Mobility Gradient Research Articles

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AbstractThe mobility of surface charge carriers is estimated based on an analysis of the photoelectromagnetic effect in three-dimensional (Bi_1 –_ x Sb_ x )_2Te_3 (0 ≤ x ≤ 0.55) topological insulators. A high degree of degeneracy of the carrier gas in combination with a low energy of the exciting terahertz quantum provide a nonequilibrium process associated exclusively with thermal heating of the carrier. Under these conditions, the photovoltage is determined by the mobility gradient of the surface and bulk carriers. The photovoltage and, consequently, the mobility gradient disappear completely with an increase in the bulk mobility up to 10^5 cm^2 V^–1 s^–1. Photovoltage is clearly observed in the samples with comparatively low bulk mobility under the same experimental conditions.

Loschmidt’s Paradox, Extended to CPT Symmetry, Bypasses Second Law

Loschmidt’s paradox is extended by replacing its assumption of time reversibility with full CPT symmetry. Mobility is identified as a means for expressing collisions or dissipation, and the cross product of its gradient with the magnetic field, for expressing parity. Three phenomena incorporating such cross products are identified. The first is the cross product of the mobility gradient with the magnetic field. The second combines this cross product with the E cross B drift. The third is the reciprocal of the Nernst effect expressed as a cross product of the temperature gradient and the magnetic field. Simulations are conducted for testing Loschmidt’s extended paradox. Onsager’s exclusion of magnetic fields and rotation from reciprocals violates CPT symmetry and is unjustified. All three cross-product phenomena skew statistics in a fashion unanticipated by Boltzmann’s assumptions in his H-Theorem. CPT symmetric systems fall outside the assumptions of the theorem which is not rendered invalid but simply limited to its domain of applicability. Therefore, these systems do not violate the second law as Boltzmann defines it. They bypass it.

The folding equilibrium of huntingtin exon 1 monomer depends on its polyglutamine tract

Expansion of the polyglutamine (polyQ) tract in exon 1 of the huntingtin protein (Httex1) leads to Huntington's disease resulting in fatal neurodegeneration. However, it remains poorly understood how polyQ expansions alter protein structure and cause toxicity. Using CD, EPR, and NMR spectroscopy, we found here that monomeric Httex1 consists of two co-existing structural states whose ratio is determined by polyQ tract length. We observed that short Q-lengths favor a largely random-coil state, whereas long Q-lengths increase the proportion of a predominantly α-helical state. We also note that by following a mobility gradient, Httex1 α-helical conformation is restricted to the N-terminal N17 region and to the N-terminal portion of the adjoining polyQ tract. Structuring in both regions was interdependent and likely stabilized by tertiary contacts. Although little helicity was present in N17 alone, each Gln residue in Httex1 enhanced helix stability by 0.03-0.05 kcal/mol, causing a pronounced preference for the α-helical state at pathological Q-lengths. The Q-length-dependent structuring and rigidification could be mimicked in proteins with shorter Q-lengths by a decrease in temperature, indicating that lower temperatures similarly stabilize N17 and polyQ intramolecular contacts. The more rigid α-helical state of Httex1 with an expanded polyQ tract is expected to alter interactions with cellular proteins and modulate the toxic Httex1 misfolding process. We propose that the polyQ-dependent shift in the structural equilibrium may enable future therapeutic strategies that specifically target Httex1 with toxic Q-lengths.

Direct Mapping of Nanoscale Viscoelastic Dynamics at Nanofiller/Polymer Interfaces

The mobility gradient of polymer chains near a solid interface plays an important role in explaining a complex behavior of relaxation dynamics and thermodynamic properties observed for various poly...

Effect of processing conditions on water mobility and cooking quality of gluten-free pasta. A Magnetic Resonance Imaging study

A new approach for producing gluten-free pasta from hydrated (50 °C, 20 min) rice kernels, skipping the grinding step, was explored. Magnetic Resonance Imaging (MRI) was used to study the hydration kinetics of rice, by monitoring the time evolution of both proton density and water transverse-relaxation rate during water diffusion. Results showed that the optimal water diffusion was reached after 180 min, allowing the extrusion of hydrated rice kernels into pasta. MRI analysis also highlighted in cooked pasta gradients of water distribution and mobility, in agreement with the high shear force that was measured using the Kramer cell (1066.5 vs 896.4 N). The high hydration in the external layers of pasta did not negatively affect the cooking quality (cooking loss, compression energy, firmness) of the product. MRI analysis provided experimental evidence for the optimization of early steps in the technological process of grains for the production of gluten-free pasta.

