Geometrical Confinement Effect Research Articles

Cryogenic Atom Probe Tomograph to Resolve Confined Electrochemical Interfaces

Atom Probe Tomography (APT) is an analysis technique capable of providing a 3-dimensional, element-sensitive map of needle-shaped samples (where the tip radius is about 50 nm) with sub-nanometre resolution [1]. APT analysis is carried out at temperatures in the range of 25-75 K to limit the thermal motion of surface atoms which improves the spatial and mass resolution. As a result, this enables the analysis of frozen liquids, and by extension solid-liquid interfaces, by so-called “cryo-APT”. While this was historically challenging, recent advancements in UHV-cryogenic transfer and sample preparation methods now make it possible for the atomic scale analysis of interfacial species in electrochemical systems [2-4].In this study, we will attempt to explore the effect of nanoscale geometrical confinement on the distribution of interfacial ions for the first time and to resolve the electric double layer, using cryo-APT. We will describe the workflow for cryo-APT sample preparation to study confined solid-liquid interfaces, using nanoporous gold as a template material with a frozen sodium iodide solution inside its pores. APT reconstructions will allow 3-dimensional mapping of aqueous interfacial species, helping to develop our understanding of the structure of the electric double layer under nanoscale geometrical confinement. The attached Figure shows an SEM image of the later stages of cryo-FIB milling of an APT tip of frozen deionised water and 50 mM sodium iodide and a segment of the related APT atom map reconstruction.

Stepwise ordering under nano-scale confinement in thin silica films

The phase transition from an amorphous 2D structure to a crystalline 3D structure under confinement is a fascinating and challenging problem in materials science and condensed matter physics. This work reports the Field Emission Scanning Electron Microscopy (FESEM) and energy dispersive analysis of X-rays (EDAX), X-ray diffraction (XRD), X-ray reflectivity (XRR), Fourier Transform Infrared Spectroscopy (FTIR) studies on pristine silica thin films. Silica was deposited on pristine Al (001) substrate (Alfa-Aesar) from tetraethyl orthosilicate (TEOS) precursor and thin films were spin-coated at different spinning frequencies under ambient conditions. The samples are highly porous and in-plane porosity varies from 30% to 60% as analyzed from both the FESEM and XRR and with increasing spinning frequency pores become more ordered. XRR shows that the samples are composed of two layers, a thick layer of aluminum oxide (Al2O3) as buffer layer and a layer of SiO2 on the top of the film. XRR shows sharp ordering rather than any layering. The film thickness varies from 90[Formula: see text]Å to 200[Formula: see text]Å and it shows a linear decay in the thickness of silica layer. In between 1000 and 1500[Formula: see text]rpm, XRR shows a sharp drop in the thickness of silica layer where thickness of this layer falls below the unit-cell dimension indicating higher-ordered crystallization under strained condition. XRD shows that there are no crystalline peaks coming from silica at lower rpm except the peaks of Al coming from the substrate. At 1000, 1200 and 1500[Formula: see text]rpm there is peak corresponding to the quartz, highly crystalline phase of SiO2 and where the substrate is slightly modified Al2O3. Reflection from (101) plane is prominent in all the films and it gives indication of formation 3D crystalline phase. Whereas FTIR shows that the Si–O stretching modes exhibit sharper and more defined reflectance peak compared to the broad bands observed in the samples in lower rpm. Observations suggest a clear indication of progressive ordering under confinement. To one extent, the results of XRR and FESEM will suggest the mechanism of transition from 2D amorphous to 3D crystalline phase under geometrical confinement effect and total confinement effect arising from thinning of the films with the increasing rpm, interfacial effect and on the other hand XRD and FTIR results will explain high ordering under confinement without any layering.

