1,252 research outputs found
The structure of fluids with impurities
The influence of dilute impurities on the structure of a fluid solvent is
investigated theoretically. General arguments, which do not rely on particular
models, are used to derive an extension of the Ornstein-Zernike form for the
solvent structure factor at small scattering vectors. It is shown that dilute
impurities can influence the solvent structure only if they are composed of
ions with significantly different sizes. Non-ionic impurities or ions of
similar size are shown to not alter the solvent structure qualitatively. This
picture is compatible with available experimental data. The derived form of the
solvent structure factor is expected to be useful to infer information on the
impurity-solvent interactions form measured scattering intensities
Phase behavior of ionic liquid crystals
Bulk properties of ionic liquid crystals are investigated using density
functional theory. The liquid crystal molecules are represented by ellipsoidal
particles with charges located in their center or at their tails. Attractive
interactions are taken into account in terms of the Gay-Berne pair potential.
Rich phase diagrams involving vapor, isotropic and nematic liquid, as well as
smectic phases are found. The dependence of the phase behavior on various
parameters such as the length of the particles and the location of charges on
the particles is studied
Bulk and interfacial properties of binary hard-platelet fluids
Interfaces between demixed fluid phases of binary mixtures of hard platelets
are investigated using density-functional theory. The corresponding excess free
energy functional is calculated within a fundamental measure theory adapted to
the Zwanzig model, in which the orientations of the particles of rectangular
shape are restricted to three orthogonal orientations. Density and
orientational order parameter profiles at interfaces between coexisting phases
as well as the interfacial tension are determined. A density inversion,
oscillatory density profiles, and a Fisher-Widom line have been found in a
mixture of large thin and small thick platelets. The lowest interfacial tension
corresponds to the mean bulk orientation of the platelets being parallel to the
interface. For a mixture of large and small thin platelets, complete wetting of
an isotropic-nematic interface by a second nematic phase is found.Comment: 7 pages, 6 figure
Relaxation dynamics in fluids of platelike colloidal particles
The relaxation dynamics of a model fluid of platelike colloidal particles is
investigated by means of a phenomenological dynamic density functional theory.
The model fluid approximates the particles within the Zwanzig model of
restricted orientations. The driving force for time-dependence is expressed
completely by gradients of the local chemical potential which in turn is
derived from a density functional -- hydrodynamic interactions are not taken
into account. These approximations are expected to lead to qualitatively
reliable results for low densities as those within the isotropic-nematic
two-phase region. The formalism is applied to model an initially spatially
homogeneous stable or metastable isotropic fluid which is perturbed by
switching a two-dimensional array of Gaussian laser beams. Switching on the
laser beams leads to an accumulation of colloidal particles in the beam
centers. If the initial chemical potential and the laser power are large enough
a preferred orientation of particles occurs breaking the symmetry of the laser
potential. After switching off the laser beams again the system can follow
different relaxation paths: It either relaxes back to the homogeneous isotropic
state or it forms an approximately elliptical high-density core which is
elongated perpendicular to the dominating orientation in order to minimize the
surface free energy. For large supersaturations of the initial isotropic fluid
the high-density cores of neighboring laser beams of the two-dimensional array
merge into complex superstructures.Comment: low-resolution figures due to file size restrictions, revised versio
Transport and dynamical properties of inertial ratchets
In this paper we discuss the dynamics and transport properties of a massive
particle, in a time dependent periodic potential of the ratchet type, with a
dissipative environment. The directional currents and characteristics of the
motion are studied as the specific frictional coefficient varies, finding that
the stationary regime is strongly dependent on this parameter. The maximal
Lyapunov exponent and the current show large fluctuations and inversions,
therefore for some range of the control parameter, this inertial ratchet could
originate a mass separation device. Also an exploration of the effect of a
random force on the system is performed.Comment: PDF, 16 pages, 7 figure
Nonequilibrium steady states in fluids of platelike colloidal particles
