Channel Interior Research Articles

Fabrication of a Microfluidic-Based Device Coated with Polyelectrolyte-Capped Titanium Dioxide to Couple High-Performance Liquid Chromatography with Inductively Coupled Plasma Mass Spectrometry for Mercury Speciation.

Mercury (Hg) is a toxic element which impacts on biological systems and ecosystems. Because the toxicity of Hg species is highly dependent on their concentration levels and chemical forms, the sensitive identification of the chemical forms of Hg-i.e., Hg speciation-is of major significance in providing meaningful information about the sources of Hg exposure. In this study, a microfluidic-based device made of high-clarity poly(methyl methacrylate) (PMMA) was fabricated. Then, titanium dioxide nanoparticles (nano-TiO2s) were attached to the treated channel's interior with the aid of poly(diallyldimethylammonium chloride) (PDADMAC). After coupling the nano-TiO2-coated microfluidic-based photocatalyst-assisted reduction device (the nano-TiO2-coated microfluidic-based PCARD) with high-performance liquid chromatography (HPLC) and inductively coupled plasma mass spectrometry (ICP-MS), a selective and sensitive, hyphenated system for Hg speciation was established. Validation procedures demonstrated that the method could be satisfactorily applied to the determination of mercury ions (Hg2+) and methylmercury ions (CH3Hg+) in both human urine and water samples. Remarkably, the zeta potential measured clearly indicated that the PDADMAC-capped nano-TiO2s with a predominance of positive charges indeed provided a steady force for firm attachment to the negatively charged device channel. The cause of the durability of the nano-TiO2-coated microfluidic-based PCARD was clarified thus.

Direct laser acceleration: A model for the electron injection from the walls of a cylindrical guiding structure.

We use analytical methods and particle-in-cell simulation to investigate the origin of electrons accelerated by the process of direct laser acceleration driven by high-power laser pulses in preformed narrow cylindrical plasma channels. The simulation shows that the majority of accelerated electrons are originally located along the interface between the channel wall and the channel interior. The analytical model based on the electron hydrodynamics illustrates the underlying physical mechanism of the release of electrons from the channel wall when irradiated by an intense laser, the subsequent electron dynamics, and the corresponding evolution of the channel density profile. The quantitative predictions of the total charge of released electrons and the average electron density inside the channel are validated by comparison with the simulation results.

Flume experiments in the development of crevasse-splay deposits: transition from asymmetric to symmetric geometry

Abstract Crevasse-splay deposits play an important role in the reconstruction of the magnitude of past flood events and in understanding the behavior of river systems. Despite the extensive studies conducted on the geometry and facies of crevasse-splay deposits, their spatiotemporal developmental processes have remained insufficiently understood. In this study, scaled flume experiments were conducted to study the relationship between the developmental processes of crevasse splays and their characteristics. An experimental flume was set up in a tank to simulate the 2019 Chikuma River flooding event in central Japan. To model the overbank flow, an opening was created on the side of the flume’s wall through which the flow flooded onto a horizontal acrylic plate. The sediment used in the experiments consisted of particles with grain sizes of approximately 0.3 and 0.1 mm, which were determined to be equivalent to bedload gravel and suspended sand in a real-scale river using dimensional analysis. The results of the experiments revealed three important findings: 1) Crevasse-splay deposits initially developed an asymmetric shape extending downstream of the main river channel but gradually showed a symmetric geometry. The river mainstream initially influenced the direction of the inundation flow, but channel bifurcations after the deposition of the sediment piles later changed the geometry of splays into a more symmetric shape. 2) Crevasse-splay deposits developed in two distinct regions (proximal and distal splay), corresponding to sediment transport by bedload and suspended load, respectively. These two regions are commonly observed in the actual field scale. 3) The original overbank flow was a sheet flow without channels, which caused coarse-grained sediments to be spread over a wide area. Subsequently, the accumulation of coarse sands in the developed channel interiors resulted in the buildup of finer-grained sediments upstream of the proximal splay. Thus, the proximal splay deposits became slightly coarse downstream, whereas they rapidly became fine at the boundary with the distal splay. These findings indicate that the characteristics of crevasse-splay deposits vary with the landform’s development stage, thus providing a basis for interpreting their depositional facies.

