Mobility Gradient Research Articles

Stress–diffusion coupling. Application to interstitial diffusion

A numerical study of the stress–diffusion coupling theory based on the Cahn–Larché method is presented in the case of interstitial diffusion. Indeed, the interaction between diffusion and stress fields has been widely studied in the past. Most of the studies focused on the effect of chemical stress due to mass transport on diffusion. The majority of the studies have been focusing on the coupling between external mechanical and chemical stresses with diffusion. In this study, the effect of stress/strain on mobility and stress gradient mobility and driving forces are presented, based on the model of Voorhees and Larché. The analytical solutions are detailed, from general equations (interstitial diffusion, elastic material) to a macroscopic model (interstitial diffusion, isotropic elastic material, infinite dilute solution, 2D axisymmetric). Numerical simulations for different samples thickness and β parameter helped to better understand and then experimentally identify the prevailing effect of stress on mobility or diffusion driving forces. These studies demonstrated the impact of constant compressive and tensile stress on diffusion kinetics through the mobility term of the equation, as well as the effects of negative and positive stress gradients via the Nernstian term. The results show that constant compressive stress and negative stress gradient hinder the diffusion. On the contrary, tensile stress and positive stress gradient promote the diffusion. This work also demonstrates the importance of coupling between mobility and Nernstian effect on the diffusion promotion.

First Successful Wireline Stress Testing in a Gas Hydrate Reservoir in the Hyuganada Sea, Japan

This study presents a stress testing operation conducted using a wireline formation tester in a newly discovered gas hydrate prospect located offshore in Japan. The campaign, which spanned from December 2021 to January 2022, involved drilling a well using logging-while-drilling technology. Subsequently, wireline formation testing and stress testing were successfully conducted at three different depths within a gas hydrate-concentrated zone. The testing was accomplished in a single riserless descent, with the primary goal of obtaining crucial data such as mobility, formation pressure, and fracture gradient for one of the prospects. This operation marked the first stress testing job performed with dual packers in an open water and deepwater environment specifically for gas hydrate reservoirs. The study also provides a comprehensive interpretation of the data gathered during the operation. Moreover, it evaluates various properties such as formation mobility, formation pressure, initial breakdown pressure, closure pressure, fracture propagation pressure, and instantaneous shut-in pressure.

Awakenings in triplicate: in ontogeny, after immobility, and to the presence of a congener. Comments on Ilan Golani's concept of the mobility gradient.

Golani's concept of the "mobility gradient" describes the unfolding of motor behavior after immobility and in ontogeny. The two processes run parallel and are similar across vertebrates. In both time scales, the trend is for the behavior to progressively become enriched, cover more dimensions (motor expansion) and become less dependent on external stimuli (stimulus-bound). This paper addresses the question whether the gradient extends into social, interactive behavior. Observation of natural groups larger than dyads may help answering that question. As an example, the natural social behavior of a fish, Pethia nigrofasciata is described in some detail. It is concluded that their motor behavior expands in the course of their daily spawning period.

Surface mobility gradient and emergent facilitation in glassy films.

Confining glassy polymers into films can substantially modify their local and film-averaged properties. We present a lattice model of film geometry with void-mediated facilitation behaviors but free from any elasticity effect. We analyze the spatially varying viscosity to delineate the transport properties of glassy films. The film mobility measurements reported by Yang et al., Science, 2010, 328, 1676 are successfully reproduced. The flow exhibits a crossover from a simple viscous flow to a surface-dominated regime as the temperature decreases. The propagation of a highly mobile front induced by the free surface is visualized in real space. Our approach provides a microscopic treatment of the observed glassy phenomena.

Local dynamics and failure of inhomogeneous polymer networks.

Inhomogeneous crosslinked polymers are powerful platforms for materials design, because they can be synthesized from materials that provide complimentary properties to the resulting gel. For example, a membrane with both glassy and rubbery domains will be mechanically robust while enabling transport. The dynamics, and mechanical and failure properties of rubbery/glassy conetworks are only beginning to be studied, and there is likely to be strong heterogeneities in the dynamics and mechanical response. In this study, we use coarse-grained molecular dynamics simulations to generate microphase separated rubbery/glassy polymer networks with a bicontinuous morphology via in silico crosslinking. We study the effect of phase boundary on the local mobility gradient, and our simulation results reveal an asymmetric shift in the local mobility gradient across the interface that extends deeper into the phase with a lower Tg when the system temperature is between the glass transition temperatures of the two phases. Moreover, by employing a model that allows bond breaking, we examine the microscopic mechanism for failure in these networks as a function of the molecular weight of polymer strands between crosslinks and the number fraction of the glassy domain. Under uniaxial extension, we find that the stress is initially larger in the glassy domain. As the deformation proceeds, the segmental dynamics of the two phases homogenize, and subsequently bond breaking begins.

