281 research outputs found
Droplet traffic in microfluidic networks: A simple model for understanding and designing
We propose a simple model to analyze the traffic of droplets in microfluidic
``dual networks''. Such functional networks which consist of two types of
channels, namely those accessible or forbidden to droplets, often display a
complex behavior characteristic of dynamical systems. By focusing on three
recently proposed configurations, we offer an explanation for their remarkable
behavior. Additionally, the model allows us to predict the behavior in
different parameter regimes. A verification will clarify fundamental issues,
such as the network symmetry, the role of the driving conditions, and of the
occurrence of reversible behavior. The model lends itself to a fast numerical
implementation, thus can help designing devices, identifying parameter windows
where the behavior is sufficiently robust for a devices to be practically
useful, and exploring new functionalities.Comment: accepted for publication in PR
Transverse electrokinetic and microfluidic effects in micro-patterned channels: lubrication analysis for slab geometries
Off-diagonal (transverse) effects in micro-patterned geometries are predicted
and analyzed within the general frame of linear response theory, relating
applied presure gradient and electric field to flow and electric current. These
effects could contribute to the design of pumps, mixers or flow detectors.
Shape and charge density modulations are proposed as a means to obtain sizeable
transverse effects, as demonstrated by focusing on simple geometries and using
the lubrication approximation.Comment: 9 pages, 7 figure
Rheological instability in a simple shear thickening model
We study the strain response to steady imposed stress in a spatially
homogeneous, scalar model for shear thickening, in which the local rate of
yielding \Gamma(l) of mesoscopic `elastic elements' is not monotonic in the
local strain l. Despite this, the macroscopic, steady-state flow curve (stress
vs. strain rate) is monotonic. However, for a broad class of \Gamma(l), the
response to steady stress is not in fact steady flow, but spontaneous
oscillation. We discuss this finding in relation to other theoretical and
experimental flow instabilities. Within the parameter ranges we studied, the
model does not exhibit rheo-chaos.Comment: 8 pages, 3 figs. Minor corrections made. To appear in Euro. Phys.
Let
Monte Carlo simulations of copolymers at homopolymer interfaces: Interfacial structure as a function of the copolymer density
By means of extensive Monte Carlo simulations of the bond fluctuation model,
we study the effect of adding AB diblock copolymers on the properties of an
interface between demixed homopolymer phases. The parameters are chosen such
that the homopolymers are strongly segregated, and the whole range of copolymer
concentrations in the two phase coexistence region is scanned. We compare the
``mushroom'' regime, in which copolymers are diluted and do not interact with
each other, with the ``wet brush'' regime, where copolymers overlap and
stretch, but are still swollen by the homopolymers. A ``dry brush'' regime is
never entered for our choice of chain lengths. ``Intrinsic'' profiles are
calculated using a block analysis method introduced by us in earlier work. We
discuss density profiles, orientational profiles and contact number profiles.
In general, the features of the profiles are similar at all copolymer
concentrations, however, the profiles in the concentrated regime are much
broader than in the dilute regime. The results compare well with
self-consistent field calculations.Comment: to appear in J. Chem. Phy
Coral relocation in Chabahar Bay, the North-east of Oman Sea
Corals are marine benthic animals typically living in compact colonies of many identical individual polyps (Barnes, 1987; Gateno et al., 1996; Sumich, 1996). Coral reefs are important for many reasons including: a) Most importantly, they provide protection and shelter for many different species of fish. b) They turn surplus carbon dioxide in the water into a limestone shell. Without coral, the amount of carbon dioxide in the water would increase dramatically and that would affect all living things on Earth. c) Similar to a barrier, the coral reefs protect coasts from strong currents and waves by slowing down the water before it gets to the shore. d) Coral reef ecosystems support a variety of human needs such as fisheries and tourism (James and Spurgeon, 1992; Moberg and Folke, 1999; Cesar, 2000). Therefore, the conservation of coral colonies is very vital for marine organisms and human. In Chabahar Bay, the coral reefs are in danger of destruction due to the development program of Shahid Beheshti Port. Since the corals are very sensitive to turbidity and suspended sediments from land reclamation and dredging projects, therefore appropriate measures should be conducted for conservation and recovery of them. At present, the coral relocation is suggested as a good method for recovery of coral reefs after a disturbance in condition of their native habitats. In our project, over 28,000 hard corals were transported to coast of Hotel Lipar (Fig. 1), an area at a distance of 3.5 km far from Shahid Beheshti Port. Also, the new techniques were used for coral reattachment and transportation
Topological Measure Locating the Effective Crossover between Segregation and Integration in a Modular Network
We introduce an easily computable topological measure which locates the
effective crossover between segregation and integration in a modular network.
