8,030 research outputs found

    An investigation of entorhinal spatial representations in self-localisation behaviours

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    Spatial-modulated cells of the medial entorhinal cortex (MEC) and neighbouring cortices are thought to provide the neural substrate for self-localisation behaviours. These cells include grid cells of the MEC which are thought to compute path integration operations to update self-location estimates. In order to read this grid code, downstream cells are thought to reconstruct a positional estimate as a simple rate-coded representation of space. Here, I show the coding scheme of grid cell and putative readout cells recorded from mice performing a virtual reality (VR) linear location task which engaged mice in both beaconing and path integration behaviours. I found grid cells can encode two unique coding schemes on the linear track, namely a position code which reflects periodic grid fields anchored to salient features of the track and a distance code which reflects periodic grid fields without this anchoring. Grid cells were found to switch between these coding schemes within sessions. When grid cells were encoding position, mice performed better at trials that required path integration but not on trials that required beaconing. This result provides the first mechanistic evidence linking grid cell activity to path integration-dependent behaviour. Putative readout cells were found in the form of ramp cells which fire proportionally as a function of location in defined regions of the linear track. This ramping activity was found to be primarily explained by track position rather than other kinematic variables like speed and acceleration. These representations were found to be maintained across both trial types and outcomes indicating they likely result from recall of the track structure. Together, these results support the functional importance of grid and ramp cells for self-localisation behaviours. Future investigations will look into the coherence between these two neural populations, which may together form a complete neural system for coding and decoding self-location in the brain

    Beam scanning by liquid-crystal biasing in a modified SIW structure

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    A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

    The Effects of Salt Precipitation During CO2 Injection into Deep Saline Aquifer and Remediation Techniques

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    The by-products of combustion from the utilisation of fossil fuels for energy generation are a source of greenhouse gas emissions, mainly Carbon dioxide (CO2). This has been attributed to climate change because of global warming. Carbon capture and storage (CCS) technology has the potential to reduce anthropogenic greenhouse gas emissions by capturing CO2 from emissions sources and stored in underground formations such as depleted oil and gas reservoirs or deep saline formations. Deep saline aquifers for disposal of greenhouse gases are attracting much attention as a result of their large storage capacity. The problem encountered during CO2 trapping in the saline aquifer is the vaporisation of water along with the dissolution of CO2. This vaporisation cause salt precipitation which eventually reduces porosity and impairs the permeability of the reservoir thereby impeding the storage capacity and efficiency of the technology. Salt precipitation during CO2 storage in deep saline aquifers can have severe consequences during carbon capture and storage operations in terms of CO2 injectivity.This work investigates and assesses, experimentally, the effects of the presence of salt precipitation on the CO2 injectivity, the factors that influence them on selected core samples by core flooding experiments, and remediation of salt precipitation during CO2 injection. The investigation also covered the determination of optimum range of deep saline aquifers for CO2 storage, and the effects of different brine-saturated sandstones during CO2 sequestration in deep saline aquifers. In this investigation, three (3) different sandstone core samples (Bentheimer, Salt Wash North, and Grey Berea) with different petrophysical properties were used for the study. This is carried out in three different phases for a good presentation.• Phase I of this study involved brine preparation and measurement of brine properties such as brine salinity, viscosity, and density. The brine solutions were prepared from different salts (NaCl, CaCl2, KCl, MgCl2), which represent the salt composition of a typical deep saline aquifer. The core samples were saturated with different brine salinities (5, 10, 15, 20, 25, wt.% Salt) and testing was conducted using the three selected core samples.• Phase II entailed the cleaning and characterisation of the core samples by experimental core analyses to determine the petrophysical properties: porosity and permeability. Helium Porosimetry and saturation methods were used for porosity determination. Core flooding was used to determine the permeability of the core samples. The core flooding process was conducted at a simulated reservoir pressure of 1500 psig, the temperature of 45 °C, with injection rates of 3.0 ml/min respectively. Interfacial tension (IFT) measurements between the CO2 and various brine salinities as used in the core flooding were also conducted in this phase. Remediation scenarios of opening the pore spaces of the core samples were carried out using the same core flooding rig and the precipitated core samples were flooded with remediation fluids (low salinity brine and seawater) under the same reservoir conditions. The petrophysical properties (Porosity, Permeability) of the core samples were measured before core flooding, after core flooding and remediation test respectively.• In phase III of the study, SEM Image analyses were conducted on the core samples before core flooding, after core flooding, and remediation test respectively. This was achieved by using the FEI Quanta FEG 250 FEG high-resolution Scanning Electron Microscope (SEM) interfaced to EDAX Energy Dispersive X-ray Analysis (EDX).xivResults from Bentheimer, Salt Wash North, and Grey Berea core samples indicated a reduction in porosity, permeability impairment, as well as salt precipitation. It was also found that, at 10 to 20 wt.% brine concentrations in both monovalent and divalent brine, a substantial volume of CO2 is sequestered, which indicates the optimum concentration ranges for storage purposes. The salting-out effect was greater in divalent salt, MgCl2 and CaCl2 as compared to monovalent salt (NaCl and KCl). Porosity decreased by 0.5% to 7% while permeability was decreased by up to 50% in all the tested scenarios. CO2 solubility was evaluated in a pressure decay test, which in turn affects injectivity. Hence, the magnitude of CO2 injectivity impairment depends on both the concentration and type of salt species. The findings from this study are directly relevant to CO2 sequestration in deep saline aquifers as well as screening criteria for carbon storage with enhanced gas and oil recovery processes. Injection of remediation fluids during remediation tests effectively opened the pore spaces and pore throats of the core samples and thereby increasing the core sample's porosity in the range of 14.0% to 28.5% and 2.2% to 12.9% after using low salinity brine and seawater remediation fluids respectively. Permeability also increases in the range of 40.6% to 68.4% and 7.4% to 17.2% after using low salinity brine and seawater remediation fluids respectively. These findings provide remediation strategies useful in dissolving precipitated salt as well as decreasing the salinity of the near-well brine which causes precipitation.The SEM images of the core samples after the flooding showed that salt precipitation not only plugged the pore spaces of the core matrix but also showed significant precipitation around the rock grains thereby showing an aggregation of the salts. This clearly proved that the reduction in the capacity of the rock is associated with salt precipitation in the pore spaces as well as the pore throats. Thus, insight gained in this study could be useful in designing a better mitigation technique, CO2 injectivity scenarios, as well as an operating condition for CO2 sequestration in deep saline aquifers

