Geometrical Confinement Effect Research Articles

Dimensional transition in Darcy-Rayleigh-Taylor mixing

The effect of geometrical confinement on Rayleigh-Taylor mixing in porous media is investigated by means of numerical simulations. We find a dimensional transition from three-dimensional to two-dimensional phenomenology when the density plumes become larger than the confining scale. In the two-dimensional regime we observe a faster mixing process and a larger value of the density Nusselt number.

Stabilization and topological transformation of magnetic bubbles in disks of a kagome magnet

Magnetic bubbles are cylinder domains found in a wide variety of uniaxial ferromagnets. They are categorized into two types based on the rotational sense of their domain walls. These two types of bubbles have the potential to be binary states in a skyrmion-bubble-based magnetic memory. Experimental realization of a single bubble along with single bubble transformations in nanostructures is important for single bit operations in such a memory but has not yet been demonstrated. In this study, effects of geometrical confinement on stabilization and transformation of magnetic bubbles in Fe3Sn2 disks were explored. We observed magnetic bubble cluster states with a tunable bubble number based on the disk size and tilted field angle. By tracking the size dependence of the bubble number, we further demonstrate how both types of single bubbles can be realized and transformed into each other in a controlled manner without bubble number variation.

Uniform Magnesium Electrodeposition via Synergistic Coupling of Current Homogenization, Geometric Confinement, and Chemisorption Effect.

Unevenly distributed magnesium (Mg) electrodeposits have emerged as a major obstacle for Mg-metal batteries. A comprehensive design matrix is reported for 3D magnesiophilic hosts, which regulate the uniform Mg electrodeposition through a synergistic coupling of homogenizing current distribution, geometric confinement, and chemisorptive interaction. Vertically aligned nitrogen- and oxygen-doped carbon nanofiber arrays on carbon cloth (denoted as "VNCA@C") are developed as a proof of concept. The evenly arranged short nanoarray architecture helps to homogenize the surface current density and the microchannels built in this 3D host allow the preferential nucleation of Mg due to their geometrical confinement effect. Besides, the nitrogen-/oxygen-doped carbon species exhibit strong chemisorptive interaction toward Mg atoms, providing preferential nucleation sites as demonstrated by first-principle calculation results. Electrochemical analysis reveals a peculiar yet highly reversible microchannel-filling growthbehavior of Mg metals, which empowers the delicately designed VNCA@C host with the ability to deliver a reduced nucleation overpotential of 429mV at 10.0mA cm-2 and an elongated Mg plating/stripping cycle life (110 cycles) under high current density of 10.0 mA cm-2 . The proposed design matrix can be extended to other metal anodes (such as lithium and zinc) for high-energy-density batteries.

Diffusiophoresis of a highly charged soft particle normal to a conducting plane.

Diffusiophoresis of a soft particle in electrolyte solutions normal to a conducting solid plane is investigated theoretically in this study, focusing on the highly charged particle in particular. A pseudo-spectral method based on Chebyshev polynomial is adopted to solve the resultant governing electrokinetic equations. It was found, among other things, that the closer the soft particle is to the plane, the faster it moves in general, provided only the chemiphoresis component of the diffusiophoresis is involved, i.e., no diffusion potential is present. The presence of the conducting plane is found to have three effects upon the particle motion nearby: the geometric boundary confinement effect, the electrostatic mirror-image force analog effect, and the hydrodynamic retarding effect. The enhancement of the double layer polarization by the first two effects leads to the seeming intriguing observation mentioned above. The particle always moves away from the plane in chemiphoresis. If a diffusion potential is present, however, then it is possible to drive the particle toward the plane. The results have potential applications in drug delivery.

Morphodynamic signatures of MDA-MB-231 single cells and cell doublets undergoing invasion in confined microenvironments

Cancer cell metastasis is a major factor in cancer-related mortality. During the process of metastasis, cancer cells exhibit migratory phenotypes and invade through pores in the dense extracellular matrix. However, the characterization of morphological and subcellular features of cells in similar migratory phenotypes and the effects of geometric confinement on cell morphodynamics are not well understood. Here, we investigate the phenotypes of highly aggressive MDA-MB-231 cells in single cell and cell doublet (an initial and simplified collective state) forms in confined microenvironments. We group phenotypically similar single cells and cell doublets and characterize related morphological and subcellular features. We further detect two distinct migratory phenotypes, fluctuating and non-fluctuating, within the fast migrating single cell group. In addition, we demonstrate an increase in the number of protrusions formed at the leading edge of cells after invasion through geometric confinement. Finally, we track the short and long term effects of varied degrees of confinement on protrusion formation. Overall, our findings elucidate the underlying morphological and subcellular features associated with different single cell and cell doublet phenotypes and the impact of invasion through confined geometry on cell behavior.

