283 research outputs found
Synthetic Chemotaxis and Collective Behavior in Active Matter
Conspectus: The ability to navigate in chemical gradients, called chemotaxis,
is crucial for the survival of microorganisms. It allows them to find food and
to escape from toxins. Many microorganisms can produce the chemicals to which
they respond themselves and use chemotaxis for signalling which can be seen as
a basic form of communication. Remarkably, the past decade has let to the
development of synthetic microswimmers like e.g. autophoretic Janus colloids,
which can self-propel through a solvent, analogously to bacteria and other
microorganims. The mechanism underlying their self-propulsion involves the
production of certain chemicals. The same chemicals involved in the
self-propulsion mechanism also act on other microswimmers and bias their
swimming direction towards (or away from) the producing microswimmer. Synthetic
microswimmers therefore provide a synthetic analogue to chemotactic motile
microorganisms. When these interactions are attractive, they commonly lead to
clusters, even at low particle density. These clusters may either proceed
towards macrophase separation, resembling Dictyostelium aggregation, or, as
shown very recently, lead to dynamic clusters of self-limited size (dynamic
clustering) as seen in experiments in autophoretic Janus colloids. Besides the
classical case where chemical interactions are attractive, this Account
discusses, as its main focus, repulsive chemical interactions, which can create
a new and less known avenue to pattern formation in active systems leading to a
variety of pattern, including clusters which are surrounded by shells of
chemicals, travelling waves and more complex continously reshaping patterns. In
all these cases `synthetic signalling' can crucially determine the collective
behavior of synthetic microswimmer ensembles and can be used as a design
principle to create patterns in motile active particles
Phase-field-crystal model for liquid crystals
Based on static and dynamical density functional theory, a
phase-field-crystal model is derived which involves both the translational
density and the orientational degree of ordering as well as a local director
field. The model exhibits stable isotropic, nematic, smectic A, columnar,
plastic crystalline and orientationally ordered crystalline phases. As far as
the dynamics is concerned, the translational density is a conserved order
parameter while the orientational ordering is non-conserved. The derived
phase-field-crystal model can serve for efficient numerical investigations of
various nonequilibrium situations in liquid crystals
DDFT calibration and investigation of an anisotropic phase-field crystal model
The anisotropic phase-field crystal model recently proposed and used by
Prieler et al. [J. Phys.: Condens. Matter 21, 464110 (2009)] is derived from
microscopic density functional theory for anisotropic particles with fixed
orientation. Further its morphology diagram is explored. In particular we
investigated the influence of anisotropy and undercooling on the process of
nucleation and microstructure formation from atomic to the microscale. To that
end numerical simulations were performed varying those dimensionless parameters
which represent anisotropy and undercooling in our anisotropic phase-field
crystal (APFC) model. The results from these numerical simulations are
summarized in terms of a morphology diagram of the stable state phase. These
stable phases are also investigated with respect to their kinetics and
characteristic morphological features.Comment: It contain 13 pages and total of 7 figure
Colloidal crystal growth at externally imposed nucleation clusters
We study the conditions under which and how an imposed cluster of fixed
colloidal particles at prescribed positions triggers crystal nucleation from a
metastable colloidal fluid. Dynamical density functional theory of freezing and
Brownian dynamics simulations are applied to a two-dimensional colloidal system
with dipolar interactions. The externally imposed nucleation clusters involve
colloidal particles either on a rhombic lattice or along two linear arrays
separated by a gap. Crystal growth occurs after the peaks of the nucleation
cluster have first relaxed to a cutout of the stable bulk crystal.Comment: 4 pages, accepted for publication in Phys. Rev. Let
Effect of many-body interactions on the solid-liquid phase-behavior of charge-stabilized colloidal suspensions
The solid-liquid phase-diagram of charge-stabilized colloidal suspensions is
calculated using a technique that combines a continuous Poisson-Boltzmann
description for the microscopic electrolyte ions with a molecular-dynamics
simulation for the macroionic colloidal spheres. While correlations between the
microions are neglected in this approach, many-body interactions between the
colloids are fully included. The solid-liquid transition is determined at a
high colloid volume fraction where many-body interactions are expected to be
strong. With a view to the Derjaguin-Landau-Verwey-Overbeek theory predicting
that colloids interact via Yukawa pair-potentials, we compare our results with
the phase diagram of a simple Yukawa liquid. Good agreement is found at high
salt conditions, while at low ionic strength considerable deviations are
observed. By calculating effective colloid-colloid pair-interactions it is
demonstrated that these differences are due to many-body interactions. We
suggest a density-dependent pair-potential in the form of a truncated Yukawa
potential, and show that it offers a considerably improved description of the
solid-liquid phase-behavior of concentrated colloidal suspensions
Soft repulsive mixtures under gravity: brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking
A binary mixture of particles interacting via long-ranged repulsive forces is
studied in gravity by computer simulation and theory. The more repulsive
A-particles create a depletion zone of less repulsive B-particles around them
reminiscent to a bubble. Applying Archimedes' principle effectively to this
bubble, an A-particle can be lifted in a fluid background of B-particles. This
"depletion bubble" mechanism explains and predicts a brazil-nut effect where
the heavier A-particles float on top of the lighter B-particles. It also
implies an effective attraction of an A-particle towards a hard container
bottom wall which leads to boundary layering of A-particles. Additionally, we
have studied a periodic inversion of gravity causing perpetual mutual
penetration of the mixture in a slit geometry. In this nonequilibrium case of
time-dependent gravity, the boundary layering persists. Our results are based
on computer simulations and density functional theory of a two-dimensional
binary mixture of colloidal repulsive dipoles. The predicted effects also occur
for other long-ranged repulsive interactions and in three spatial dimensions.
They are therefore verifiable in settling experiments on dipolar or charged
colloidal mixtures as well as in charged granulates and dusty plasmas.Comment: 10 pages, 11 figure
Attraction between like-charged colloidal particles induced by a surface a density - functional analysis
We show that the first non-linear correction to the linearised
Poisson-Boltzman n (or DLVO) theory of effective pair interactions between
charge-stabilised, co lloidal particles near a charged wall leads to an
attractive component of entro pic origin. The position and depth of the
potential compare favourably with rec ent experimental measurementsComment: 12 pages including 2 figures. submitted to physical review letter
Tension and stiffness of the hard sphere crystal-fluid interface
A combination of fundamental measure density functional theory and Monte
Carlo computer simulation is used to determine the orientation-resolved
interfacial tension and stiffness for the equilibrium hard-sphere crystal-fluid
interface. Microscopic density functional theory is in quantitative agreement
with simulations and predicts a tension of 0.66 kT/\sigma^2 with a small
anisotropy of about 0.025 kT and stiffnesses with e.g. 0.53 kT/\sigma^2 for the
(001) orientation and 1.03 kT/\sigma^2 for the (111) orientation. Here kT is
denoting the thermal energy and \sigma the hard sphere diameter. We compare our
results with existing experimental findings
Field theory fo charged fluids and colloids
A systematic field theory is presented for charged systems. The one-loop
level corresponds to the classical Debye-H\"uckel (DH) theory, and exhibits the
full hierarchy of multi-body correlations determined by pair-distribution
functions given by the screened DH potential. Higher-loop corrections can lead
to attractive pair interactions between colloids in asymmetric ionic
environments. The free energy follows as a loop-wise expansion in half-integer
powers of the density; the resulting two-phase demixing region shows pronounced
deviations from DH theory for strongly charged colloids.Comment: 4 pages, 2 ps figs; new version corrects some minor typo
- …