Nonequilibrium steady states in an open system connecting two reservoirs of
platelike colloidal particles are investigated by means of a recently proposed
phenomenological dynamic density functional theory [M. Bier and R. van Roij,
Phys. Rev. E 76, 021405 (2007)]. The platelike colloidal particles are
approximated within the Zwanzig model of restricted orientations, which
exhibits an isotropic-nematic bulk phase transition. Inhomogeneities of the
local chemical potential generate a diffusion current which relaxes to a
nonvanishing value if the two reservoirs coupled to the system sustain
different chemical potentials. The relaxation process of initial states towards
the steady state turns out to comprise two regimes: a smoothening of initial
steplike structures followed by an ultimate relaxation of the slowest diffusive
mode. The position of a nonequilibrium interface and the particle current of
steady states depend nontrivially on the structure of the reservoirs due to the
coupling between translational and orientational degrees of freedom of the
fluid
Ecosystem size-induced environmental fluctuations affect the temporal dynamics of community assembly mechanisms
Understanding processes that determine community membership and abundance is important for many fields from theoretical community ecology to conservation. However, spatial community studies are often conducted only at a single timepoint despite the known influence of temporal variability on community assembly processes. Here we used a spatiotemporal study to determine how environmental fluctuation differences induced by mesocosm volumes (larger volumes were more stable) influence assembly processes of aquatic bacterial metacommunities along a press disturbance gradient. By combining path analysis and network approaches, we found mesocosm size categories had distinct relative influences of assembly process and environmental factors that determined spatiotemporal bacterial community composition, including dispersal and species sorting by conductivity. These processes depended on, but were not affected proportionately by, mesocosm size. Low fluctuation, large mesocosms primarily developed through the interplay of species sorting that became more important over time and transient priority effects as evidenced by more time-delayed associations. High fluctuation, small mesocosms had regular disruptions to species sorting and greater importance of ecological drift and dispersal limitation indicated by lower richness and higher taxa replacement. Together, these results emphasize that environmental fluctuations influence ecosystems over time and its impacts are modified by biotic properties intrinsic to ecosystem size
Feasiblity study for a 34 GHz (Ka band) gyroamplifier
The feasibility of using a gyroklystron power tube as the final amplifier in a 400 kW CW 34 GHz transmitter on the Goldstone Antenna is investigated. A conceptual design of the gyroklystron and the transmission line connecting it with the antenna feed horn is presented. The performance characteristics of the tube and transmission line are compared to the transmitter requirements for a deep space radar system. Areas of technical risk for a follow-on hardware development program for the gyroklystron amplifier and overmoded transmission line components are discussed
A random walker on a ratchet
We analyze a model for a walker moving on a ratchet potential. This model is
motivated by the properties of transport of motor proteins, like kinesin and
myosin. The walker consists of two feet represented as two particles coupled
nonlinearly through a bistable potential. In contrast to linear coupling, the
bistable potential admits a richer dynamics where the ordering of the particles
can alternate during the walking. The transitions between the two stable states
on the bistable potential correspond to a walking with alternating particles.
We distinguish between two main walking styles: alternating and no alternating,
resembling the hand-over-hand and the inchworm walking in motor proteins,
respectively. When the equilibrium distance between the two particles divided
by the periodicity of the ratchet is an integer, we obtain a maximum for the
current, indicating optimal transport.Comment: 10 pages, 5 figure
Separation quality of a geometric ratchet
We consider an experimentally relevant model of a geometric ratchet in which
particles undergo drift and diffusive motion in a two-dimensional periodic
array of obstacles, and which is used for the continuous separation of
particles subject to different forces. The macroscopic drift velocity and
diffusion tensor are calculated by a Monte-Carlo simulation and by a
master-equation approach, using the correponding microscopic quantities and the
shape of the obstacles as input. We define a measure of separation quality and
investigate its dependence on the applied force and the shape of the obstacles
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