Stokes flow solutions in infinite and semi‐infinite porous channels

AbstractWe derive a class of exact solutions for Stokes flow in infinite and semi‐infinite channel geometries with permeable walls. These simple, explicit, series expressions for both pressure and Stokes flow are valid for all permeability values. At the channel walls, we impose a no‐slip condition for the tangential fluid velocity and a condition based on Darcy's law for the normal fluid velocity. Fluid flow across the channel boundaries is driven by the pressure drop between the channel interior and exterior; we assume the exterior pressure to be constant. We show how the ground state is an exact solution in the infinite channel case. For the semi‐infinite channel domain, the ground‐state solutions approximate well the full exact solution in the bulk and we derive a method to improve their accuracy at the transverse wall. This study is motivated by the need to quantitatively understand the detailed fluid dynamics applicable in a variety of engineering applications including membrane‐based water purification, heat and mass transfer, and fuel cells.

Self-regulation of the nuclear pore complex enables clogging-free crowded transport

Nuclear pore complexes (NPCs) are the main conduits for macromolecular transport into and out of the nucleus of eukaryotic cells. The central component of the NPC transport mechanism is an assembly of intrinsically disordered proteins (IDPs) that fills the NPC channel. The channel interior is further crowded by large numbers of simultaneously translocating cargo-carrying and free transport proteins. How the NPC can efficiently, rapidly, and selectively transport varied cargoes in such crowded conditions remains ill understood. Past experimental results suggest that the NPC is surprisingly resistant to clogging and that transport may even become faster and more efficient as the concentration of transport protein increases. To understand the mechanisms behind these puzzling observations, we construct a computational model of the NPC comprising only a minimal set of commonly accepted consensus features. This model qualitatively reproduces the previous experimental results and identifies self-regulating mechanisms that relieve crowding. We show that some of the crowding-alleviating mechanisms-such as preventing saturation of the bulk flux-are "robust" and rely on very general properties of crowded dynamics in confined channels, pertaining to a broad class of selective transport nanopores. By contrast, the counterintuitive ability of the NPC to leverage crowding to achieve more efficient single-molecule translocation is "fine-tuned" and relies on the particular spatial architecture of the IDP assembly in the NPC channel.

Joint Channel Access and Power Control Optimization in Large-Scale UAV Networks: A Hierarchical Mean Field Game Approach

With the development of the new generation communications, unmanned aerial vehicle base stations (UAV-BSs) have been widely used to enhance the terrestrial communications. Because of the increasing demand for communication, the scale of the networks is constantly expanding. This paper investigates a joint channel access and power control optimization in large-scale UAV coverage networks, and proposes a hierarchical mean field game approach. Firstly, considering the quality of service (QoS) requirement and energy consumption, the cost function of UAV-BSs is designed. A joint channel access and power control optimization framework is built to improve the efficiency of resource utilization by minimizing the cost function. Then, the joint optimization problem is formulated as a hierarchical game model whose exterior is a channel access game and interior is a multiple mean fields game (MMFG). The channel access game is proved to be a potential game and have at least one Nash Equilibrium (NE). As feedback for channel access optimization, power control policy is obtained by MMFG. A near-optimal algorithm for joint optimizations is designed to achieve the NE. Besides, aiming to the networks with limited computing capability, we also propose a fast optimization approach. The original optimization problem is divided into two independent sub-problems. The channel optimization sub-problem is formulated as a local altruistic game model, then the power policy sub-problem is solved by MMFG. A distributed algorithm based on spatial adaptive play and better response is proposed. Finally, the simulation results verify the validity of the proposed algorithms. The near-optimal optimization method has a better performance while the fast optimization method has a higher speed of the convergence. Compared with the traditional approaches, both of them improve resources utilization efficiency greatly.

Radial position effects of externally mounted hollow cathode on a magnetically shielded Hall thruster

A magnetically shielded Hall thruster (MSHT) has a significantly longer service life than a conventional unshielded Hall thruster (USHT) as it shifts the magnetic field of maximum strength from the channel interior to the exterior. In this study, it was experimentally determined that the influence of the radial position of an externally mounted hollow cathode on the performance of an MSHT is quite different from that of a USHT. The measured plasma parameters demonstrate that in addition to the coupling voltage loss and plume divergence loss, the electron cross-field transport loss (not considered previously and introduced by the outward shifting of the magnetic field) is also important to explaining the performance variation in the MSHT. This study refines the coupling theory between the cathode and Hall thruster and provides a valuable reference for determining the radial position of an externally mounted hollow cathode in an MSHT.

Artificial Water Channels—Progress Innovations and Prospects

Artificial Water Channels—Progress Innovations and Prospects

Chanalyzer: A Computational Geometry Approach for the Analysis of Protein Channel Shape and Dynamics.

