15,400 research outputs found
Self-Organized Vortices of Circling Self-Propelled Particles and Curved Active Flagella
Self-propelled point-like particles move along circular trajectories when
their translocation velocity is constant and the angular velocity related to
their orientation vector is also constant. We investigate the collective
behavior of ensembles of such circle swimmers by Brownian dynamics simulations.
If the particles interact via a "velocity-trajectory coordination" rule within
neighboring particles, a self-organized vortex pattern emerges. This vortex
pattern is characterized by its particle-density correlation function ,
the density correlation function of trajectory centers, and an order
parameter representing the degree of the aggregation of the particles.
Here, we systematically vary the system parameters, such as the particle
density and the interaction range, in order to reveal the transition of the
system from a light-vortex-dominated to heavy-vortex-dominated state, where
vortices contain mainly a single and many self-propelled particles,
respectively. We also study a semi-dilute solution of curved,
sinusoidal-beating flagella, as an example of circling self-propelled particles
with explicit propulsion mechanism and excluded-volume interactions. Our
simulation results are compared with previous experimental results for the
vortices in sea-urchin sperm solutions near a wall. The properties of the
vortices in simulations and experiments are found to agree quantitatively.Comment: 14 pages, 15 figure
Computational studies of biomembrane systems: Theoretical considerations, simulation models, and applications
This chapter summarizes several approaches combining theory, simulation and
experiment that aim for a better understanding of phenomena in lipid bilayers
and membrane protein systems, covering topics such as lipid rafts, membrane
mediated interactions, attraction between transmembrane proteins, and
aggregation in biomembranes leading to large superstructures such as the light
harvesting complex of green plants. After a general overview of theoretical
considerations and continuum theory of lipid membranes we introduce different
options for simulations of biomembrane systems, addressing questions such as:
What can be learned from generic models? When is it expedient to go beyond
them? And what are the merits and challenges for systematic coarse graining and
quasi-atomistic coarse grained models that ensure a certain chemical
specificity
Theoretical study of stimulated and spontaneous Hawking effects from an acoustic black hole in a hydrodynamically flowing fluid of light
We propose an experiment to detect and characterize the analog Hawking
radiation in an analog model of gravity consisting of a flowing
exciton-polariton condensate. Under a suitably designed coherent pump
configuration, the condensate features an acoustic event horizon for sound
waves that at the semiclassical level is equivalent to an astrophysical black
hole horizon. We show that a continuous-wave pump-and-probe spectroscopy
experiment allows to measure the analog Hawking temperature from the dependence
of the stimulated Hawking effect on the pump-probe detuning. We anticipate the
appearance of an emergent resonant cavity for sound waves between the pump beam
and the horizon, which results in marked oscillations on top of an overall
exponential frequency dependence. We finally analyze the spatial correlation
function of density fluctuations and identify the hallmark features of the
correlated pairs of Bogoliubov excitations created by the spontaneous Hawking
process, as well as novel signatures characterizing the emergent cavity
Cooperation of Sperm in Two Dimensions: Synchronization, Attraction and Aggregation through Hydrodynamic Interactions
Sperm swimming at low Reynolds number have strong hydrodynamic interactions
when their concentration is high in vivo or near substrates in vitro. The
beating tails not only propel the sperm through a fluid, but also create flow
fields through which sperm interact with each other. We study the hydrodynamic
interaction and cooperation of sperm embedded in a two-dimensional fluid by
using a particle-based mesoscopic simulation method, multi-particle collision
dynamics (MPC). We analyze the sperm behavior by investigating the relationship
between the beating-phase difference and the relative sperm position, as well
as the energy consumption. Two effects of hydrodynamic interaction are found,
synchronization and attraction. With these hydrodynamic effects, a multi-sperm
system shows swarm behavior with a power-law dependence of the average cluster
size on the width of the distribution of beating frequencies
Rods Near Curved Surfaces and in Curved Boxes
We consider an ideal gas of infinitely rigid rods near a perfectly repulsive
wall, and show that the interfacial tension of a surface with rods on one side
is lower when the surface bends towards the rods. Surprisingly we find that
rods on both sides of surfaces also lower the energy when the surface bends. We
compute the partition functions of rods confined to spherical and cylindrical
open shells, and conclude that spherical shells repel rods, whereas cylindrical
shells (for thickness of the shell on the order of the rod-length) attract
them. The role of flexibility is investigated by considering chains composed of
two rigid segments.Comment: 39 pages including figures and tables. 12 eps figures. LaTeX with
REVTe
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