Why we need to look beyond the glass transition temperature to characterize the dynamics of thin supported polymer films

There is significant variation in the reported magnitude and even the sign of [Formula: see text] shifts in thin polymer films with nominally the same chemistry, film thickness, and supporting substrate. The implicit assumption is that methods used to estimate [Formula: see text] in bulk materials are relevant for inferring dynamic changes in thin films. To test the validity of this assumption, we perform molecular simulations of a coarse-grained polymer melt supported on an attractive substrate. As observed in many experiments, we find that [Formula: see text] based on thermodynamic criteria (temperature dependence of film height or enthalpy) decreases with decreasing film thickness, regardless of the polymer-substrate interaction strength ε. In contrast, we find that [Formula: see text] based on a dynamic criterion (relaxation of the dynamic structure factor) also decreases with decreasing thickness when ε is relatively weak, but [Formula: see text] increases when ε exceeds the polymer-polymer interaction strength. We show that these qualitatively different trends in [Formula: see text] reflect differing sensitivities to the mobility gradient across the film. Apparently, the slowly relaxing polymer segments in the substrate region make the largest contribution to the shift of [Formula: see text] in the dynamic measurement, but this part of the film contributes less to the thermodynamic estimate of [Formula: see text] Our results emphasize the limitations of using [Formula: see text] to infer changes in the dynamics of polymer thin films. However, we show that the thermodynamic and dynamic estimates of [Formula: see text] can be combined to predict local changes in [Formula: see text] near the substrate, providing a simple method to infer information about the mobility gradient.

Boundary Formation through a Direct Threshold-Based Readout of Mobile Small RNA Gradients

Small RNAs have emerged as a new class of mobile signals. Here, we investigate their mechanism of action and show that mobile small RNAs generate sharply defined domains of target gene expression through an intrinsic and direct threshold-based readout of their mobility gradients. This readout is highly sensitive to small RNA levels at the source, allowing plasticity in the positioning of a target gene expression boundary. Besides patterning their immediate targets, the readouts of opposing small RNA gradients enable specification of robust, uniformly positioned developmental boundaries. These patterning properties of small RNAs are reminiscent of those of animal morphogens. However, their mode of action and the intrinsic nature of their gradients distinguish mobile small RNAs from classical morphogens and present a unique direct mechanism through which to relay positional information. Mobile small RNAs and their targets thus emerge as highly portable, evolutionarily tractable regulatory modules through which to create pattern.

Impact of spinal rod stiffness on porcine lumbar biomechanics: Finite element model validation and parametric study.

A three-dimensional finite element model of the porcine lumbar spine (L1-L6) was used to assess the effect of spinal rod stiffness on lumbar biomechanics. The model was validated through a comparison with in vitro measurements performed on six porcine spine specimens. The validation metrics employed included intervertebral rotations and the nucleus pressure in the first instrumented intervertebral disc. The numerical results obtained suggest that rod stiffness values as low as 0.1 GPa are required to reduce the mobility gradient between the adjacent and instrumented segments and the nucleus pressures across the porcine lumbar spine significantly. Stiffness variations above this threshold value have no significant effect on spine biomechanics. For such low-stiffness rods, intervertebral rotations in the instrumented zone must be monitored closely in order to guarantee solid fusion. Looking ahead, the proposed model will serve to examine the transverse process hooks and variable stiffness rods in order to further smooth the transition between the adjacent and instrumented segments, while preserving the stability of the instrumented zone, which is needed for fusion.

Levels and patterns of internal migration in Europe: A cohort perspective

Europe displays important variations in the level of internal migration, with a clear spatial gradient of high mobility in northern and western Europe but lower mobility in the south and east. However, cross-national variation in levels of internal migration remains poorly understood, because it is analysed almost exclusively using cross-sectional data and period measures. This paper seeks to advance understanding of cross-national variation in migration levels in 14 European countries by drawing on a recently proposed suite of migration cohort measures, coupled with internationally comparable retrospective residential histories. It shows that differences in migration levels are mainly attributable to variation in the extent of repeat movement, which is underpinned by the differences in mean ages at first and last move that together delineate the average length of migration careers. Cohort analysis provides a robust foundation for exploring the demographic mechanisms underpinning variation in migration levels across countries and over time.

Mobility Gradient of Poly(ethylene terephthalate) Chains near a Substrate Scaled by the Thickness of the Adsorbed Layer

The question of how to scale the mobility gradient of polymer chains near a substrate in supported ultrathin polymer films is a great challenge. In this paper, a mobility gradient of poly(ethylene terephthalate) (PET) chains near a substrate is characterized by cold crystallization. We found that either decreasing the PET film thickness or increasing the absorbed layer thickness consistently reveals three characteristic film thicknesses, which are all linearly dependent on the adsorbed-layer thickness. At the first thickness, the low-temperature peak of the top surface crystallization starts to shift toward the high-temperature peak of the bulk-like polymer crystallization; at the second thickness, it arrives there; and at the third thickness, crystallization is completely suppressed. The three kinds of film thicknesses characterize the depth profile of the local dynamics, reflecting the long-range effects of the substrate, which could be scaled by the thickness of the adsorbed layer.

