Search results for "Diffusion"

Statics and Dynamics of Bidisperse Polymer Melts:  A Monte Carlo Study of the Bond-Fluctuation Model

1998

As a first step toward the computer simulation of polydisperse polymeric melts, a lattice model containing two types of chains with lengths N1 = 20 − x and N2 = 20 + 4x (0 ≤ x ≤ 10 ) is studied. This variation of x, together with the fixed composition of 80% of short and 20% of long chains, leads to a polydispersity of 1 ≤ Nw/Nn ≤ 2 (Nw, Nn:  weight-, number-average chain lengths). To represent dense melts, the bond-fluctuation model at a volume fraction, φ = 1/2, of occupied lattice sites is used. The simulation treats both the athermal case (chain connectivity and excluded volume interaction only) and a thermal case, where additionally a choice for the bond length and bond angle potential…

1,3,5-triisopropylbenzene diffusion in polystyrene solutions

1996

Self-diffusion coefficients of 1,3,5-triisopropylbenzene (TIB) in binary solutions with polystyrene and ternary mixtures comprised of TIB, toluene, and polystyrene have been measured by static gradient nuclear magnetic resonance (SG-NMR) techniques. These data, as well as mutual-diffusion coefficient data measured by capillary column inverse gas chromatography (CCIGC), have been analyzed here with the Vrentas-Duda free-volume diffusion model. Although both binary and ternary diffusion coefficient data can be accurately correlated with the model, the results contradict those of an earlier investigation, which suggested that TIB diffuses as a single unit in polystyrene solutions. The new data…

Analysis of a parabolic cross-diffusion population model without self-diffusion

2006

Abstract The global existence of non-negative weak solutions to a strongly coupled parabolic system arising in population dynamics is shown. The cross-diffusion terms are allowed to be arbitrarily large, whereas the self-diffusion terms are assumed to disappear. The last assumption complicates the analysis since these terms usually provide H 1 estimates of the solutions. The existence proof is based on a positivity-preserving backward Euler–Galerkin approximation, discrete entropy estimates, and L 1 weak compactness arguments. Furthermore, employing the entropy–entropy production method, we show for special stationary solutions that the transient solution converges exponentially fast to its…

Inhibition of neuronally mediated secretion in rat colonic mucosa by prostaglandin D2

1992

Abstract The effect of prostaglandin D 2 (PGD 2 ) on ion transport across the mucosa of the descending colon was studied in rats. PGD 2 dose-dependently decreased baseline short-circuit current of mucosa-submucosal preparations mounted either in the Ussing chamber or mounted as an everted sac. However, with the everted sac technique, the tissue was about 1000 times more sensitive to PGD 2 . Concomitant with the decrease in short-circuit current, PGD 2 increased the mucosal-to-serosal fluxes of sodium and chloride and decreased the serosal-tomucosal flux of chloride. PGD 2 inhibited the secretory action of the PGI 2 analogue iloprost, PGD 2α , and neurotensin. The action of these secretagogu…

Misure di agglomerazione spaziale nei Sistemi Locali del Lavoro siciliani

2009

Spatial agglomeration indicators in the Sicilian Local Labour Systems Objectives - This paper examines the links among productive specialization, variety of the economic environment and knowledge spillovers on the basis of the territorial analyses present in the current literature. With the aim of identifying localization benefits in a particular area, some aspects of business settling are compared with trends in the development of economic activities and, particularly, with agglomeration process of the firms. Methods and Results The empirical analysis will be carried out pointing to the dynamics of employment consequent to the effects of the externalities above mentioned. Employment data a…

Two-Sided Estimates of the Solution Set for the Reaction–Diffusion Problem with Uncertain Data

2009

We consider linear reaction–diffusion problems with mixed Dirichlet–Neumann–Robin conditions. The diffusion matrix, reaction coefficient, and the coefficient in the Robin boundary condition are defined with an uncertainty which allow bounded variations around some given mean values. A solution to such a problem cannot be exactly determined (it is a function in the set of “possible solutions” formed by generalized solutions related to possible data). The problem is to find parameters of this set. In this paper, we show that computable lower and upper bounds of the diameter (or radius) of the set can be expressed throughout problem data and parameters that regulate the indeterminacy range. Ou…

Existence of a traveling wave solution in a free interface problem with fractional order kinetics

2021

Abstract In this paper we consider a system of two reaction-diffusion equations that models diffusional-thermal combustion with stepwise ignition-temperature kinetics and fractional reaction order 0 α 1 . We turn the free interface problem into a scalar free boundary problem coupled with an integral equation. The main intermediary step is to reduce the scalar problem to the study of a non-Lipschitz vector field in dimension 2. The latter is treated by qualitative topological methods based on the Poincare-Bendixson Theorem. The phase portrait is determined and the existence of a stable manifold at the origin is proved. A significant result is that the settling time to reach the origin is fin…

Indagini sul virus della tristezza degli agrumi (CTV) e sui viroidi degli agrumi.

2007

L'INTERVENTO DEL FARMACISTA NELL’EDUCAZIONE SANITARIA

2013

Il fenomeno della diffusione delle sostanze di abuso tra i giovani ha assunto aspetti preoccupanti. Lo spaccio di droga avviene in tutte le strutture a cui accedono i giovani, nessuna esclusa e oggi anche nel web, una delle nuove frontiere del traffico illecito. All’interno di questo scenario la figura del farmacista ha dato contributi validi nella divulgazione, per tentare di salvaguardare la salute dei giovani anche attraverso la promozione diretta di campagne di educazione/sensibilizzazione che forniscano all’utente le informazioni necessarie a difendere la propria salute. Materiali e metodi: Abbiamo realizzato due conferenze inerenti l’informazione sulle sostanze di abuso e la nuova dif…

Nuclear Magnetic Resonance investigation of PVA-GTA FRicke gel dosimeters exposed to clinical photon beams

2016

Fricke Xylenol Gel (FXG) dosimetric system is based on the radiation induced oxidation of ferrous (Fe2+) to ferric (Fe3+) ions. The application of Fricke gels for ionizing radiation dosimetry is continuously increasing worldwide due to their many favorable properties. However, one of their shortcomings is that ferrous and ferric ions diffuse in the gel matrix. To maintain the spatial integrity of the dose distribution, Fricke gels must be undergoing measurement within a few hours of their irradiation, so that ferric ions remain close to their point of production. Thus, the spatial integrity of the dose distribution in the Fricke gel is maintained. The gel matrix also contributes to the oxid…