Peptide-Induced Fusion of Dynamic Membrane Nanodomains: Implications in a Viral Entry.

Enveloped viruses infect host cells via protein-mediated membrane fusion. However, insights into the microscopic rearrangement induced by the viral proteins and peptides have not yet emerged. Here, we report a new methodology to extract viral fusion peptide (FP)-mediated biomembrane dynamical nanodomain fusion parameter, λ, based on stimulated emission depletion microscopy coupled with fluorescence correlation spectroscopy. We also define another dynamical parameter membrane gradient, defined in terms of the ratio of average lipid diffusion coefficients across dynamic crossover length scales, ξ. Significantly, we observe that λ as well as these mobility gradients are larger in the stiffer liquid-ordered (Lo) phase compared to the liquid-disordered phase and are more effective at the smaller nanodomain interfaces, which are only present in the Lo phase. The results could possibly help to resolve a long-standing puzzle about the enhanced fusogenicity of FP in the Lo phase. Results obtained from the diffusion results have been correlated with the human immunodeficiency virus gp41 FP-induced membrane fusion.

Anomalous Hall effect in naturally oxidized normal-metal Al/Cu double films

An unexpectedly larger current-induced spin–orbit torque in oxidized Cu (CuO x )/ferromagnet (FM) than heavy-metal/FM has recently attracted intense attention in spintronic studies. Although the two mechanisms, interfacial Rashba Edelstein effect and spin-vorticity coupling, have been put forward based on the two different conductive features of CuO x , i.e. electrical insulator and gradient of electrical mobility, the detailed investigation of transport of CuO x is still lacking. Here we experimentally report the positive and negative anomalous Hall effect (AHE) in naturally oxidized normal-metal Al/Cu double films. We found that the onset temperature of AHE corresponds to magnetic transition temperature of CuO x . Furthermore, by comparing Hall resistance of the crystalline and amorphous Cu/Al double films, we identify that the positive anomalous Hall resistance attributes to magnetic moment of CuO x itself, while the negative anomalous Hall resistance can originate from the spin or orbital currents generated at the CuO x /AlO x interface interact with magnetization of CuO x and its inverse process.

Molecular Dynamics Simulation of the Influence of Temperature and Salt on the Dynamic Hydration Layer in a Model Polyzwitterionic Polymer PAEDAPS.

We investigate the hydration of poly(3-[2-(acrylamido) ethyldimethylammonio] propanesulfonate) over a range of temperatures in pure water and with the inclusion of 0.1 mol/L NaCl using atomistic molecular dynamics simulation. Drawing on concepts drawn from the field of glass-forming liquids, we use the Debye-Waller parameter (<u2>) for describing the water mobility gradient around the polybetaine backbone extending to an overall distance ≈18 Å. The water mobility in this layer is defined through the mean-square water molecule displacement at a time on the order of water's β-relaxation time. The brushlike topology of polybetaines leads to two regions in the dynamic hydration layer. The inner region of ≈10.5 Å is explored by pendant group conformational motions, and the outer region of ≈7.5 Å represents an extended layer of reduced water mobility relative to bulk water. The dynamic hydration layer extends far beyond the static hydration layer, adjacent to the polymer.

The diversity of pool-riffle morphologies

Pool-riffle sediment and flow dynamics have been studied for many years, but relatively little work has investigated how variations in flow and sediment regimes might lead to a diversity of pool-riffle morphologies. In this letter we analyze a database of quantitative and qualitative measurements from sites where pool-riffle morphologies have been studied and assess the sites relative to morphologic transitions and mobility thresholds. Results show that pool-riffle sites are stretched over a morphologic gradient of predicted bar types from scrolls to chutes and a modelled mobility gradient from threshold to low transport channels. Flow- and sediment-driven phenomenon that are relevant for pool-riffle mechanics are discussed in this context.