Commensurability between Element Symmetry and the Number of Skyrmions Governing Skyrmion Diffusion in Confined Geometries

AbstractMagnetic skyrmions are topological magnetic structures, which exhibit quasi‐particle properties and can show enhanced stability against perturbation from thermal noise. Recently, thermal Brownian diffusion of these quasi‐particles has been found in continuous films, and unconventional computing applications have received significant attention, requiring structured elements. Thus, as the next necessary step, skyrmion diffusion in confined geometries is studied, and it is found to be qualitatively different: The diffusion is governed by the interplay between the total number of skyrmions and the structure geometry. In particular, the effect of circular and triangular geometrical confinement is ascertained. It is found that for triangular geometries, the behavior is drastically different for the cases when the number of skyrmions in the element is either commensurate or incommensurate with a symmetric filling of the element. This influence of commensurability is corroborated by simulations of a quasi‐particle model.

Effects of Geometric Confinement on Caging and Dynamics of Polymer-Tethered Nanoparticle Suspensions

Well-dispersed polymer-tethered silica nanoparticles exhibit soft glassy rheology and caging behaviors due to chain interpenetration driven by entropic attraction and geometric confinement. In this...

Effects of Geometric Confinement on Zero-Gravity Droplets between Two Parallel Planes.

Investigation of geometric effects on confined droplets is important for the fundamental understanding of the ubiquitous wetting phenomena in nature as well as for a variety of practical applications. In this study, the effect of geometric confinement on the wetting behavior of a droplet confined between two parallel rigid planes was investigated. The closed solutions of the Young-Laplace equation were derived through an analytical method. These solutions are applicable for arbitrary values of the contact angles and the ratio of the size of the droplet to the separation of the planes. For completely nonwetting and wetting cases, an asymptotic method was employed, and the sophisticated analytical solutions of the Laplace pressure, droplet volume, surface energy, and capillary force were expressed as functions of the size and the separation of the droplet in a simple way. In order to check the applicability of the results, experiments were designed to counteract the gravity of the droplets. The asymptotic results not only quantitatively agree well with the theoretical and experimental results over a large range of the parameter space, but also provide a straightforward view for reflecting the effect of the geometric confinement on the wetting behaviors of droplets.

Magnetic Field Dependence of Excitonic and Optical Properties of InP/ZnS Core/Shell Nanostructure

The effect of a magnetic field on the excitonic and optical properties of an InP/ZnS core/shell nanodot (type I) including the influence of the geometrical confinement effect has been investigated. The exciton binding energy, oscillator strength, linear and nonlinear electronic and optical absorption coefficients, electronic dipole moment, and optical gain are studied for different ratios of the core/shell dot radii with and without the application of a magnetic field. The variational formalism is employed to obtain the electronic properties, whereas a compact density matrix method is applied to determine the optical properties. It is observed that the exciton binding energy is enhanced as the core/shell ratio tends to unity, attempting to reach its well-known two-dimensional (2D) limit. The application of a magnetic field shifts the peak to higher energies. The combined effects of a magnetic field and spatial confinement lead to control over the electronic as well as optical properties. These results will be useful for enhancing the performance of such structures for any desired potential application in optical devices.

