1,015 research outputs found
Phase behavior and interfacial properties of nonadditive mixtures of Onsager rods
Within a second virial theory, we study bulk phase diagrams as well as the
free planar isotropic-nematic interface of binary mixtures of nonadditive thin
and thick hard rods. For species of the same type the excluded volume is
determined only by the dimensions of the particles, whereas for dissimilar ones
it is taken to be larger or smaller than that, giving rise to a nonadditivity
that can be positive or negative. We argue that such a nonadditivity can result
from modelling of soft interactions as effective hard-core interactions. The
nonadditivity enhances or reduces the fractionation at isotropic-nematic ()
coexistence and may induce or suppress a demixing of the high-density nematic
phase into two nematic phases of different composition ( and ),
depending on whether the nonadditivity is positive or negative. The interfacial
tension between co-existing isotropic and nematic phases show an increase with
increasing fractionation at the interface, and complete wetting of the
interface by the phase upon approach of the triple point
coexistence. In all explored cases bulk and interfacial properties of the
nonadditive mixtures exhibit a striking and quite unexpected similarity with
the properties of additive mixtures of different diameter ratio.Comment: 12 pages, revised version, submitted to JC
Dynamical Monte Carlo Study of Equilibrium Polymers (II): The Role of Rings
We investigate by means of a number of different dynamical Monte Carlo
simulation methods the self-assembly of equilibrium polymers in dilute,
semidilute and concentrated solutions under good-solvent conditions. In our
simulations, both linear chains and closed loops compete for the monomers,
expanding on earlier work in which loop formation was disallowed. Our findings
show that the conformational properties of the linear chains, as well as the
shape of their size distribution function, are not altered by the formation of
rings. Rings only seem to deplete material from the solution available to the
linear chains. In agreement with scaling theory, the rings obey an algebraic
size distribution, whereas the linear chains conform to a Schultz--Zimm type of
distribution in dilute solution, and to an exponentional distribution in
semidilute and concentrated solution. A diagram presenting different states of
aggregation, including monomer-, ring- and chain-dominated regimes, is given
Osmotic Pressure of Solutions Containing Flexible Polymers Subject to an Annealed Molecular Weight Distribution
The osmotic pressure in equilibrium polymers (EP) in good solvent is
investigated by means of a three dimensional off-lattice Monte Carlo
simulation. Our results compare well with real space renormalisation group
theory and the osmotic compressibility K \propto \phi \upd \phi/\upd P from
recent light scattering study of systems of long worm-like micelles. We confirm
the scaling predictions for EP based on traditional physics of quenched
monodisperse polymers in the dilute and semidilute limit. Specifically, we find
and, hence, in the semidilute
regime --- in agreement with both theory and experiment. At higher
concentrations where the semidilute blobs become too small and hard-core
interactions and packing effects become dominant, a much stronger increase %
\log(P/\phi)\approx \log(\Nav^2/\phi) \propto \phi is evidenced and,
consequently, the compressibility decreases much more rapidly with than
predicted from semidilute polymer theory, but again in agreement with
experiment.Comment: 7 pages, 4 figures, LATE
Dynamic behaviour of ISFET-based sensor-actuator systems
Rapid acid-base titrations can be performed at the surface of a noble-metal electrode with coulometrically generated ions. An ISFET is used as an indicator electrode to detect the equivalence point in the resulting titration curve. The time needed to reach the equivalence point is typically 0.5 to 10 s for acid/base concentrations ranging from 0.5 × 10−3 to 20 × 10−3 mol l−1.\ud
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A model is presented describing the concentration profiles which appear during the coulometric generation of ions. The result of this model is in good agreement with corresponding measurements. These measurements are carried out with two different actuator electrodes, of which the processing steps are described
Quasiuniversal connectedness percolation of polydisperse rod systems
The connectedness percolation threshold (eta_c) and critical coordination
number (Z_c) of systems of penetrable spherocylinders characterized by a length
polydispersity are studied by way of Monte Carlo simulations for several aspect
ratio distributions. We find that (i) \eta_c is a nearly universal function of
the weight-averaged aspect ratio, with an approximate inverse dependence that
extends to aspect ratios that are well below the slender rod limit and (ii)
that percolation of impenetrable spherocylinders displays a similar
quasiuniversal behavior. For systems with a sufficiently high degree of
polydispersity, we find that Z_c can become smaller than unity, in analogy with
observations reported for generalized and complex networks.Comment: 5 pages with 3 figures + 2 pages and 4 figures of supplemental
materia
Osmotic compression of droplets of hard rods: A computer simulation study
By means of computer simulations we study how droplets of hard, rod-like
particles optimize their shape and internal structure under the influence of
the osmotic compression caused by the presence of spherical particles that act
as depletion agents. At sufficiently high osmotic pressures the rods that make
up the drops spontaneously align to turn them into uniaxial nematic liquid
crystalline droplets. The nematic droplets or "tactoids" that are formed this
way are not spherical but elongated, resulting from the competition between the
anisotropic surface tension and the elastic deformation of the director field.
In agreement with recent theoretical predictions we find that sufficiently
small tactoids have a uniform director field, whilst large ones are
characterized by a bipolar director field. From the shape and director-field
transformation of the droplets we are able to estimate the surface anchoring
strength and an average of the elastic constants of the hard-rod nematic
Modelling of non-steady-state concentration profiles at ISFET-based coulometric sensor—actuator systems
Acid or base concentrations can be determined very rapidly by performing an acid—base titration with coulometrically generated OH− or H+ ions at a noble metal actuator electrode in close proximity to the pH-sensitive gate of an ion-sensitive field effect transistor (ISFET). The ISFET is used as the indicator electrode to detect the equivalence point in the titration curve. Typical values for the time needed to reach the equivalence point are 0.5–10 s for acid or base concentrations ranging from 0.5 × 10−3 to 20 × 10−3 mol l−1.\ud
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A model is presented, giving an analytical description of the diffusion phenomena that occur with the sensor—actuator system. The results of this analytical model description, using linearized concentration gradients, are presented together with the results of numerical simulations. Both results are in good agreement with measurements
Modelling of the migration effect occurring at an ISFET-based coulometric sensor-actuator system
The migration effect, in addition to diffusion, occurring at an ion-selective field-effect transistor (ISFET)-based coulometric sensor-actuator system has been studied. A diffusion-migration model is presented, based on the numerical solution of the Nernst-Planck equations of which a digital simulation is realized. Corresponding experiments were carried out and compared with the simulation. The results are in good agreement with the simulation.\ud
Typical titration times of this system were found to be 0.5–10 s, corresponding to fully dissociated acid concentrations of 0.5×10−3−6.5×10−3 mol 1−1 with excess of supporting electrolyte. Both the simulation and experimental results show that if the concentration of the supporting electrolyte is 20 times higher than that of the species to be titrated, the deviation caused by migration is less than 5% and within the experimental error when pure diffusion is considered. At relatively low concentrations of supporting electrolyte, the migration effect should be taken into account to determine the concentrations of titrated species
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