Segregation corresponds to the degree of network modularity, while integration
is expressed in terms of the algebraic connectivity of an associated
hyper-graph. The rigorous treatment of the simplified case of cliques of equal
size that are gradually rewired until they become completely merged, allows us
to show that this topological crossover can be made to coincide with a
dynamical crossover from cluster to global synchronization of a system of
coupled phase oscillators. The dynamical crossover is signaled by a peak in the
product of the measures of intra-cluster and global synchronization, which we
propose as a dynamical measure of complexity. This quantity is much easier to
compute than the entropy (of the average frequencies of the oscillators), and
displays a behavior which closely mimics that of the dynamical complexity index
based on the latter. The proposed toplogical measure simultaneously provides
information on the dynamical behavior, sheds light on the interplay between
modularity vs total integration and shows how this affects the capability of
the network to perform both local and distributed dynamical tasks
Entropy production and Kullback-Leibler divergence between stationary trajectories of discrete systems
The irreversibility of a stationary time series can be quantified using the
Kullback-Leibler divergence (KLD) between the probability to observe the series
and the probability to observe the time-reversed series. Moreover, this KLD is
a tool to estimate entropy production from stationary trajectories since it
gives a lower bound to the entropy production of the physical process
generating the series. In this paper we introduce analytical and numerical
techniques to estimate the KLD between time series generated by several
stochastic dynamics with a finite number of states. We examine the accuracy of
our estimators for a specific example, a discrete flashing ratchet, and
investigate how close is the KLD to the entropy production depending on the
number of degrees of freedom of the system that are sampled in the
trajectories.Comment: 14 pages, 7 figure
Cooperative Transport of Brownian Particles
We consider the collective motion of finite-sized, overdamped Brownian
particles (e.g., motor proteins) in a periodic potential. Simulations of our
model have revealed a number of novel cooperative transport phenomena,
including (i) the reversal of direction of the net current as the particle
density is increased and (ii) a very strong and complex dependence of the
average velocity on both the size and the average distance of the particles.Comment: 4 pages, 5 figure
Random walks of molecular motors arising from diffusional encounters with immobilized filaments
Movements of molecular motors on cytoskeletal filaments are described by
directed walks on a line. Detachment from this line is allowed to occur with a
small probability. Motion in the surrounding fluid is described by symmetric
random walks. Effects of detachment and reattachment are calculated by an
analytical solution of the master equation in two and three dimensions. Results
are obtained for the fraction of bound motors, their average velocity and
displacement. The diffusion coefficient parallel to the filament becomes
anomalously large since detachment and subsequent reattachment, in the presence
of directed motion of the bound motors, leads to a broadening of the density
distribution.
The occurrence of protofilaments on a microtubule is modeled by internal
states of the binding sites. After a transient time all protofilaments become
equally populated.Comment: 20 pages Phys Rev E format + 11 figure
Information and maximum power in a feedback controlled Brownian ratchet
Closed-loop or feedback controlled ratchets are Brownian motors that operate
using information about the state of the system. For these ratchets, we compute
the power output and we investigate its relation with the information used in
the feedback control. We get analytical expressions for one-particle and
few-particle flashing ratchets, and we find that the maximum power output has
an upper bound proportional to the information. In addition, we show that the
increase of the power output that results from changing the optimal open-loop
ratchet to a closed-loop ratchet also has an upper bound that is linear in the
information.Comment: LaTeX, 6 pages, 4 figures, improved version to appear in Eur. Phys.
J.
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