Temperature effects on the electrohydrodynamic and electrokinetic behaviour of ion-selective nanochannels

A non-isothermal formulation of the Poisson–Nernst–Planck with Navier–Stokes equations is used to study the influence of heating effects in the form of Joule heating and viscous dissipation and imposed temperature gradients on a microchannel/nanochannel system. The system is solved numerically under various cases in order to determine the influence of temperature-related effects on ion-selectivity, flux and fluid flow profiles, as well as coupling between these phenomena. It is demonstrated that for a larger reservoir system, the effects of Joule heating and viscous dissipation only become relevant for higher salt concentrations and electric field strengths than are compatible with ion-selectivity due to Debye layer overlap. More interestingly, it is shown that using different temperature reservoirs can have a strong influence on ion-selectivity, as well as the induced electrohydrodynamic flows

Enhanced ion transport using geometrically structured charge selective interfaces

A microfluidic platform containing charged hydrogels is used to investigate the effect of geometry on charge transport in electrodialysis applications. The influence of heterogeneity on ion transport is determined by electrical characterization and fluorescence microscopy of three different hydrogel geometries. We found that electroosmotic transport of ions towards the hydrogel is enhanced in heterogeneous geometries, as a result of the inhomogeneous electric field in these systems. This yields higher ionic currents for equal applied potentials when compared to homogeneous geometries. The contribution of electroosmotic transport is present in all current regimes, including the Ohmic regime. We also found that the onset of the overlimiting current occurs at lower potentials due to the increased heterogeneity in hydrogel shape, owing to the non-uniform electric field distribution in these systems. Pinning of ion depletion and enrichment zones is observed in the heterogeneous hydrogel systems, due to electroosmotic flows and electrokinetic instabilities. Our platform is highly versatile for the rapid investigation of the effects of membrane topology on general electrodialysis characteristics, including the formation of ion depletion zones on the micro-scale and the onset of the overlimiting current

Influence of temperature gradients on mono- and divalent ion transport in electrodialysis at limiting currents

Temperature gradients in electrodialysis (ED) stacks can potentially enhance the efficiency of charge separation and the selective transport of ions. We have previously investigated temperature gradients in the Ohmic regime but not in the limiting current regime, where diffusion of ions towards the membrane determines the transport rate and temperature gradients potentially have the largest influence. In this research, commercial ion exchange membranes (FAS and FKS, FUMATECH, Germany) are used for the investigation of temperature gradients in the limiting current regime. In contrast to the Ohmic regime, we find that heating the diluted stream increases the current obtained (at a constant applied potential) when compared to heating the concentrate stream in systems containing monovalent KCl and NaCl solutions. For mixtures of mono- and divalent ions, the temperature gradient has a larger influence on the selectivity of the separation. If the desalinated stream is heated, divalent Mg2+ ions show a higher transport than the monovalent K+ and Na+ ions. This is due to the enhanced competitive transport of the mono- and divalent ions under the application of a temperature gradient. These results show the potential application and relevance of temperature gradients to enhance the selective separation of mono- and divalent ions

Influence of temperature gradients on charge transport in asymmetric nanochannels

Charge selective asymmetric nanochannels are used for a variety of applications, such as nanofluidic sensing devices and energy conversion applications. In this paper, we numerically investigate the influence of an applied temperature difference over tapered nanochannels on the resulting charge transport and flow behavior. Using a temperature-dependent formulation of the coupled Poisson-Nernst-Planck and Navier-Stokes equations, various nanochannel geometries are investigated. Temperature has a large influence on the total ion transport, as the diffusivity of ions and viscosity of the solution are strongly affected by temperature. We find that the selectivity of the nanochannels is enhanced with increasing asymmetry ratios, while the total current is reduced at higher asymmetry cases. Most interestingly, we find that applying a temperature gradient along the electric field and along the asymmetry direction of the nanochannel enhances the selectivity of the tapered channels even further, while a temperature gradient countering the electric field reduces the selectivity of the nanochannel. Current rectification is enhanced in asymmetric nanochannels if a temperature gradient is applied, independent of the direction of the temperature difference. However, the degree of rectification is dependent on the direction of the temperature gradient with respect to the channel geometry and the electric field direction. The enhanced selectivity of nanochannels due to applied temperature gradients could result in more efficient operation in energy harvesting or desalination applications, motivating experimental investigations

Structured computer-based training in the interpretation of neuroradiological images

Computer-based systems may be able to address a recognised need throughout the medical profession for a more structured approach to training. We describe a combined training system for neuroradiology, the MR Tutor that differs from previous approaches to computer-assisted training in radiology in that it provides case-based tuition whereby the system and user communicate in terms of a well-founded Image Description Language. The system implements a novel method of visualisation and interaction with a library of fully described cases utilising statistical models of similarity, typicality and disease categorisation of cases. We describe the rationale, knowledge representation and design of the system, and provide a formative evaluation of its usability and effectiveness

Liquid-liquid displacement in slippery liquid-infused membranes (SLIMs)