Role of various dielectric environment matrices of InP/ZnS core/shell quantum dot on optical gain coefficient

Theoretical studies on excitonic and optical properties of a single exciton confined in an InP/ZnS heterostructure core/shell quantum dot embedded in various dielectric environments are investigated. The energies are found with and without the inclusion of dielectric mismatch employing single band effective mass approximation using variational formulism. The solution of Poisson-Schrodinger wave equations for the attractive term between the electron and hole is carried out using a self consistent approach in the Hartree approximation. The binding energy due to an exciton and oscillator strength is found with the effect of geometrical confinement. The total absorption coefficients, the injection current density for the optical output and the corresponding threshold optical pump intensity studied in the presence of various dielectric environments are investigated in the InP/ZnS heterostructure core/shell quantum dot. The ratio of core/shell hetero-structured quantum dot radius for various values of dielectric matrices immersed is found. The results show that the obtained properties are considerably enhanced with the incorporation of dielectric environment matrices immersed in the core/shell quantum dot particularly the highest dielectric constant will bring out the better results. It is hoped that to the present study will contribute the understanding of excitonic and optical properties in the group II–VI core/shell heterostructure quantum dots for the potential applications in photovoltaic and light emitting diodes.

Geometrical confinement effects on fundamental thermal properties of rutile and anatase TiO2 cylindrical and tubular nanostructures

In this work, we investigated the influence of the geometrical confinement effects on the fundamental thermal properties of rutile and anatase TiO2 for both cylindrical nanostructures (CNSs) and nanotubular structures (NTSs), respectively. Calculations of energy levels are developed in the framework of effective mass approximation by generalizing the resolution of Schrödinger equation in a truncated cylinder. The energy spectrum is then used in the determination of thermodynamic properties by using the Boltzmann-Gibbs distribution. Numerical computations done for both rutile and anatase TiO2 nanomaterials reveal a strong localization of the electron orbitals along to the lateral surface for all the studied are CNS and NTS. The average energy, heat capacity, entropy, and Helmholtz free energy calculated for different thicknesses for NTS and different cross-sections of CNS. Our numerical investigation shows that all thermodynamic properties depend on the temperature, the cross-section for the CNS, and the shell thickness for the NTS. We demonstrated that for low thickness, the heat capacity shows a Schottky-like anomaly at low temperatures. We also show that the Rutile structure is more stable than anatase. We hope that the thermodynamic properties concluded from this study can be considered as useful information for understanding the thermodynamic properties of TiO2 nanofibers.

Three-Dimensional MOF-Assisted Self-Polarized Ferroelectret: An Effective Autopowered Remote Healthcare Monitoring Approach.

In recent years, flexible and sensitive pressure sensors are of extensive interest in healthcare monitoring, artificial intelligence, and national security. In this context, we report the synthetic procedure of a three-dimensional (3D) metal-organic framework (MOF) comprising cadmium (Cd) metals as nodes and isoniazid (INH) moieties as organic linkers (CdI2-INH═CMe2) for designing self-polarized ferroelectret-based highly mechano-sensitive skin sensors. The as-synthesized MOF preferentially nucleates the stable piezoelectric β-phase in poly(vinylidene fluoride) (PVDF) and also gives rise to a porous ferroelectret composite film. Benefiting from the porous structure of 3D MOFs, composite ferroelectret film-based ultrasensitive pressure sensor (mechano-sensitivity of 8.52 V/kPa within 1 kPa pressure range) as well as high-throughput ( power density of 32 μW/cm2) mechanical energy harvester (MEH) has been designed. Simulation-based finite element method (FEM) analysis indicates that the geometrical stress confinement effect within the interpore region of the ferroelectret structure synergistically influences the mechano-electrical property of the MEH. In addition, 143 pC/N (∼4.5 times higher than commercial piezoelectric PVDF films) piezoelectric charge coefficient (d33) magnitude and superior response time (tr ∼ 8 ms) of this composite ferroelectret film enable the detection of different physiological signals such as coughing, pronunciation, and gulping behavior, making it a promising candidate for early intervention of healthcare, which may play a significant role in accurate alert of influenza and chronic obstructive pulmonary disease (COPD)-related symptoms. In addition, MEH enables the tracking of the subtle pressure change in the wrist pulse, indicating its usefulness in effective mechano-sensitivity. Since the cardiovascular signal is one of the vital parameters that can determine the on-going physiological conditions, the wireless transmission of the detected wrist pulse signal has been demonstrated. All of these features coupled with wireless data transmission indicate the promising application of MOF-assisted composite ferroelectret films in noninvasive real-time remote healthcare monitoring.