Morphological analysis of protein channels is a key step for a thorough understanding of their biological function and mechanism. In this respect, molecular dynamics (MD) is a very powerful tool, enabling the description of relevant biological events at the atomic level, which might elude experimental observations, and pointing to the molecular determinants thereof. In this work, we present a computational geometry-based approach for the characterization of the shape and dynamics of biological ion channels or pores to be used in combination with MD trajectories. This technique relies on the earliest works of Edelsbrunner and on the NanoShaper software, which makes use of the alpha shape theory to build the solvent-excluded surface of a molecular system in an aqueous solution. In this framework, a channel can be simply defined as a cavity with two entrances on the opposite sides of a molecule. Morphological characterization, which includes identification of the main axis, the corresponding local radius, and the detailed description of the global shape of the cavity, is integrated with a physico-chemical description of the surface facing the pore lumen. Remarkably, the possible existence or temporary appearance of fenestrations from the channel interior towards the outer lipid matrix is also accounted for. As a test case, we applied the present approach to the analysis of an engineered protein channel, the mechanosensitive channel of large conductance.

Turbulence suppression and regeneration in a magnetohydrodynamic duct flow due to influence of arbitrary electrically conductive walls

In the current study, we present a series of numerical simulations describing a turbulent magnetohydrodynamic flow subjected to a transverse magnetic field in a square duct with arbitrary electrically conductive walls. The characteristic flow and electromagnetic non-dimensional parameters (Reynolds and Hartmann number, respectively) were fixed, while the wall conductivity ratio (Cd) was varied from the perfectly electrically insulated (Cd = 0) to perfectly electrically conducting (Cd→∞). We have assumed the one-way coupling between the flow of an electrically conducting fluid and the imposed magnetic field. The influence of the electrically conducting walls was imposed by simulating additional finite solid domains coupled with the channel interior. The turbulence was simulated by applying the large eddy simulation approach with the dynamic Smagorinsky sub-grid scale model. The obtained results confirmed a significant impact of the conductivity of the surrounding walls on the flow and turbulence reorganization. We have observed that the initially fully developed turbulence regime was gradually suppressed in the 0≤Cd<0.15 range, while the fully laminarized state was obtained at Cd=0.15. We found that the process of turbulence suppression was accompanied by the appearance of the patterned turbulence phenomenon in the proximity of the walls parallel to the magnetic field direction. With a further increase in the wall conductivity parameter (0.15<Cd<∞), we have observed a complete turbulence regeneration. We found that this turbulence regeneration was caused by the local reorganization of the total current density loops near the electrically conducting walls.

Optically Accessible Microfluidic Flow Channels for Noninvasive High-Resolution Biofilm Imaging Using Lattice Light Sheet Microscopy.

Imaging platforms that enable long-term, high-resolution imaging of biofilms are required to study cellular level dynamics within bacterial biofilms. By combining high spatial and temporal resolution and low phototoxicity, lattice light sheet microscopy (LLSM) has made critical contributions to the study of cellular dynamics. However, the power of LLSM has not yet been leveraged for biofilm research because the open-on-top imaging geometry using water-immersion objective lenses is not compatible with living bacterial specimens; bacterial growth on the microscope’s objective lenses makes long-term time-lapse imaging impossible and raises considerable safety concerns for microscope users. To make LLSM compatible with pathogenic bacterial specimens, we developed hermetically sealed, but optically accessible, microfluidic flow channels that can sustain bacterial biofilm growth for multiple days under precisely controllable physical and chemical conditions. To generate a liquid- and gas-tight seal, we glued a thin polymer film across a 3D-printed channel, where the top wall had been omitted. We achieved negligible optical aberrations by using polymer films that precisely match the refractive index of water. Bacteria do not adhere to the polymer film itself, so that the polymer window provides unobstructed optical access to the channel interior. Inside the flow channels, biofilms can be grown on arbitrary, even nontransparent, surfaces. By integrating this flow channel with LLSM, we were able to record the growth of S. oneidensis MR-1 biofilms over several days at cellular resolution without any observable phototoxicity or photodamage.

Geomorphological characterization of the 2014–2015 Holuhraun lava flow-field in Iceland