Numerical simulations of the sedimentation of soft particles in confined Newtonian liquids

The gravitational sedimentation of solid particles in liquids is important in several applications, many of which involve soft particles (e.g., microplastics, microgels, and biological cells). However, there is still little knowledge on the sedimentation of deformable beads. In this work, we employ fully three-dimensional finite-element numerical simulations to investigate the sedimentation of an initially spherical elastic bead in a Newtonian liquid contained inside a square-cross section vessel. In particular, we carry out a wide parametric analysis to understand the effects of inertia, particle deformability, and geometrical confinement on the particle sedimentation dynamics and settling velocity. In addition, the sedimentation of a particle in a Newtonian liquid close to a vertical wall is studied. When the particle sediments at the center of the vessel, the settling velocity increases at increasing inertia and decreases at increasing confinement and deformability. However, at intermediate inertia, the settling velocity passes through a minimum and then rises as the particle gets softer. Correspondingly, the equilibrium deformed shape of the particle shows the appearance of a concavity at the rear. When the sedimentation occurs close to a vertical wall, the closer the particle to the solid surface, the slower its sedimentation. On the other hand, the hydrodynamic interaction between the particle and the wall makes the particle move away from the wall.

Ion transport, mechanical properties and relaxation dynamics in structural battery electrolytes consisting of an imidazolium protic ionic liquid confined into a methacrylate polymer

The effect of confining a liquid electrolyte into a polymer matrix was studied by means of Raman spectroscopy, differential scanning calorimetry, temperature-modulated differential scanning calorimetry, dielectric spectroscopy, and rheology. The polymer matrix was obtained from thermal curing ethoxylated bisphenol A dimethacrylate while the liquid electrolyte consisted of a protic ionic liquid based on the ethyl-imidazolium cation [C2HIm] and the bis(trifluoromethanesulfonyl)imide [TFSI] anion, doped with LiTFSI salt. We report that the confined liquid phase exhibits the following characteristics: (i) a distinctly reduced degree of crystallinity; (ii) a broader distribution of relaxation times; (iii) reduced dielectric strength; (iv) a reduced cooperativity length scale at the liquid-to-glass transition temperature (T g); and (v) up-speeded local T g-related ion dynamics. The latter is indicative of weak interfacial interactions between the two nanophases and a strong geometrical confinement effect, which dictates both the ion dynamics and the coupled structural relaxation, hence lowering Tg by about 4 K. We also find that at room temperature, the ionic conductivity of the structural electrolyte achieves a value of 0.13 mS/cm, one decade lower than the corresponding bulk electrolyte. Three mobile ions (Im+, TFSI-, and Li+) contribute to the measured ionic conductivity, implicitly reducing the Li+ transference number. In addition, we report that the investigated solid polymer electrolytes exhibit the shear modulus needed for transferring the mechanical load to the carbon fibers in a structural battery. Based on these findings, we conclude that optimized microphase-separated polymer electrolytes, including a protic ionic liquid, are promising for the development of novel multifunctional electrolytes for use in future structural batteries.

Lithium metal deposition under the geometrical confinement effect: Dendritic copper foam current collector

Dendrite-free lithium is the ultimate goal in achieving the commercial application of lithium metal anode. In this paper, Dendritic Copper Foam Current Collector (DCFC) is prepared by an electrometallurgy method to realize the homogeneous deposition of lithium metal. The deposition behavior of Cu and Li on different interfaces was studied. Firstly, the Cu dendrites constructed on the surface of Cu foam are induced and formed by the potential oscillation generated by the hydrogen evolution reaction. Secondly, the heterogeneous nucleation model reveals that the reason for the dendrite-free lithium deposition on the dendritic copper foam is the geometrical confinement effect. Compared with commercial copper foam, DCFC with hierarchical structure significantly reduces the nucleation energy and deposition overpotential of lithium metal. The in situ optical microscope observation and finite element simulation results confirm our ideas. Electrochemically, DCFC works stably after 1200 h at a lower potential of 36 mV in a DCFC@Li symmetric battery, which is much superior to 75 mV for just 340 h in Cu foam. The ideal collector with a dendritic structure has broad prospects for the practical application of lithium metal anode.

