593 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
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
\ud
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
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
\ud
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
Compound redistribution due to droplet evaporation on a thin polymeric film: theory
A thin polymeric film in contact with a fluid body may leach
low-molecular-weight compounds into the fluid. If this fluid is a small
droplet, the compound concentration within the liquid increases due to ongoing
leaching in combination with the evaporation of the droplet. This may
eventually lead to an inversion of the transport process and a redistribution
of the compounds within the thin film. In order to gain an understanding of the
compound redistribution, we apply a macroscopic model for the evaporation of a
droplet and combine that with a diffusion model for the compound transport. In
the model, material deposition and the resulting contact line pinning are
associated with the precipitation of a fraction of the dissolved material. We
find three power law regimes for the size of the deposit area as a function of
the initial droplet size, dictated by the competition between evaporation,
diffusion and the initial compound concentrations in the droplet and the thin
film. The strength of the contact line pinning determines the deposition
profile of the precipitate, characterised by a pronounced edge and a linearly
decaying profile towards the centre of the stain. Our predictions for the
concentration profile within the solid substrate resemble patterns found
experimentally.Comment: 12 pages, 10 figure
Development of a microprocessor-controlled coulometric system for stable ph control
The coulometric pH control system utilizes a programmable coulostat for controlling the pH of a certain volume of unbuffered solution. Based on theoretical considerations, conditions are established which guarantee stable operation with maximum suppression of disturbances from the dissolution of carbon dioxide, for example. It is shown that the dynamic properties of the control system depend greatly on the response time of the pH sensor which measures the actual pH. The best results are therefore obtained by using an ISFET as pH sensor
Transient nucleation driven by solvent evaporation
We theoretically investigate homogeneous crystal nucleation in a solution
containing a solute and a volatile solvent. The solvent evaporates from the
solution, thereby continuously increasing the concentration of the solute. We
view it as an idealized model for the far-out-of-equilibrium conditions present
during the liquid-state manufacturing of organic electronic devices. Our model
is based on classical nucleation theory, taking the solvent to be a source of
the transient conditions in which the solute drops out of solution. Other than
that, the solvent is not directly involved in the nucleation process itself. We
approximately solve the kinetic master equations using a combination of Laplace
transforms and singular perturbation theory, providing an analytical expression
for the nucleation flux, predicting that (i) the nucleation flux lags slightly
behind a commonly used quasi-steady-state approximation, an effect that is
governed by two counteracting effects originating from the solvent evaporation:
while a faster evaporation rate results in an increasingly larger influence of
the lag time on the nucleation flux, this lag time itself we find to decrease
with increasing evaporation rate, (ii) the nucleation flux and the
quasi-steady-state nucleation flux are never identical, except trivially in the
stationary limit and (iii) the initial induction period of the nucleation flux,
which we characterize with a generalized induction time, decreases weakly with
the evaporation rate. This indicates that the relevant time scale for
nucleation also decreases with increasing evaporation rate. Our analytical
theory compares favorably with results from numerical evaluation of the
governing kinetic equations
- …