The 2014–2015 Holuhraun eruption extruded >1 km3 of lava in a barren region of the Icelandic highlands. Due to its large volume and the abundance of data for this eruption, Holuhraun is an ideal site to investigate fissure-fed eruption products for comparison with other large lava flow-fields on Earth and other planetary bodies. To characterize lava morphologies associated with this event, we used 0.01–0.5 m/pixel image data, acquired from aerial surveys and small Unoccupied Aircraft Systems (sUAS) to create a 1:800-scale facies map that was ground-truthed using field observations during the summers of 2015–2019. Each facies region exhibits similar attributes in the remote sensing data, including albedo, surface texture, and geomorphology. However, at our mapping scale of 1:800, the facies typically include mixtures of lava types. Results show that transitional lava types (e.g., rubbly pāhoehoe, slabby pāhoehoe, and spiny pāhoehoe) dominate the 2014–2015 Holuhraun lava flow-field (83.82 km2), rather than the traditional end-members of ʻaʻā and classical pāhoehoe. At 1:800-scale, we distinguish the following eight facies (with the percentage of total flow-field area shown in parentheses): rubbly (57.35%), spiny (25.96%), undifferentiated rubbly–spiny (9.59%), shelly (5.58%), pāhoehoe (1.24%), flat-lying–knobby (0.58%), vent-proximal edifice (0.19%), and channel interior (0.16%). Field observations show that initial coherent pāhoehoe surfaces were episodically disrupted to produce slabby and rubbly textures that resemble ʻaʻā in remote sensing data. Our observations also show that continued solidification of the lava beneath brecciated surfaces can cause the surfaces of disrupted lobes to stabilize, or restabilize and undergo inflation. These factors complicate the use of surface texture as a direct indicator of emplacement style, which can change over the course of an eruption. This complexity has important implications for reconstructing the emplacement history of flow-fields on Earth and other planetary bodies.

Vortex generation due to internal solitary wave propagation past a sidewall constriction

Abstract

Determination of Long-Range Internal Stresses in Cyclically Deformed Copper Single Crystals Using Convergent Beam Electron Diffraction

Understanding long range internal stresses (LRIS) may be crucial for elucidating the basis of the Bauschinger effect, plastic deformation in fatigued metals, and plastic deformation in general. Few studies have evaluated LRIS using convergent beam electron diffraction (CBED) in cyclically deformed single crystals oriented in single slip and there are no such studies carried out on cyclically deformed single crystals in multiple slip. In our earlier and recent study, we assessed the LRIS in a cyclically deformed copper single crystal in multiple slip via measuring the maximum dislocation dipole heights. Nearly equal maximum dipole heights in the high dislocation density walls and low dislocation density channels suggested a uniform stress state across the labyrinth microstructure. Here, we evaluate the LRIS by determining the lattice parameter in the channels and walls of the labyrinth dislocation structure using CBED. Findings of this work show that lattice parameters obtained were almost equal near the walls and within the channels. Thus, a homogenous stress state within the heterogeneous dislocation microstructure is again suggested. Although the changes in the lattice parameter in the channels are minimal (less than 10−4 nm), CBED chi-squared analysis suggests that the difference between the lattice parameter values of the cyclically deformed and unstrained copper are slightly higher in the proximity of the walls in comparison with the channel interior. These values are less than 6.5% of the applied stress. It can be concluded that the dominant characteristics of the Bauschinger effect may need to include the Orowan-Sleeswyk mechanism type of explanation since both the maximum dipole height measurements and the lattice parameter assessment through CBED analysis suggest a homogenous stress state. This work complements our earlier work that determined LRIS based on dipole heights by assessing LRIS through a different methodology, carried out on a cyclically deformed copper single crystal oriented for multiple slip.

Analysis: Overcoming Human Factors Challenges of Endoscope Culturing with Turbulent Fluid Flow.

Analysis: Overcoming Human Factors Challenges of Endoscope Culturing with Turbulent Fluid Flow.

A SiN Microcalorimeter and a Non-Contact Precision Method of Temperature Calibration

A dual-channel flow-through microcalorimeter device is presented along with a non-contact precision method of temperature calibration. A microfluidic channel with a volume of ≈550 pL in a spiral layout was fabricated from low-stress nitride suspended over a cavity in a silicon substrate. The thin-film heater and resistive temperature detector (RTD) on the channel were defined by a patterned Ti layer. The device exhibited a rapid thermal response with the following thermal characteristics: a time constant of ≈10.5 ms, a heat conductance of $\approx 152~\mu ~\text{W}\,\cdot ~{\mathrm {K}}^{-{1}} $ and a heat capacitance of $\approx 1.6~\mu \text{J}\,\cdot {\mathrm {K}}^{-{1}} $ on average based on the measurements of nine separate units fabricated on the same wafer. Further, the melting curve analysis (MCA) of a double-stranded DNA was proposed as a non-contact method of microcalorimeter RTD calibration. The method revealed that the measurement by the RTD underestimates the temperature of the channel interior by an amount of nearly 15%, at ≈ 9 mW. At this dissipated power level, the method also revealed a spatial nonuniformity of ≈ ± 0.8 °C across the microcalorimeter. [2020-0173]

Investigation on the phase split characteristics of slug and annular flow in a metal foam-filled T-junction