The Effect of Electroplating Nickel on the Mechanical Properties of Brittle Mg-Based Bulk Metallic Glasses

Magnesium-based bulk metallic glasses (BMGs) are typical intrinsic brittle lightweight BMG alloys, and their improvement in plasticity has attracted widespread attention in the field of BMGs. We used the electroplating method to modify the surface of Mg59.5Cu22.9Ag11Gd6.6 BMGs and investigated the geometric confinement effect of the Ni coating on the mechanical properties of the BMG. The results show that under the plating conditions of adding 1 g/L nano Al2O3 to the plating solution, adjusting the plating temperature to 50 °C, and plating time to 3 h, a smooth and dense nickel coating with a thickness of about 150 μm can be formed on the surface of the Mg-based BMG. The uniaxial compression tests showed that the average fracture strength of the BMG was increased from 565 MPa to 598 MPa by a 50 μm Ni coating, and the fluctuation range of strength was decreased from 429 MPa to 265 MPa, a reduction of 36%. The Weibull analysis showed that the Weibull modulus m was increased from 4.3 to 4.8 by the coating, and the safety stress was increased from 54 MPa to 235 MPa, indicating that electroplating nickel could improve the reliability of the Mg-based BMG alloy. However, no significant improvement of the compression plasticity was found, which indicated that improving the room temperature plasticity of brittle Mg-based BMG alloys by the geometric confinement of electroplating Ni was limited. The influence of the thickness of the Ni coating on the maximum stress level and stress distribution in the BMG samples was analyzed by ANSYS finite element simulation. It was found that when the thickness of the coating was 30% of the radius of the cylindrical compressed sample, the stress distribution caused by the Ni coating was the most uniform, and the maximum stress level was relatively reduced, which is beneficial for improving the geometric confinement effect. As a result, the Mg-based BMG sample coated with a Ni coating of 150 μm thickness exhibited ~0.3% macroscopic compressive plasticity. This is of great significance for understanding the plastic deformation mechanism of brittle BMGs improved by geometric confinement.

Effects of geometrical confinement on the generation of droplets at microfluidics T-junctions with rectangle channels

Effects of geometrical confinement on the generation of droplets at microfluidics T-junctions with rectangle channels

Tuning the Diamagnetic Susceptibility and Electron Average Position in a GaAs Hemi-Spherical Quantum Dot with an Off-Center Donor Atom

The Langevin theory of diamagnetism is used to examine the effects of geometric confinement and hydrogenic impurity location on the diamagnetic susceptibility in a GaAs hemispherical quantum dot with an infinite confinement potential considered as vacuum. Using the finite difference approach and the effective mass approximation, the electron-donor Schrödinger equations are derived. As a function of the size of the hemi-spherical quantum dot, the mean value of the electron location and electron to ionized donor atom distance are investigated, taking into account the various impurity positions. The results show that shrinking the size of the hemi-spherical quantum dot improves the diamagnetic susceptibility by reducing the electron-donor distance. The major findings show that the donor impurity location has a significant impact on the diamagnetic susceptibility. We believe that the findings from our work into the diamagnetic susceptibility of quantum dots will be crucial in determining how well optoelectronic devices will operate.

Optical properties of donor impurity in Yukawa like potential: application to SiGe/Si and Si/SiGe

We have investigated the effect of temperature and geometrical confinement on the behavior of spherical cavities with Yukawa potential presence inside the cavity within effective mass approximation. Using the Finite-element method, we have calculated the energy eigenvalue with the geometric and temperature effects into consideration with consideration of temperature and position-dependent mass, and position-dependent dielectric constant. The optical transition from the ground to the excited state shows a blueshift in optical absorption coefficient, refractive index changes, and second and third harmonic generation with an increase in temperature. The increase in the geometric size of the spherical cavity causes a redshift in the optical resonance peaks.

Dimensional crossover in the nearest-neighbor statistics of random points in a quasi-low-dimensional system

In this work, we study the effects of geometric confinement on the point statistics in a quasi-low-dimensional system. Specifically, we focus on the nearest-neighbor statistics. Accordingly, we have performed comprehensive numerical simulations of binomial point process on quasi-one-dimensional rectangle strips for different values of the confinement ratio defined as the ratio of the strip width to the mean nearest-neighbor distance. We found that the nearest-neighbor distance distributions (NNDDs) conform to an extreme value Weibull distribution with the shape parameter depending on the confinement ratio, while the process intensity remains constant. This finding reveals the reduction of effective spatial degrees of freedom in a quasi-low-dimensional system under the geometric confinement. The scale dependence of the number of effective spatial degrees of freedom is found to obey the crossover ansatz. We stress that the functional form of the crossover ansatz is determined by the nature of the studied point process. Accordingly, different physical processes in the quasi-low-dimensional system obey different crossover ansatzes. The relevance of these results for quasi-low-dimensional systems is briefly highlighted.