Metastable Polymer Adsorption Dictates the Dynamical Gradients at Interfaces

Polymer adsorption proceeds through a wide variety of metastable states due to kinetic constraints originating from the pinning of segments on solid interfaces, creating an interfacial adsorption layer composed of flattened trains, loosely adsorbed loops, and tails of chains. In this work, we show that the metastable structures of adsorption layers developed via thermal annealing at a temperature above glass-transition temperature Tg determine the length scale of interfacial mobility gradient dynamics (ξdyn). Specifically, we find that 1) the evolution of ξdyn mimics the trend in the slow kinetics of polymer absorption and 2) ξdyn grows linearly with the thickness of the overall interfacial adsorption layer. The findings reinforce the idea that the interfacial dynamical gradients are relevant to polymer adsorption and also imply that extension of the loosely adsorbed chains into the film interior could facilitate the propagation of suppressed dynamics originating at the interface.

Confined Brownian suspensions: Equilibrium diffusion, thermodynamics, and rheology

We examine the impact of confinement on the structure, dynamics, and rheology of spherically confined macromolecular suspensions, with a focus on the role played by entropic forces, by comparing the limits of strong hydrodynamics and no hydrodynamics. We present novel measurements of the osmotic pressure, intrinsic viscosity, and long-time self-diffusivity in spherical confinement and find confinement induces strong structural correlations and restrictions on configurational entropy that drive up osmotic pressure and viscosity and drive down self-diffusion. Even in the absence of hydrodynamics, confinement produces distinct short-time and long-time self-diffusion regimes. This finding revises the previous understanding that short-time self-diffusion is a purely hydrodynamic quantity. The entropic short-time self-diffusion is proportional to an entropic mobility, a direct analog to the hydrodynamic mobility. A caging plateau following the short-time regime is stronger and more durable without hydrodynamics, and entropic drift—a gradient in volume fraction—drives particles out of their cages. The distinct long-time regime emerges when an entropic mobility gradient arising from heterogeneous distribution of particle volume drives particles out of local cages. We conclude that entropic mobility gradients produce a distinct long-time dynamical regime in confinement and that hydrodynamic interactions weaken this effect. From a statistical physics perspective, confinement restricts configurational entropy, driving up confined osmotic pressure, viscosity, and (inverse) long-time dynamics as confinement tightens. We support this claim by rescaling the volume fraction as the distance from confinement-dependent maximum packing, which collapses the data for each rheological measure onto a single curve.

FGOR: Flow-Guided Opportunistic Routing for Intrabody Nanonetworks

The advancement of nano communication has opened the door for the development of intrabody medical application services. Flow-guided nano-communication networks have gained major attraction in recent years as an effective solution for intrabody sensing and actuation. This article builds a three-layer vertical network structure for intrabody nanonetworks, i.e., nano nodes, nano routers, and gateway, where data packets generated by nano nodes are relayed to the gateway through nano routers or other nodes. However, how to guarantee the data transmission through the way of multiple hops in such a scenario is an unsolved challenge. In order to improve the throughput and reduce the energy consumption of intrabody nanonetworks in a single-flow environment where the nano devices are restricted, a flow-guided opportunistic routing (FGOR) protocol is proposed. In FGOR, a relative position (RP) model is proposed to formulate the criterion for candidate relay selection (CRS) and enable the nodes’ direction awareness to the gateway. Moreover, the CRS criterion is redesigned through a mobility gradient (MG) model further derived from the RP model. The candidate nodes are prioritized based on node ID, available energy, and RP information of nodes to perform backoff forwarding for decreasing transmission redundancy. Simulation results show that the RP model improves the throughput and significantly extends the lifecycle of intrabody nanonetwork by reducing the energy consumption. Compared with the RP model, the MG model performs better in terms of delay and successful transmission rate, especially within the circulation environment of intrabody.

