Papers by Keyword: Mass Transport

Abstract: Nanobubbles are gaseous vesicles under 1 micrometer in size, widely used in medical imaging and drug delivery. Their compressibility enhances ultrasound imaging, while their small size allows for extravasation into tumors via the enhanced permeation and retention effect. However, drug delivery remains challenging due to high interstitial pressure within tumors, which limits penetration to the core. To address this, nanobubbles can serve as drug delivery devices, with controlled release triggered by focused ultrasound. Understanding their transport behavior is crucial, particularly in the tumor microenvironment. This study investigates the effect of ultrasound and presence of shear on the permeation of nanobubbles through the tumor interstitium. Rhodamine-loaded nanobubbles, mimicking drug-loaded delivery device, were introduced into a membrane insert system containing an agarose-gel layer simulating the tumor interstitium. Nanobubble concentration in the receiver well was monitored via fluorescence intensity, assessing transport efficiency under different conditions. Comparisons with non-echogenic fluorescent dextrans provided insights into distinct nanoparticle behaviors. Results showed that ultrasound significantly enhanced nanobubble permeation, whereas the presence of shear primarily benefited dextran transport. These findings highlight the unique responsiveness of nanobubbles to ultrasound, offering a promising strategy for improving drug delivery in solid tumors.

Abstract: We compared the evolution of three different SiC sources during standard PVT growth runs. The evolution of the growing crystal and the morphological changes in the SiC source were visualized using in-situ X-ray visualization. Computer simulation was used to calculate the temperature field distributions. It is found that the densification and shrinkage of the SiC source material during the growth process can affect the growth conditions in such a way that the convexity of the growth interface is increased in an unfavorable manner. While unfavorable growth conditions can be related to thermal properties due to less favorable SiC powder evolution, predicting such behavior is a rather complex task that still relies on the support of experimental methods.

Abstract: The fracture mechanics assessment of materials exposed to harmful environments requires the understanding of the interaction between the soluted species and the affected mechanical behaviour. With the introduction of a mass transport mechanism the entire problem is subjected to a time frame that dictates the time-dependent action of soluted species on mechanical properties. A numerical framework within the phase field approach is presented with an embrittlement-based coupling mechanism showing the influence on crack patterns and fracture toughness. Within the phase field approach the modeling of sharp crack discontinuities is replaced by a diffusive crack model facilitating crack initiation and complex crack topologies such as curvilinear crack patterns, without the requirement of a predefined crack path. The isotropic hardening of the elasto-plastic deformation model and the local fracture criterion are affected by the species concentration. This allows for embrittlement and leads to accelerated crack propagation. An extended mass transport equation for hydrogen embrittlement, accounting for mechanical stresses and deformations, is implemented. For stabilisation purposes a staggered scheme is applied to solve the system of partial differential equations. The simulation of showcases demonstrates crack initiation and crack propagation aiming for the determination of stress-intensity factors and crack-resistance curves.

Abstract: In this study, mass transport properties of liquid Cu-Ag alloys are investigated over wide temperature and composition ranges. The calculations are performed within the framework of the Green-Kubo (GK) formalism by using equilibrium molecular dynamics (MD) simulations along with one of the most reliable embedded-atom method potentials for this system developed by [P. Williams et al.: Modell. Simul. Mater. Sci. Eng. vol. 14 (2006), p. 817]. The approach employed allows for evaluation of the components’ self-diffusion coefficients as well as the phenomenological coefficient for mass transport Lcc. The results obtained in this study can be used to predict the kinetics of solidification of real liquid Cu-Ag alloys.

Abstract: Environmentally-assisted material degradation involves mass transport and mechanical processes interacting in the material. A well-known example is hydrogen-induced stress-corrosion cracking. One major challenge within this scope is the quantification of the coupling mechanisms in question. The computational modeling of environmentally-assisted cracks is the key objective ofthis investigation and realised within the theory of gradient-extended dissipative continua with length-scales. The modeling of sharp crack discontinuities is replaced by a diffusive crack model based onthe introduction of a crack phase-field to maintain the evolution of complex crack topologies. Withina thermodynamical framework allowing for mechanical and mass transport processes the crack phase-field is capable to model crack initiation and propagation bythe finite element method. As complexcrack situations such as crack initiation, curvilinear crack patterns and crack branching are usuallyhard to realise with sharp crack models, they can be assessedwithout the requirement of a predefinedcrack path within this method. The numerical modeling of a showcase demonstrates a crack initiationas well as a crack propagation situation with respect to the determination of stress-intensity factors; acrack deviation situation with a curvilinear crack path is modeled by the introduction of a geometricalperturbation and a locally enhanced species concentration