Interband optical Raman gain in a strained oxide quantum dot with Hylleraas co-ordinates

Exciton Raman scattering of a three-level system is observed in a CdO/ZnO quantum dot using the size dependent Smorodinsky–Winternitz potential. Resonance Raman excitation for the fundamental optical transition is concentrated in the present work. The differential cross section of Raman optical intensity associated with the exciton is computed with the incident photon energy and the geometrical confinement effect. Raman differential cross section is found by the third-order harmonic generation. The built-in internal fields are incorporated in the heterostructure. They arise between the interior and outer core/shell materials in the heterostructure which contains the spontaneous polarization and piezoelectric polarization. The Hamiltonian of the exciton is included with the dielectric mismatch effect and the variational approach with Hylleraas co-ordinates are employed to obtain the energy eigen values numerically. The energy dependent effective mass of electron and the size related strain-induced potentials for the conduction and valence bands are incorporated in the Hamiltonian. The nonlinear optical properties are obtained with the density matrix approach. The results show that the Raman intensities are more influenced in the strong confinement region. The resonance peak Raman intensity is observed as 6.84 (arb.units) for a 60 A spherical dot. The Raman gain is optimized by varying the dot size. And, the Raman optical gain as high as 240/cm is achieved for the dot radius, 22 A of CdO/ZnO spherical quantum dot. This can be employed for the potential applications in optical devices.

Field-free spin\u2013orbit torque perpendicular magnetization switching in ultrathin nanostructures

Magnetic-field-free current-controlled switching of perpendicular magnetization via spin–orbit torque (SOT) is necessary for developing a fast, long data retention, and high-density SOT magnetoresistive random access memory (MRAM). Here, we use both micromagnetic simulations and atomistic spin dynamics (ASD) simulations to demonstrate an approach to field-free SOT perpendicular magnetization switching without requiring any changes in the architecture of a standard SOT-MRAM cell. We show that this field-free switching is enabled by a synergistic effect of lateral geometrical confinement, interfacial Dyzaloshinskii–Moriya interaction (DMI), and current-induced SOT. Both micromagnetic and atomistic understanding of the nucleation and growth kinetics of the reversed domain are established. Notably, atomically resolved spin dynamics at the early stage of nucleation is revealed using ASD simulations. A machine learning model is trained based on ~1000 groups of benchmarked micromagnetic simulation data. This machine learning model can be used to rapidly and accurately identify the nanomagnet size, interfacial DMI strength, and the magnitude of current density required for the field-free switching.

Effect of geometrical confinement on the flow of soft microgel particle pastes

In this paper, we determine the effect of confinement on the shear flow of concentrated soft microgel particle suspensions. Utilizing a flexure-based microgap rheometer (FMR), aqueous suspensions of swollen P(NIPAM-co-AA) microgel particles of 4μ m diameter at different volume fractions larger than 64 vol% are subjected to shearing deformations between plates separated by gaps of 5–120μ m while monitoring the stress response at different rates. Describing the stress evolution from a balance of elastohydrodynamic lubrication and elastic forces following the approach of Cloitre and Bonnecaze, it could be shown that already below a critical confinement level on the order of 10 particle diameters an increase of the interparticle pressure leads to a rising shear stress at a given rate in the yielded regime. This effect is strongest for particle volume fractions close to the critical closest packing, with two orders of magnitude increase in flow resistance when approaching the single-particle confinement level, whereas this confinement effect on the flow decreases when raising the concentration. Furthermore, it could be shown that, at the onset of the yielding of the suspensions, the confinement is increasing the effective modulus, but not the yield strain.

Numerical modelling of engineered cementitious composites-concrete encased steel composite columns

Recently, a new section form of concrete encased steel composite columns with Engineered Cementitious Composites (ECC) confinement (ECC-CES), which outperforms conventional CES columns in terms of failure behaviour, ductility and toughness has been introduced. Numerical modelling of the post-peak behaviours of such encased composite columns is a challenging task due to brittle failure of concrete, interface bond failure and buckling of steel section. This study presents a finite element modelling procedure for the ECC-CES columns. Both material and geometric nonlinearities are incorporated in the model and inelastic behaviour of ECC, concrete and steel were described by using appropriate material constitutive laws. Moreover, effects of initial geometric imperfections, contact interactions and confinement by steel and ECC encasement on concrete core were considered. Concrete damaged plasticity model was employed to describe the failure of ECC and concrete. The FE model was validated against experimental results and found to give good predictions of compressive behaviour of ECC-CES columns compared with the test results. Finally, a small scale parametric study was conducted using the validated numerical model to study the effect of the materials' strength on the column compressive behaviour.