Explicit-solute implicit-solvent molecular simulation with binary level-set, adaptive-mobility, and GPU

Coarse-grained modeling and efficient computer simulations are critical to the study of complex molecular processes with many degrees of freedom and multiple spatiotemporal scales. Variational implicit-solvent model (VISM) for biomolecular solvation is such a modeling framework, and its initial success has been demonstrated consistently. In VISM, an effective free-energy functional of solute-solvent interfaces is minimized, and the surface energy is a key component of the free energy. In this work, we extend VISM to include the solute mechanical interactions, and develop fast algorithms and GPU implementation for the extended variational explicit-solute implicit-solvent (VESIS) molecular simulations to determine the underlying molecular equilibrium conformations. We employ a fast binary level-set method for minimizing the solvation free energy of solute-solvent interfaces and construct an adaptive–mobility gradient descent method for solute atomic optimization. We also implement our methods on the integrated GPU. Numerical tests and applications to several molecular systems verify the accuracy, stability, and efficiency of our methods and algorithms. It is found that our new methods and GPU implementation improve the efficiency of the molecular simulation significantly over the CPU implementation. Our fast computational techniques may enable us to simulate very large systems such as protein-protein interactions and membrane dynamics for which explicit-solvent all-atom molecular dynamics simulations can be very expensive.

The role of intramolecular relaxations on the structure and stability of vapor-deposited glasses.

Stable glasses (SGs) are formed through surface-mediated equilibration (SME) during physical vapor deposition (PVD). Unlike intermolecular interactions, the role of intramolecular degrees of freedom in this process remains unexplored. Here, using experiments and coarse-grained molecular dynamics simulations, we demonstrate that varying dihedral rotation barriers of even a single bond, in otherwise isomeric molecules, can strongly influence the structure and stability of PVD glasses. These effects arise from variations in the degree of surface mobility, mobility gradients, and mobility anisotropy, at a given deposition temperature (Tdep). At high Tdep, flexible molecules have access to more configurations, which enhances the rate of SME, forming isotropic SGs. At low Tdep, stability is achieved by out of equilibrium aging of the surface layer. Here, the poor packing of rigid molecules enhances the rate of surface-mediated aging, producing stable glasses with layered structures in a broad range of Tdep. In contrast, the dynamics of flexible molecules couple more efficiently to the glass layers underneath, resulting in reduced mobility and weaker mobility gradients, producing unstable glasses. Independent of stability, the flattened shape of flexible molecules can also promote in-plane orientational order at low Tdep. These results indicate that small changes in intramolecular relaxation barriers can be used as an approach to independently tune the structure and mobility profiles of the surface layer and, thus, the stability and structure of PVD glasses.

Social Gradient in Physical Mobility: An Investigation Among the OlderPopulation in India

In densely populated developing countries like India, the constant rise in the older population has worsened the burden of palliative care. Palliative care is primarily required for older people who are physically disabled. To address the issue of palliative care, a thorough understanding of physical immobility among the older population is required. Therefore, the present study aims at an in-depth understanding of the trajectory of mobility decline as well as the critical nexus between physical immobility and social status in late life.The study uses nationally representative unit-level secondary data from two rounds of the National Sample Survey of India conducted in 2004 and 2014 to examine the underlying association between socioeconomic class and physical mobility loss among older individuals. The findings suggest that physical immobility among older adults is closely related to the social gradient. Socioeconomic factors like socioreligious identity, income, education level, household type, living environment, and gender significantly influence the physical immobility in old age. The results of this study contribute to understanding the dynamics and variability of physical immobility among older individuals and help in developing policies to reduce the future burden of palliative care.

Surface diffusion of a glassy discotic organic semiconductor and the surface mobility gradient of molecular glasses.

Surface diffusion has been measured in the glass of an organic semiconductor, MTDATA, using the method of surface grating decay. The decay rate was measured as a function of temperature and grating wavelength, and the results indicate that the decay mechanism is viscous flow at high temperatures and surface diffusion at low temperatures. Surface diffusion in MTDATA is enhanced by 4 orders of magnitude relative to bulk diffusion when compared at the glass transition temperature Tg. The result on MTDATA has been analyzed along with the results on other molecular glasses without extensive hydrogen bonds. In total, these systems cover a wide range of molecular geometries from rod-like to quasi-spherical to discotic and their surface diffusion coefficients vary by 9 orders of magnitude. We find that the variation is well explained by the existence of a steep surface mobility gradient and the anchoring of surface molecules at different depths. Quantitative analysis of these results supports a recently proposed double-exponential form for the mobility gradient: log D(T, z) = log Dv(T) + [log D0 - log Dv(T)]exp(-z/ξ), where D(T, z) is the depth-dependent diffusion coefficient, Dv(T) is the bulk diffusion coefficient, D0 ≈ 10-8m2/s, and ξ ≈ 1.5nm. Assuming representative bulk diffusion coefficients for these fragile glass formers, the model reproduces the presently known surface diffusion rates within 0.6 decade. Our result provides a general way to predict the surface diffusion rates in molecular glasses.