Abstract: This paper studied the combined effect of anode and cathode current collectors on the behavior of water and methanol crossover in micro direct methanol fuel cells (μDMFC). A two-phase, two-dimensional, isothermal model was built to analyze the mass transport behavior in the membrane electrolyte assembly (MEA). The results show that the distribution of methanol exhibits uniform concentration feature over the anode porous area with the asymmetric anode and cathode current collectors. Also, lower water content gradient across the membrane is found. Comparing experiments were conducted to verify the simulation results. Two passive μDMFCs were fabricated and tested. The cells presents almost the same performance at relatively low methanol concentration. However, an increase of 10% cell performance (from 27.5 mW.cm^-2 for symmetric current collectors to 30.3 mW.cm^-2 for asymmetric current collectors) at 5 mol.L^-1 methanol solution was observed. This reveals that the couple of asymmetric current collectors induced the methanol to distribute uniformly and largely suppressed the water and methanol crossover.

Abstract: In a wetted-wall column of pilot-scale sulphur dioxide removal efficiencies were measured at 20°C and atmospheric pressure with aqueous ammonia solutions as the scrubbing liquid for different sulphur dioxide inlet concentration in the range of 1000-3000 mg/m³. A mathematical model with the effects of a chemical enhancement factor and sulfite concentration in the liquid phase developed to interpret laboratory experiments was adapted and the calculated values were in reasonable agreement with the experimental values. It appears that this model can provide good predictions of the absorption performances of industrial columns and be helpful to design scrubbers for SO₂ abatement with aqueous ammonia solutions.

Abstract: The pioneering works in the area of mass transport in porous media go back to the end of last century. The partial differential equations governing the mass and heat transfer can be solved using numerical techniques, and in this paper we solve them analytically under different boundary conditions including time-periodic boundary conditions. The nature of these solutions is discussed. Analytic solutions provide valuable physical insight and are usually easier to compute. In addition, these solutions may help to experimentally determine the parameters in a setting where both the mass and temperature gradients are present, without resorting to a simplified set of equations that govern heat and mass transfer separately.

Abstract: This paper presents a level set framework for the modelling of doping effect during surfacediffusion phenomena in a granular packing. The molecular flux of the doped compound is related tothe chemical potentials of all the diffusion species. The evolution of the grain compact is simulatedin three dimensions, based on the resulting kinetic law relating the surface diffusion velocity to thethermodynamic driving force. An anisotropic adaptive mesh, based on the level set function propertiesis used to refine the mesh in the surroundings of the grain surface. The simulations have been perfomedby using parallel computing strategy.

Abstract: Although the exact causes for presbyopia are not known with certainty, there is the general understanding that it is associated with aging. This is one of the major paradigms of science because the analysis of the symptoms observed during the treatment of presbyopia in the first author of this paper indicates that presbyopia is associated with metabolic dehydrated secretions that have accumulated in various parts of the eyes. As a process of accumulation it is not difficult to associate it statistically to aging. The metabolic secretions produced by the cells inside the human eye are accumulated simultaneously in the cornea, trabecular meshwork, in the lens and retina, subject only to its origin, however, the symptomatology is well characterized by the location of accumulation. Genetic factors are directly related to how the clumps of metabolic secretions occur and the accumulation form is very specific according to the movement of the eyes based on the activities performed by the patient, so any outcome is unpredictable. This paper introduces the hypothesis that macular degeneration may be caused by transport mechanism failure by forced convection of nutrients and metabolic mass produced by the retina. The hypothesis examined in this paper considers the symptoms observed during treatment of presbyopia the first author, the analysis of some clinical procedures to diagnose the degeneration, the symptoms described in the publications on this subject as well as showing how some genetic factor may anticipate the disease. It is not hard to make a statistical association of macular degeneration with aging in the hypothesis that it is caused by of metabolic accumulation of secretions produced in the retina.