Spontaneous helical flows in active nematics lying on a cylindrical surface.

Within the framework of the two-dimensional Ericksen-Leslie model, we explore the effect of geometric confinement on the spontaneous flow of active nematic gels. The nematic particles are assumed to flow on a cylindrical surface, while a degenerate tangential anchoring is enforced. Using the linear approximation of the motion equations, we show that there is a close interplay among extrinsic curvature, flow, director alignment, and activity. We find that the extrinsic curvature promotes the director alignment parallel to the cylindrical axis and is responsible for raising the critical threshold with respect to the flat case. Our analysis reveals a very rich scenario where the key quantities are the activity coefficient, the tumbling parameter, and the anisotropic viscosity ratio. Thus, solutions can exhibit a double periodicity in both the azimuthal and axial variables. As a consequence, the velocity field can make a finite angle with the cylinder axis and the active flow winds on the surface with a helical pattern, while the director oscillates around the cylinder generators. Our results can be validated on thin layers of nematic gels placed between two concentric cylinders and suggest which material properties are most suited for the design of active microfluidic devices.

Indentation behavior of highly confined elasto-plastic materials

The effect of geometric confinement is well-known from hardness measurements of thin films on stiff substrates and has been modeled both phenomenologically and using e.g. Finite Element Analysis. However, these models are mainly focused on a specific experiment or a certain material family. In the present work, Finite Element Analysis is used to gain a better understanding of the interplay between geometric constraints in various microstructures and a wide range of materials properties. It is shown that a very simple model can be used to replicate thin film hardness data where the film is softer than the substrate as well as how materials properties alter the indentation behavior of materials confined in one to three dimensions. It is shown that qualitative agreement with nanoindentation of the metallic binder phase in the complex 3D-microstructure of a cemented carbide is achieved using an axisymmetric “pill-box” model with classical plasticity. It is also shown that the effect of higher-order confinement can be described by the Korsunsky thin film hardness model by re-optimizing the fitting parameters.

Differential Adsorption of l‐ and d‐Lysine on Achiral MFI Zeolites as Determined by Synchrotron X‐Ray Powder Diffraction and Thermogravimetric Analysis

AbstractReported here is the first crystallographic observation of stereospecific bindings of l‐ and d‐lysine (Lys) in achiral MFI zeolites. The MFI structure offers inherent geometric and internal confinement effects for the enantiomeric difference in l‐ and d‐Lys adsorption. Notable differences have been observed by circular dichroism (CD) spectroscopy and thermogravimetric analysis (TGA). Distinct l‐ and d‐Lys adsorption behaviours on the H‐ZSM‐5 framework have been revealed by the Rietveld refinement of high‐resolution synchrotron X‐ray powder diffraction (SXRD) data and the density‐functional theory (DFT) calculations. Despite demonstrating the approach for l‐ and d‐Lys over MFI zeolites at an atomistic resolution, the differential adsorption study sheds light on the rational engineering of molecular interaction(s) with achiral microporous materials for chiral separation purposes.

Differential Adsorption of l- and d-Lysine on Achiral MFI Zeolites as Determined by Synchrotron X-Ray Powder Diffraction and Thermogravimetric Analysis.

Reported here is the first crystallographic observation of stereospecific bindings of l- and d-lysine (Lys) in achiral MFI zeolites. The MFI structure offers inherent geometric and internal confinement effects for the enantiomeric difference in l- and d-Lys adsorption. Notable differences have been observed by circular dichroism (CD) spectroscopy and thermogravimetric analysis (TGA). Distinct l- and d-Lys adsorption behaviours on the H-ZSM-5 framework have been revealed by the Rietveld refinement of high-resolution synchrotron X-ray powder diffraction (SXRD) data and the density-functional theory (DFT) calculations. Despite demonstrating the approach for l- and d-Lys over MFI zeolites at an atomistic resolution, the differential adsorption study sheds light on the rational engineering of molecular interaction(s) with achiral microporous materials for chiral separation purposes.