    Thermal transport in sodium boiling flows for concentrating solar thermal applications

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    Understanding heat and mass transfer phenomena in nucleate boiling of liquid metals such as sodium is an emerging field of study, in particular for the development of next generation concentrating solar thermal power plants with boiling sodium as the heat transfer fluid. The research presented in this doctoral project is focused on advancing the knowledge of sodium boiling by developing comprehensive physics-based bubble growth models. Such models can highlight the governing heat transfer and hydrodynamic phenomena dominating the bubble growth process in sodium, thus aiding the development of efficient sodium boiling systems. In the first part of this work, two numerical heat transfer models are developed with the aim of quantifying the influence of heat transfer mechanisms on the growth of a bubble in sodium pool boiling. In the first model the governing mass, momentum and energy conservation equations are solved to compute the evaporative heat flux from a region where the liquid-vapour interface of the bubble meets the wall, referred to as the contact line region. The model accounts for the influence of an electron pressure component on the evaporation of the fluid film in the contact line region in sodium. The results show that for the same wall superheat, the heat flux from sodium is six times larger compared to a high Prandtl number fluid, here FC-72, due to the high thermal conductivity of the liquid metal. The second numerical model predicts the growth rate of a sodium bubble based on the heat transferred from a microlayer (which is a thin layer of fluid formed underneath a bubble), the thermal boundary layer, and the bulk liquid surrounding the bubble. The model accounts for the variation in the wall temperature below the bubble as the liquid in the microlayer and the thermal boundary layer evaporates. Predictions from the model for a bubble growing with a constant contact angle indicate that the microlayer evaporation is the dominant heat transfer mechanism during the initial phase of bubble growth after nucleation. In addition, a parametric study conducted to study the effect of wall superheat indicated that the larger the wall superheat, the larger is the growth rate and radius of a sodium bubble. The development of a comprehensive mechanistic bubble growth model accounting for the variation in the contact angle and the shape of a bubble is pursued next. The heat transfer model that was developed based on the evaporation of the microlayer in the first part of this project is coupled to a force and a contact angle sub-model to study the complete bubble growth process from nucleation to departure in pool boiling. A novel methodology is presented to approximate the balloon-like shape of a bubble prior to departure as a truncated sphere atop a conical bottleneck. The model is extensively verified and validated against high-fidelity CFD simulations and experimental data on pool boiling of water and methanol from literature, and shows good agreement. The validated mechanistic model is then used to simulate the bubble growth process in sodium and to investigate the effects of wall superheat, contact angle rate, bulk liquid temperature and the accommodation coefficient on the bubble growth and departure characteristics. It is found that a sodium bubble is typically large with departure radius on the order of a few centimetres. In addition, it is observed that smaller the wall superheat, the greater is the tendency of the bubble to have a balloon-like shape at departure