The application of metal foam in the flow boiling phase of transformation heat can greatly increase the nuclear nucleation rate, increase the fluid disturbance, and increase the heat exchange area, thereby greatly improving the heat exchange efficiency. However, the metal frame structure can increase the complexity of the channel interior, which may cause serious water shortages in some branches. Aiming at this problem, this paper investigated the characteristics related to the flow moved through a small heat transfer channel filled with metal foam at a T-junction. Experiments focused on the relationships between the phase split and the flow pattern as well as the two-phase superficial velocity and the pores per inch (PPI) effect of the metal foam. A 2.5 × 2.5 mm rectangular channel dividing at a T-junction filled with metal foam was used in the experiment. Air is the gas medium and water is the liquid medium. Results showed that after the channel was filled with metal foam, the liquid taken off was better than when the channel was not filled with metal foam. The liquid taken off from side branch decreased with an increase in PPI. When the PPI number reach 10, the liquid taken off gets the best result. At last, four predictive models of phase split characteristics for the two flow patterns with different PPI were developed, which was verified by experimental results showing high accuracy.

Molecular dynamics simulations of ion separation in nano-channel water flows using an electric field

ABSTRACTRemoval of undesired substances from water is a field of investigation recently focused at the nanoscale. Towards this direction, molecular dynamics simulations are conducted in this paper to investigate unwanted ion removal in nanochannel flows. The simulation method incorporates a Poiseuille-like water/ion flow system at the nanoscale where an electric field, of various magnitudes in the range of E = 0.25–1.5 V/Å, is applied perpendicular to the flow, leading anions and cations close to the wall regions, similar to the Capacitive De-Ionization method. The time needed for ions to reach equilibrium, i.e. to flow in the region near the walls while pure water flows in the channel interior, is t = 1.3 ns when E = 1.5 V/Å and t = 4.0 ns when E = 0.25 V/Å, showing a dependency on the value of the electric field. Calculations on density, velocity, and temperature values report on fluid properties to be used in the proposed desalination configuration and could act as a basis to guide novel technological applications and extend to higher scales.

2D (r–z) numerical simulations of the plasma and channel erosion in a 100 kW class nested Hall thruster**© 2018. California Institute of Technology. Government sponsorship acknowledged.

Results from the first numerical simulations of a nested Hall thruster (NHT) are presented. The X3 is a 100 kW class thruster with one center-mounted hollow cathode and three acceleration channels none of which is currently magnetically shielded. The simulations were conducted with the r–z axisymmetric code Hall2De and focused on operation with all three channels firing, at discharge voltages of 300, 400 and 800 V. The corresponding discharge power levels are 38, 49.6 and 100 kW. A recent experimental campaign provided measurements of the thrust for a wide range of channel combinations and operating conditions, but not at 800 V. At 300 and 400 V, with all three channels operating, the computed thrust is underestimated by 8.4% and 4.7%, respectively. The simulations also allowed for the first estimates of erosion by ion sputtering in a NHT. The maximum erosion rates along the boron nitride channel walls are found to be in the order of 1 mm kh−1 (103 μm kh−1) at 300 V. The rates at 800 V are a few times higher than those at 300 V. Mechanisms driving the erosion are identified, in part through comparisons with measurements and simulations of a similar single-channel unshielded Hall thruster that has been extensively studied in the past. It is found that anomalous collisions, now widely accepted to drive electron transport in these devices, and the manner in which they develop spatially in the channel interior can play a significant role on the channel erosion.

Electrostatic attraction of weak monoacid anions increases probability for protonation and passage through aquaporins

A positive electrostatic field emanating from the center of the aquaporin (AQP) water and solute channel is responsible for the repulsion of cations. At the same time, however, a positive field will attract anions. In this regard, l-lactate/lactic acid permeability has been shown for various isoforms of the otherwise highly water and neutral substrate selective AQP family. The structural requirements rendering certain AQPs permeable for weak monoacids and the mechanism of conduction have remained unclear. Here, we show by profiling pH-dependent substrate permeability, measurements of media alkalization, and proton decoupling that AQP9 acts as a channel for the protonated, neutral monocarboxylic acid species. Intriguingly, the obtained permeability rates indicate an up to 10 times higher probability of passage via AQP9 than given by the fraction of the protonated acid substrate at a certain pH. We generated AQP9 point mutants showing that this effect is independent from properties of the channel interior but caused by the protein surface electrostatics. Monocarboxylic acid-conducting AQPs thus employ a mechanism similar to the family of formate-nitrite transporters for weak monoacids. On a more general basis, our data illustrate semiquantitatively the contribution of surface electrostatics to the interaction of charged molecule substrates or ligands with target proteins, such as channels, transporters, enzymes, or receptors.