Liquid-infused membranes inspired by slippery liquid-infused porous surfaces (SLIPS) have been recently introduced to membrane technology. The gating mechanism of these membranes is expected to give rise to anti-fouling properties and multi-phase transport capabilities. However, the long-term retention of the infusion liquid has not yet been explored. To address this issue, we investigate the retention of the infusion liquid in slippery liquid-infused membranes (SLIMs) via liquid-liquid displacement porometry (LLDP) experiments combined with microscopic observations of the displacement mechanism. Our results reveal that pores will be opened corresponding to the capillary pressure, leading to preferential flow pathways for water transport. The LLDP results further suggest the presence of liquid-lined pores in SLIM. This hypothesis is analyzed theoretically using an interfacial pore flow model. We find that the displacement patterns correspond to capillary fingering in immiscible displacement in porous media. The related physics regarding two-phase flow in porous media is used to confirm the permeation mechanism appearing in SLIMs. In order to experimentally observe liquid-liquid displacement, a microfluidic chip mimicking a porous medium is designed and a highly ramified structure with trapped infusion liquid is observed. The remaining infusion liquid is retained as pools, bridges and thin films around pillar structures in the chip, which further confirms liquid-lining. Fractal dimension analysis, along with evaluation of the fluid (non-wetting phase) saturation, further confirms that the fractal patterns correspond to capillary fingering, which is consistent with an invasion percolation with trapping (IPT) model

Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy

We review HB stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters (GCs) in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the second-parameter problem is presented. A technique is proposed to estimate the HB types of extragalactic GCs on the basis of integrated far-UV photometry. The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyrae, is also revisited, giving a distance modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are studied. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and Space Scienc

Radioactive ages from the precambrian rocks in australia

The central nucleus of the Australian Precambrian shield lies in the western half of the continent and is characterized by striking similarities in structure and age over a vast area. Ages in the vicinity of 2,700 m.y. have been determined at many localities by the Rb/Sr and K/A methods. Flanking and cutting the shield are much younger belts of Precambrian rocks which have been fused onto the older shield. On the south‐eastern side, the deep‐seated Fraser Fault separates the Goldfields rocks (2,700 m.y.) from pegmatized basic charnockites (1,300 m.y.). In this region the shield rocks also show a metamorphism at 2,400 m.y. A similar structure separates the nucleus on the southern side from the east‐trending rocks of the south coast which were pegmatized about 1,400 m.y. ago. The western margin of the shield bounded by the Darling Fault shows evidence of vigorous recrystallization (about 650 m.y.) and regional magmatic activity (900 to 1,100 m.y.). A long E.‐W. 1,000 m.y. belt in central Australia may continue westward to divide the central nucleus of the shield. So far no basement rocks with ages in excess of 2,000 rn.y. have been found on this continent outside Western Australia. Granites in northern Australia appear to be at least 1,650 m.y. old, with a uranium mineralization at 500 m.y. In South Australia two periods of uranium mineralization have been recognized at 500 and 1,500 m.y. The 500 m.y. event is particularly well documented at Myponga, where there is agreement between the U/Pb, K/A and Rb/Sr ages of minerals from the main uranium lode channel. Lead mineralization at Broken Hill and Mount Isa also appears to have occurred in the vicinity of 1,500 m.y. ago. The major age divisions in Canada and Australia appear to be closely comparable

Electrocatalytic reaction-driven flow

Immobilized electrocatalytic surfaces are capable of generating reaction-driven fluid flow by electrochemical energy conversion. A well-known system concerns the gold-platinum bimetallic motor driven by hydrogen peroxide conversion. In this work, we focus on experimental and numerical analyses that provide fundamental insight on the key elements that control the resulting transport characteristics in this system, including the generated electric field, reaction kinetics, and diffusio-electro-osmotic phenomena. The current between the electrodes and the induced potential that governs the reactive fluxes are measured electrochemically, while the fluid flow is analyzed using particle tracking velocimetry. Numerical simulations based on the Poisson-Nernst-Planck and Navier-Stokes equations reveal the interplay of the individual electrode surface reactivity, represented by the dimensionless Damköhler numbers, with the electrokinetic phenomena

Connecting experimental degradation kinetics to theoretical models for photocatalytic reactors: The influence of mass transport limitations

Catalytic microreactors offer great opportunities to measure reaction kinetics, and parameters influencing the reaction. Although microreactors are quite useful for characterizing catalysts, it is important to understand the relative contributions of mass transport and intrinsic kinetics to the apparent reaction rate. In this paper, we demonstrate the importance of accounting for mass transport limitations in the photocatalytic degradation of Bisphenol A over titanium dioxide. Using analytical scaling laws available from literature and numerical simulations, we provide guidelines for the use of microreactors in characterizing (photo)catalysts. These guidelines identify the mass transport limited regime, or the reaction rate limited regime. The photocatalytic degradation of Bisphenol A was found to be mass transport limited at high light intensities (photon fluxes of above 25 mW/cm2). Neglecting the influence of mass transfer limitations in fitting kinetic data resulted in the exponent of reaction rate (β) with respect to light intensity to be β~0.25, while including these effects gave an exponent directly proportional to the light intensity (β~1). These findings stress the importance of a correct inclusion of mass transport limitations. A simple analysis of the transverse Péclet number and second Damköhler number, to quantify the transport and reaction rates, is presented for our laminar flow reactor to illustrate the different limiting regimes