1,121 research outputs found
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
Effect of bending flexibility on the phase behavior and dynamics of rods
We study by means of molecular and Brownian dynamics simulations the influence of bending flexibility on the phase behavior and dynamics of monodisperse hard filamentous particles with an aspect ratio of 8 and persistence lengths equal to 3 and 11 times the particle length. Although our particles are much shorter, the latter corresponds to the values for wild-type and mutant fd virus particles that have been subject of a recent experimental study, where the diffusion of these particles in the nematic and smectic-A phase was investigated by means of video fluorescence microscopy [E. Pouget, E. Grelet, and M. P. Lettinga, Phys. Rev. E84, 041704 (2011)]. In agreement with theoretical predictions and simulations, we find that for the more flexible particles (shorter persistence length) the nematic (N) to smectic-A (Sm-A) phase transition shifts to larger values of the particle density. Interestingly, we find that for the more rigid particles (larger persistence length), the smectic layer-to-layer distance decreases monotonically with increasing density, whereas for the more flexible ones, it first increases, reaches a maximum and then decreases. For our more flexible particles, we find a smectic-B phase at sufficiently high densities. Moreover, in line with experimental observations and theoretical predictions, we find heterogeneous dynamics in the Sm-A phase, in which particles hop between the smectic layers. We compare the diffusion of our two types of particle at identical values of smectic order parameter, and find that flexibility does not change the diffusive behavior of particles along the director yet significantly slows down the diffusion perpendicular to it. In our simulations, the ratio of diffusion constants along and perpendicular to the director decreases just beyond the N-Sm-A phase transition for both our stiff and more flexible particles
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
Percolation in suspensions of polydisperse hard rods : quasi-universality and finite-size effects
We present a study of connectivity percolation in suspensions of hard
spherocylinders by means of Monte Carlo simulation and connectedness
percolation theory. We focus attention on polydispersity in the length, the
diameter and the connectedness criterion, and invoke bimodal, Gaussian and
Weibull distributions for these. The main finding from our simulations is that
the percolation threshold shows quasi universal behaviour, i.e., to a good
approximation it depends only on certain cumulants of the full size and
connectivity distribution. Our connectedness percolation theory hinges on a
Lee-Parsons type of closure recently put forward that improves upon the
often-used second virial approximation [ArXiv e-prints, May 2015, 1505.07660].
The theory predicts exact universality. Theory and simulation agree
quantitatively for aspect ratios in excess of 20, if we include the
connectivity range in our definition of the aspect ratio of the particles. We
further discuss the mechanism of cluster growth that, remarkably, differs
between systems that are polydisperse in length and in width, and exhibits
non-universal aspects.Comment: 7 figure
Stochastic Lag Time in Nucleated Linear Self-Assembly
Protein aggregation is of great importance in biology, e.g., in amyloid
fibrillation. The aggregation processes that occur at the cellular scale must
be highly stochastic in nature because of the statistical number fluctuations
that arise on account of the small system size at the cellular scale. We study
the nucleated reversible self-assembly of monomeric building blocks into
polymer-like aggregates using the method of kinetic Monte Carlo. Kinetic Monte
Carlo, being inherently stochastic, allows us to study the impact of
fluctuations on the polymerisation reactions. One of the most important
characteristic features in this kind of problem is the existence of a lag phase
before self-assembly takes off, which is what we focus attention on. We study
the associated lag time as a function of the system size and kinetic pathway.
We find that the leading order stochastic contribution to the lag time before
polymerisation commences is inversely proportional to the system volume for
large-enough system size for all nine reaction pathways tested. Finite-size
corrections to this do depend on the kinetic pathway
Inhibitory Deficits in reading disability depend on subtype: guessers but not spellers
In this study, children with the guessing subtype of dyslexia (who read fast and inaccurately) were compared with children with the spelling subtype (who read slowly and accurately) on three aspects of executive functioning (EF): response inhibition, susceptibility to interference from irrelevant information, and planning. It was found that guessers were impaired in their ability to inhibit inappropriate responding on all tasks used to assess EF (the stop signal task, the Stroop task, and the Tower of London task). This raises the question of whether the specific reading disorder of guessers may be linked to the same executive deficits which underlie ADHD. In order to unite a fast/inaccurate reading style with executive deficiencies, an attempt is made to incorporate the concept of executive control into models of lexical activation
The isfet in analytical chemistry
The fast chemical response of the pH-ISFET makes the device an excellent detector in analytical chemistry. The time response of ISFETs, with Al2O3 at the pH-sensitive gate insulator, is determined in a flow injection analysis system. Application of an ISFET and a glass electrode are compared in rapid acid-base titrations and in a coulometric system for stable pH control. Finally, a method is described that combines both the stability of the glass electrode and the fast response of the ISFET
Entropy of water and the temperature-induced stiffening of amyloid networks
In water, networks of semi-flexible fibrils of the protein -synuclein
stiffen significantly with increasing temperature. We make plausible that this
reversible stiffening is a result of hydrophobic contacts between the fibrils
that become more prominent with increasing temperature. The good agreement of
our experimentally observed temperature dependence of the storage modulus of
the network with a scaling theory linking network elasticity with reversible
crosslinking enables us to quantify the endothermic binding enthalpy and an
estimate the effective size of hydrophobic patches on the fibril surface.Comment: 13 pages, 6 figure
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