Phase separation and aging dynamics of binary liquids in porous media

We employ the state-of-the-art molecular dynamics simulations to study the kinetics of phase separation and aging phenomena of segregating binary fluid mixtures imbibed in porous materials. Different random porous structures are considered to understand the effect of pore morphology on coarsening dynamics. We find the effect of complex geometrical confinement resulting in the dramatic slowing down in the phase separation dynamics. The domain growth follows the power law with an exponent dependent on the porous host structure. After the transient period, a crossover to a slower domain growth is observed when the domain size becomes comparable to the pore size. Due to the geometric confinement, the correlation function and structure factor modify to a non-Porod behavior and violate the superuniversality hypothesis. The role of porous host structure on the nonequilibrium aging dynamics is studied qualitatively by computing the two-time order-parameter autocorrelation function. This quantity exhibits scaling laws with respect to the ratio of the domain length at the observation time and the age of the system. We find the scaling laws hold well for such confined segregating fluid mixtures.

Strong bulk-surface interaction dominated in-plane anisotropy of electronic structure in GaTe

Recently, intriguing physical properties have been unraveled in anisotropic layered semiconductors, in which the in-plane electronic band structure anisotropy often originates from the low crystallographic symmetry and thus a thickness-independent character emerges. Here, we apply high-resolution angle-resolved photoemission spectroscopy to directly image the in-plane anisotropic energy bands in monoclinic gallium telluride (GaTe). Our first-principles calculations reveal the in-plane anisotropic energy band structure of GaTe measured experimentally is dominated by a strong bulk-surface interaction rather than geometric factors, surface effect and quantum confinement effect. Furthermore, accompanied by the thickness of GaTe increasing from mono- to few-layers, the strong interlayer coupling of GaTe induces direct-indirect-direct band gap transition and the in-plane anisotropy of hole effective mass is reversed. Our results shed light on the physical origins of in-plane anisotropy of electronic structure in GaTe, providing guidance to further improving the performance of electronic and optoelectronic devices based on the layered anisotropic semiconductors.

Crossover from two- to one-dimensional Fickian diffusion in a quasi-one-dimensional system

In this work, we study the effects of geometric confinement on random walks and diffusion processes in systems of reduced dimensionality. Extensive Monte Carlo simulations of Gaussian random walks were performed on rectangular strips of infinite length. A special emphasis is made on the crossover from two- to one-dimensional diffusion in the Fickian regime. We found that the crossover behavior is controlled by the ratio of the strip width to the standard deviation of the walker step length distribution. Specifically, the characteristic time of crossover behavior scales quadratically with this ratio. Furthermore, the time dependence of the number of effective spatial degrees of freedom of the random walker on the strip is found to obey an ansatz characterized by the universal power-law exponent. This allows us to formulate the diffusion equation with the time dependent number of effective spatial degrees of freedom in the quasi-one-dimensional system.

Insight into the liquid adsorption of tobacco specific nitrosamines on ZIF-8

Liquid adsorption of TSNA by ZIF-8 was studied in this paper for the first time, applying the deformable molecular sieve to adsorb and separate TSNA. ZIF-8 can adsorb the bulky NNK molecules whose size exceeds its pore size, whereas NaA and KA zeolites cannot though they have similar pore sizes. Moreover, ZIF-8 selectively adsorbed NNK in solution and reached adsorption equilibrium within 5 min. In contrast, NNN was hard to be adsorbed and easy to be eluted by water, which is useful for the separation of TSNA. The influence of solid-liquid (S/L) ratio and other factors on TSNA liquid adsorption was investigated in detail. When S/L was 0.2, ZIF-8 exerted the highest efficiency and the adsorption capacity per unit surface area reached 12–15 nmol. Through the comparison of adsorption of NNK, NNN and NPYR, it is found that the adsorption of TSNA by ZIF-8 depended on π-π electron interaction mainly, rather than geometric confinement effect. The liquid adsorption curve of NNK conforms to Langmuir isothermal adsorption model, and its saturated adsorption capacity Qm was estimated to be 4.34 mg g−1, but actually it could reach 3.19 mg g−1 only. In addition, ZIF-8 had a mixing effect in liquid adsorption of TSNA: the presence of NNK promoted the adsorption of NNN slightly. ZIF-8 has the characteristics of fast adsorption and easy elution of TSNA in solution, which is beneficial for removal of TSNA to protect the environment and public health.

