Number Of Active Pores Research Articles

Nernst\u2013Planck modeling of multicomponent ion transport in a Nafion membrane at high current density

A mathematical model of multicomponent ion transport through a cation-exchange membrane is developed based on the Nernst–Planck equation. A correlation for the non-linear potential gradient is derived from current density relation with fluxes. The boundary conditions are determined with the Donnan equilibrium at the membrane–solution interface, taking into account the convective flow. Effective diffusivities are used in the model based on the correlation of tortuosity and ionic diffusivities in free water. The model predicts the effect of an increase in current density on the ion concentrations inside the membrane. The model is fitted to the previously published experimental data. The effect of current density on the observed increase in voltage drop and the decrease in permselectivity has been analyzed using the available qualitative membrane swelling theories. The observed non-linear behavior of the membrane voltage drop versus current density can be explained by an increase in membrane pore diameter and an increase in the number of active pores. We show how the membrane pore diameter increases and dead-end pores open up when the current density is increased.Graphical

Estimating the risk of coalescence in membrane emulsification

In order to optimise the membrane emulsification process, there is a need for a model that can be used to keep the production time of en emulsion to a minimum, without impairing its quality. This was the aim of our work. For that, we have studied the influence of transmembrane pressure on the fraction of active pores and on the drop diameter and the drop diameter distribution. We have found that an increase in transmembrane pressure makes it possible to increase the dispersed phase flux by a rise in the number of active pores (from 1 to 13% for the 0.2 μm membrane and from 2 to 45 and 49% for the 0.5 and 0.8 μm membranes, respectively). Furthermore, the increase in transmembrane pressure leads to an increase in the diameter of the droplets produced. For the 0.5 and 0.8 μm pore diameter, we have even found an increase in the polydispersity of the emulsion produced. A thorough study allowed us to locate the origin of this polydispersity: we ruled out post-production coalescence both in the bulk and in the laminar sub-layer. We showed the possibility of the expansion of the three-phase contact line over more than one pore, phenomenon we referred to as membrane coalescence. Based on these results, we built a model which allows to evaluate the risk of membrane coalescence under experimental conditions of dispersed phase flux and transmembrane pressure: the number of active pores at a distance d d/2 from the centre of a given active pore must be <2. For the three pore diameter membranes studied, our model matched the experimental results and could explain the polydispersity observed.

Interfacial aspects of water drop formation at micro-engineered orifices

The formation of emulsions with micro-engineered silicon based arrays of micro-orifices is a relatively new technique. Until now, only the preparation of oil-in-water emulsions was studied due to the hydrophilic nature of silicon. This work evaluates the emulsification of water into n-hexadecane with hydrophobized arrays of micro-orifices. We have studied the drop formation rate, the number of active pores and the drop size. In contrast to conventional macroporous membranes used for membrane emulsification, we observed high dispersed phase fluxes up to 4600 L h −1 m −2 bar −1 while all pores being active at applied pressures below 2 times the critical pressure. The drop diameter was independent from the applied pressure difference. We observed a pressure dependent lag time between drop formations at low emulsification pressures. The lag time is related to the rate of surfactant diffusion to the water–oil interface causing a reduction of the interfacial tension. A significant influence of the used hydrophobization agents, perfluorinated octyltrichlorosilane (FOTS) and octyltrichlorosilane (OTS), was found for the resulting drop sizes and the number of active pores.

The Electrochemical Impedance of Porous Nickel Electrodes in Alkaline Media: II . Nonuniform Transmission Line Analysis

Nonuniform transmission line models have been developed to provide an accurate account of the impedance characteristics of sintered porous nickel electrodes when charging and discharging under stress and low earth orbit cycling regimes (described in Part I which is the preceding article). This model represents a single pore in a porous electrode by a nonuniform transmission line involving both the charged and discharged phases. The most accurate description of the impedance characteristics was obtained by assuming a conical geometry and a χ2 distribution for the penetration depth of down the pore on discharge. The modeling results indicate that charging and discharging a sintered electrode over 596 cycles increases both the number of active pores and the fraction of pores that are more deeply reduced. These two trends account for the observed increase in capacity of the electrode with cycle number.