Confinement Effects on the Spatially Inhomogeneous Dynamics in Metallic Glass Films.

This work develops the elastically collective nonlinear Langevin equation theory to investigate, for the first time, the glassy dynamics in capped metallic glass thin films. Finite-size effects on the spatial gradient of structural relaxation time and glass transition temperature (Tg) are calculated at different temperatures and vitrification criteria. Molecular dynamics is significantly slowed down near rough solid surfaces, and the dynamics at location far from the interfaces is sped up. In thick films, the mobility gradient normalized by the bulk value obeys the double-exponential form since interference effects between two surfaces are weak. Reducing the film thickness induces a strong dynamic coupling between two surfaces and flattens the relaxation gradient. The normalized gradient of the glass transition temperature is independent of vitrification time scale criterion and can be fitted by a superposition function as the films are not ultrathin. The local fragility is found to remain unchanged with location. This finding suggests that one can use Angell plots of bulk relaxation time and the Tg spatial gradient to characterize glassy dynamics in metallic glass films. Our computational results agree well with experimental data and simulation.

Activation free energy gradient controls interfacial mobility gradient in thin polymer films.

We examine the mobility gradient in the interfacial region of substrate-supported polymer films using molecular dynamics simulations and interpret these gradients within the string model of glass-formation. No large gradients in the extent of collective motion exist in these simulated films, and an analysis of the mobility gradient on a layer-by-layer basis indicates that the string model provides a quantitative description of the relaxation time gradient. Consequently, the string model indicates that the interfacial mobility gradient derives mainly from a gradient in the high-temperature activation enthalpy ΔH0 and entropy ΔS0 as a function of depth z, an effect that exists even in the high-temperature Arrhenius relaxation regime far above the glass transition temperature. To gain insight into the interfacial mobility gradient, we examined various material properties suggested previously to influence ΔH0 in condensed materials, including density, potential and cohesive energy density, and a local measure of stiffness or u2(z)-3/2, where u2(z) is the average mean squared particle displacement at a caging time (on the order of a ps). We find that changes in local stiffness best correlate with changes in ΔH0(z) and that ΔS0(z) also contributes significantly to the interfacial mobility gradient, so it must not be neglected.

The regional geography of social mobility in Mexico

ABSTRACT We study intergenerational social mobility in Mexico at the subnational level to determine whether poorer regions have similar rates of social mobility as richer ones. We approximate socioeconomic status with an index that captures the variability across households in durable goods, services and education of household heads for both adult children and their parents. The estimated indicators suggest a clear north–south gradient in social mobility: the children of poor parents in the northern regions experience greater upward mobility than in the central and southern regions.

Glasses denser than the supercooled liquid

When aged below the glass transition temperature, [Formula: see text], the density of a glass cannot exceed that of the metastable supercooled liquid (SCL) state, unless crystals are nucleated. The only exception is when another polyamorphic SCL state exists, with a density higher than that of the ordinary SCL. Experimentally, such polyamorphic states and their corresponding liquid-liquid phase transitions have only been observed in network-forming systems or those with polymorphic crystalline states. In otherwise simple liquids, such phase transitions have not been observed, either in aged or vapor-deposited stable glasses, even near the Kauzmann temperature. Here, we report that the density of thin vapor-deposited films of N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) can exceed their corresponding SCL density by as much as 3.5% and can even exceed the crystal density under certain deposition conditions. We identify a previously unidentified high-density supercooled liquid (HD-SCL) phase with a liquid-liquid phase transition temperature ([Formula: see text]) ∼35 K below the nominal glass transition temperature of the ordinary SCL. The HD-SCL state is observed in glasses deposited in the thickness range of 25 to 55 nm, where thin films of the ordinary SCL have exceptionally enhanced surface mobility with large mobility gradients. The enhanced mobility enables vapor-deposited thin films to overcome kinetic barriers for relaxation and access the HD-SCL state. The HD-SCL state is only thermodynamically favored in thin films and transforms rapidly to the ordinary SCL when the vapor deposition is continued to form films with thicknesses more than 60 nm.