrodynamic interactions.
This novel synchronization mechanism could account for the synchronization of a
pair of flagella, e.g. in the green algae Chlamydomonas. Here, we discuss in
detail how swimming and synchronization depend on the geometry of the model
swimmer and compute the swimmer design for optimal synchronization. Our
analysis highlights the role of broken symmetries for swimming and
synchronization.Comment: 25 pages, 4 color figures, provisionally accepted for publication in
the New Journal of Physic
The Impact of Trust on Reforms
In a constantly changing economic environment a country's ability to undertake institutional reforms is crucial to maintain economic growth and to promote the welfare of its citizens. A wide range of determinants for institutional reforms have been identified. However, the impact of trust on reforms has so far never been addressed. We provide theoretical arguments why trust should influence institutional changes and test the relationship empirically. We find a significant positive relation between trust and reforms with regard to government size, the legal system, and deregulation of private businesses and the labor market. The results in other policy fields are ambiguous. --Trust,Economic Freedom,Policy Reforms
Absence of photoemission from the Fermi level in potassium intercalated picene and coronene films: structure, polaron or correlation physics?
The electronic structure of potassium intercalated picene and coronene films
has been studied using photoemission spectroscopy. Picene has additionally been
intercalated using sodium. Upon alkali metal addition core level as well as
valence band photoemission data signal a filling of previously unoccupied
states of the two molecular materials due to charge transfer from potassium. In
contrast to the observation of superconductivity in K_xpicene and K_xcoronene
(x ~ 3), none of the films studied shows emission from the Fermi level, i.e. we
find no indication for a metallic ground state. Several reasons for this
observation are discussed.Comment: 15 pages, 6 figure
Nematic order by elastic interactions and cellular rigidity sensing
We predict spontaneous nematic order in an ensemble of active force
generators with elastic interactions as a minimal model for early nematic
alignment of short stress fibers in non-motile, adhered cells. Mean-field
theory is formally equivalent to Maier-Saupe theory for a nematic liquid.
However, the elastic interactions are long-ranged (and thus depend on cell
shape and matrix elasticity) and originate in cell activity. Depending on the
density of force generators, we find two regimes of cellular rigidity sensing
for which orientational, nematic order of stress fibers depends on matrix
rigidity either in a step-like manner or with a maximum at an optimal rigidity.Comment: 12 pages, 4 figure
Flagellar swimmers oscillate between pusher- and puller-type swimming
Self-propulsion of cellular microswimmers generates flow signatures, commonly
classified as pusher- and puller-type, which characterize hydrodynamic
interactions with other cells or boundaries. Using experimentally measured beat
patterns, we compute that flagellated alga and sperm oscillate between pusher
and puller. Beyond a typical distance of 100 um from the swimmer, inertia
attenuates oscillatory micro-flows. We show that hydrodynamic interactions
between swimmers oscillate in time and are of similar magnitude as stochastic
swimming fluctuations.Comment: 12 pages, 4 color figure
Deterministic Random Walks on Regular Trees
Jim Propp's rotor router model is a deterministic analogue of a random walk
on a graph. Instead of distributing chips randomly, each vertex serves its
neighbors in a fixed order.
Cooper and Spencer (Comb. Probab. Comput. (2006)) show a remarkable
similarity of both models. If an (almost) arbitrary population of chips is
placed on the vertices of a grid and does a simultaneous walk in the
Propp model, then at all times and on each vertex, the number of chips on this
vertex deviates from the expected number the random walk would have gotten
there by at most a constant. This constant is independent of the starting
configuration and the order in which each vertex serves its neighbors.
This result raises the question if all graphs do have this property. With
quite some effort, we are now able to answer this question negatively. For the
graph being an infinite -ary tree (), we show that for any
deviation there is an initial configuration of chips such that after
running the Propp model for a certain time there is a vertex with at least
more chips than expected in the random walk model. However, to achieve a
deviation of it is necessary that at least vertices
contribute by being occupied by a number of chips not divisible by at a
certain time.Comment: 15 pages, to appear in Random Structures and Algorithm
Quasirandom Rumor Spreading: An Experimental Analysis
We empirically analyze two versions of the well-known "randomized rumor
spreading" protocol to disseminate a piece of information in networks. In the
classical model, in each round each informed node informs a random neighbor. In
the recently proposed quasirandom variant, each node has a (cyclic) list of its
neighbors. Once informed, it starts at a random position of the list, but from
then on informs its neighbors in the order of the list. While for sparse random
graphs a better performance of the quasirandom model could be proven, all other
results show that, independent of the structure of the lists, the same
asymptotic performance guarantees hold as for the classical model. In this
work, we compare the two models experimentally. This not only shows that the
quasirandom model generally is faster, but also that the runtime is more
concentrated around the mean. This is surprising given that much fewer random
bits are used in the quasirandom process. These advantages are also observed in
a lossy communication model, where each transmission does not reach its target
with a certain probability, and in an asynchronous model, where nodes send at
random times drawn from an exponential distribution. We also show that
typically the particular structure of the lists has little influence on the
efficiency.Comment: 14 pages, appeared in ALENEX'0
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