Electrohydrodynamic Pulling Consolidated High-Efficiency 3D Printing to Architect Unusual Self-Polarized β-PVDF Arrays for Advanced Piezoelectric Sensing.

Piezoelectric pressure sensors are important for applications in robotics, artificial intelligence, communication devices, etc. The hyperboloid is theoretically predicted to be an unusual 3D structure that allows concerted piezoelectric enhancement owing to its synergistic effects of geometrical stress confinement and stress concentration, but has not been experimentally fulfilled due to a lack of efficient architecting techniques. In this work, a 3D hyperboloidal arrayed self-polarized PVDF piezoelectric energy harvester (PEH) is successfully fabricated by incorporating electrohydrodynamic (EHD) pulling technology into fused deposition modeling (FDM) 3D printing. This strategy not only simplifies the layer-by-layer constructing procedure for arrays, but simultaneously realizes a self-polarized and high β-phase (92%) PVDF PEH in a single electric-pulling step, saving posttreatment such as poling and removing excessive additives. Such a PEH delivers a significantly enhanced piezoelectric potential which is around 8 times that of a 2D flat film sensor. Moreover, this PEH featuring excellent linearity within a wide pressure regime, enables the sensing of human activities in a relatively large force range, which is otherwise difficult for traditional film sensors to differentiate. This work demonstrates a potential roadmap to advanced piezoelectric sensors exploiting unusual 3D structures enabled by the unique EHD pulling coupled 3D printing technique.

Gadolinium-Coated Mesoporous Silica Nanoparticle for Magnetic Resonance Imaging.

Magnetic resonance molecular imaging can provide anatomic, functional and molecular information. However, because of the intrinsically low sensitivity of magnetic resonance imaging (MRI), high-performance MRI contrast agents are required to generate powerful image information for image diagnosis. Herein, we describe a novel T 1 contrast agent with magnetic-imaging properties facilitated by the gadolinium oxide (Gd2O3) doping of mesoporous silica nanoparticles (MSN). The size, morphology, composition, MRI relaxivity (r 1 ), surface area and pore size of these nanoparticles were evaluated following their conjugation with Gd2O3 to produce Gd2O3@MSN. This unique structure led to a significant enhancement in T 1 contrast with longitudinal relaxivity (r 1 ) as high as 51.85 ± 1.38 mM−1s−1. Gd2O3@MSN has a larger T 1 relaxivity than commercial gadolinium diethylene triamine pentaacetate (Gd-DTPA), likely due to the geometrical confinement effect of silica nanoparticles. These results suggest that we could successfully prepare a novel high-performance T 1 contrast agent, which may be a potential candidate for in-vivo MRI.

Guided Self-Assembly of ES-Derived Lung Progenitors into Biomimetic Tube Structures That Impact Cell Differentiation

Chemically directed differentiation of pluripotent stem cells (PSCs) into defined cell types is a potent strategy for creating regenerative tissue models and cell therapies. In vitro observations suggest that physical cues can augment directed differentiation. We recently demonstrated that confining human PSC-derived lung progenitor cells in a tube with a diameter that mimics those observed during lung development results in the alteration of cell differentiation towards SOX2−SOX9+ lung cells. Here we set out to assess the robustness of this geometric confinement effect with respect to different culture parameters in order to explore the corresponding changes in cell morphometry and determine the feasibility of using such an approach to enhance directed differentiation protocols. Culture of progenitor cells in polydimethylsiloxane (PDMS) tubes reliably induced self-organization into tube structures and was insensitive to a variety of extracellular matrix coatings. Cellular morphology and differentiation status were found to be sensitive to the diameter of tube cells that were cultured within but not to seeding density. These data suggest that geometric cues impose constraints on cells, homogenize cellular morphology, and influence fate status.