    Feature Papers in Compounds

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    This book represents a collection of contributions in the field of the synthesis and characterization of chemical compounds, natural products, chemical reactivity, and computational chemistry. Among its contents, the reader will find high-quality, peer-reviewed research and review articles that were published in the open access journal Compounds by members of the Editorial Board and the authors invited by the Editorial Office and Editor-in-Chief

    Measurement of the Environmental Impact of Materials

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    Throughout their life cycles—from production, usage, through to disposal—materials and products interact with the environment (water, soil, and air). At the same time, they are exposed to environmental influences and, through their emissions, have an impact on the environment, people, and health. Accelerated experimental testing processes can be used to predict the long-term environmental consequences of innovative products before these actually enter the environment. We are living in a material world. Building materials, geosynthetics, wooden toys, soil, nanomaterials, composites, wastes and more are research subjects examined by the authors of this book. The interactions of materials with the environment are manifold. Therefore, it is important to assess the environmental impact of these interactions. Some answers to how this task can be achieved are given in this Special Issue

    Developing active biomaterials for implantable devices: platforms to investigate capacitive charge based control of biofouling

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    Implantable devices, in particular biosensors, have clear utility within medicine, but face a hurdle to long-term function due to adsorption of biomolecules (biofouling) and subsequent immune re- sponse to implants, the foreign body response (FBR). Strategies to control this immune reaction have included material selection, drug release and, more recently, engineered surface properties. The increasing use of embedded electronics within many classes of implanted devices presents an opportunity to exploit electromagnetic phenomena at the device surface to mitigate biofouling and FBR. Such active biomaterials would allow dynamic modification of the apparent material properties of an implanted device. A hypothesis was developed that biological interaction with a biomaterial surface can be altered by capacitive charging. A platform was constructed to test this and related hypotheses around cell and protein surface interactions in vitro and adapted into a second platform for initial characterisa- tion work on an early in vivo model using chick eggs. These platforms were designed to be easy to fabricate and to provide multiple electrical connections into a substrate in contact with biological solutions or tissue. Electrodes were fabricated from fluoropolymer coated tantalum pentoxide, a high-κ dielectric, and compared against adjacent, identically coated, silicon dioxide regions. Cells from the MDA- MB-231 cancer cell line were cultured on these regions under electrical stimulation. A voltage de- pendent reduction of cell attachment and spreading was detected on capacitively charged surfaces compared to uncharged controls. The tentative results, suggest capacitively charged surfaces hold promise as active biomaterials. A second cell type MCF-7 did not reproduce the effect, implying a more coherent understanding is required of the mechanisms behind cell surface interactions on these surfaces. Multiple independent bioelectrochemical cell-surface interactions were observed using the plat- form and several quantification techniques were successfully employed. It is therefore argued that the platform may have wide applicability as a future research tool

    Turbulent flows over porous lattices: alteration of near-wall turbulence and pore-flow amplitude modulation

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    Turbulent flows over porous lattices consisting of rectangular cuboid pores are investigated using scale-resolving direct numerical simulations. Beyond a certain threshold which is primarily determined by the wall-normal Darcy permeability, Ky+{K_y}^+, near-wall turbulence transitions from its canonical regime, marked by the presence of streak-like structures, to another marked by the presence of spanwise coherent structures reminiscent of the Kelvin-Helmholtz (K-H) type of instability. This permeability threshold agrees well with that previously established in studies where permeable-wall boundary conditions had been used as surrogates for a porous substrate. None of the substrates investigated demonstrate any drag reduction relative to smooth-wall turbulent flow. At the permeable surface, a significant component of the flow is that which adheres to the pore geometry and undergoes amplitude modulation (AM). This pore-coherent flow remains notable within the substrates, highlighting the importance of the porous substrate's microstructure when the overlying flow is turbulent, an aspect which cannot be accounted for when using continuum-based approaches to model porous media flows or effective representations such as wall boundary conditions. The severity of the AM is enhanced in the K-H-like regime, which has implications when designing porous substrates for transport processes. This suggests that the surface of the substrate can have a geometry which is different than the rest of it and tailored to influence the overlying flow in a particular way
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