Quantitative prediction of elongation deformation and shape relaxation of a red blood cell under tensile and shear stresses

The characteristic time of stress relaxation is a key viscoelastic property of cell membrane that controls time-dependent processes such as shape recovery. Although many experimental studies have been devoted to the measurement of characteristic relaxation time, considerable uncertainty still stands because existing methods rely on different experimental designs and analyses. Here, we present a mesoscopic computational study to investigate the elastic deformation and relaxation characteristics of an isolated red blood cell (RBC) under both tensile and shear stresses. We examine the elastic response and relaxation behavior of the RBC under static tensile stretching and dynamic shear stress. When the cell deformation index responding fluid shear stress is equivalent to the one responding external tensile stretching, we find that the characteristic relaxation time for the RBC in planar flows is longer than that for the RBC under tensile stretching. We also subject the RBC to confined tube/channel flows to probe the effect of geometric confinement on its elastic deformation and relaxation dynamics. Our simulations show that the computed characteristic relaxation time is further increased when compared to those obtained under tensile stretching or planar flows, indicating that the confinement would slow down the cell relaxation process, especially under strong confinement conditions. These findings may facilitate a better understanding of variable relaxation time observed in different experiments.

Effects of early geometric confinement on the transcriptomic profile of human cerebral organoids

BackgroundHuman cerebral organoids (hCO) are attractive systems due to their ability to model important brain regions and transcriptomics of early in vivo brain development. To date, they have been used to understand the effects of genetics and soluble factors on neurodevelopment. Interestingly, one of the main advantages of hCOs are that they provide three dimensionality that better mimics the in vivo environment; yet, despite this central feature it remains unclear how spatial and mechanical properties regulate hCO and neurodevelopment. While biophysical factors such as shape and mechanical forces are known to play crucial roles in stem cell differentiation, embryogenesis and neurodevelopment, much of this work investigated two dimensional systems or relied on correlative observations of native developing tissues in three dimensions. Using hCOs to establish links between spatial factors and neurodevelopment will require the use of new approaches and could reveal fundamental principles of brain organogenesis as well as improve hCOs as an experimental model.ResultsHere, we investigated the effects of early geometric confinements on transcriptomic changes during hCO differentiation. Using a custom and tunable agarose microwell platform we generated embryoid bodies (EB) of diverse shapes mimicking several structures from embryogenesis and neurodevelopment and then further differentiated those EBs to whole brain hCOs. Our results showed that the microwells did not have negative gross impacts on the ability of the hCOs to differentiate towards neural fates, and there were clear shape dependent effects on neural lineage specification. In particular we observed that non-spherical shapes showed signs of altered neurodevelopmental kinetics and favored the development of medial ganglionic eminence-associated brain regions and cell types over cortical regions. Transcriptomic analysis suggests these mechanotransducive effects may be mediated by integrin and Wnt signaling.ConclusionsThe findings presented here suggest a role for spatial factors in brain region specification during hCO development. Understanding these spatial patterning factors will not only improve understanding of in vivo development and differentiation, but also provide important handles with which to advance and improve control over human model systems for in vitro applications.

Dielectric Susceptibility of Water in the Interface.

It has long been anticipated that dielectric constants of polar liquids are reduced in the interfacial layer. Recent experiments and computer simulations support these expectations. A strong reduction of the dielectric constant is found in the direction perpendicular to a planar substrate, while the parallel response is bulk-like. This Perspective highlights recent theoretical calculations and simulations with an eye on relating them to properties observable in the laboratory. The average interface dielectric constant computed from simulations connects to thin films experiments, but this cannot be extended to screening of charges. In contrast to dielectric theories where a single dielectric constant gauges both the polarization energy and screening, these two signatures of dielectric polarization diverge on the molecular scale. The reduction of the dielectric constant of water in thin films is currently viewed as a combined effect of geometric confinement imposed by the substrate and the reconstruction of water hydrogen bonds in the surface layer.