Moving Destination Prediction Using Sparse Dataset

Moving destination prediction offers an important category of location-based applications and provides essential intelligence to business and governments. In existing studies, a common approach to destination prediction is to match the given query trajectory with massive recorded trajectories by similarity calculation. Unfortunately, due to privacy concerns, budget constraints, and many other factors, in most circumstances, we can only obtain a sparse trajectory dataset. In sparse dataset, the available moving trajectories are far from enough to cover all possible query trajectories; thus the predictability of the matching-based approach will decrease remarkably. Toward destination prediction with sparse dataset, instead of searching similar trajectories over the sparse records, we alternatively examine the changes of distances from sampling locations to final destination on query trajectory. The underlying idea is intuitive: It is directly motivated by travel purpose, people always get closer to the final destination during the movement. By borrowing the conception of gradient descent in optimization theory, we propose a novel moving destination prediction approach, namely MGDPre. Building upon the mobility gradient descent, MGDPre only investigates the behavior characteristics of query trajectory itself without matching historical trajectories, and thus is applicable for sparse dataset. We evaluate our approach based on extensive experiments, using GPS trajectories generated by a sample of taxis over a 10-day period in Shenzhen city, China. The results demonstrate that the effectiveness, efficiency, and scalability of our approach outperform state-of-the-art baseline methods.

Spatially Distributed Rheological Properties in Confined Polymers by Noncontact Shear.

When geometrically confined to the nanometer length scale, a condition in which a large portion of the material is in the nanoscale vicinity of interfaces, polymers can show astonishing changes in physical properties. In this investigation, we employ a unique noncontact capillary nanoshearing method to directly probe nanoresolved gradients in the rheological response of ultrathin polymer films as a function of temperature and stress. Results show that ultrathin polymer films, in response to an applied shear stress, exhibit a gradient in molecular mobility and viscosity that originates at the interfaces. We demonstrate, via molecular dynamics simulations, that these gradients in molecular mobility reflect gradients in the average segmental relaxation time and the glass-transition temperature.

Dynamical heterogeneity in a vapor-deposited polymer glass.

Recently, there has been great interest in "ultrastable" glasses formed via vapor deposition, both because of emerging engineering applications of these materials (e.g., active layers in light-emitting diodes and photovoltaics) and, theoretically, as materials for probing the equilibrium properties of glassy materials below their glass transition, based on the conjecture that these materials are equivalent to glassy materials aged over astronomical time scales. We use molecular dynamics simulations to examine the properties of ultrastable vapor-deposited and ordinary polymer glasses. Based on the difference in the energy of the deposited and ordinary films, we estimate the effective cooling rate for the vapor deposited films to be 1 to 3 orders of magnitude larger than that of the ordinary film, depending on the deposition temperature. Similarly, we find an increase in the average segmental relaxation time of the vapor-deposited film compared to the ordinary glass. On the other hand, the normal mode spectrum is essentially identical for the vapor-deposited and the ordinary glass film, suggesting that the high-frequency dynamics should be similar. In short, the segmental relaxation dynamics of the polymer vapor-deposited glass are consistent with those of an ordinary polymer glass with a somewhat slower effective cooling rate. Of course, one would expect a larger effect on dynamics approaching the experimental glass transition, where the cooling rates are much slower than accessible in simulation. To more precisely probe the relationship between the dynamics of these glasses, we examine dynamical heterogeneity within the film. Due to the substantial mobility gradient in the glassy films, we find that it is crucial to distinguish the dynamics of the middle part of the film from those of the entire film. Considering the film as a whole, the average dynamical heterogeneity is dominated by the mobility gradient, and as a consequence the heterogeneity is nearly indistinguishable between the ordinary and vapor deposited glass films. In contrast, in the middle part of the film, where there is almost no mobility gradient, we find the dynamical heterogeneity within the deposited film is somewhat larger than that of the ordinary film at the same temperature. We further show that the scale of the interfacial region grows on cooling in the equilibrium film, but this trend reverses in the glass state. We attribute this reversal in part to a shrinking ratio of the relaxation time in the middle of the film to that of the interfacial layer in the non-equilibrium state. The dynamics in this mobile interfacial layer for the ordinary and deposited film are nearly the same, suggesting that the interfacial region is always in a near-equilibrium state. These results emphasize the importance of distinguishing between interfacial and internal relaxation processes in this emerging class of materials.

Impact of anchor type on porcine lumbar biomechanics: Finite element modelling and in-vitro validation

BackgroundRigid posterior implants used for spinal stabilization can be anchored to the vertebrae using pedicle screws or screws combined with transverse process hooks. In the present study, a finite element model of a porcine lumbar spine instrumented with screws and hooks is presented and validated. MethodsThe porcine lumbar spine model was validated using in-vitro measurements on six porcine specimens. Validation metrics included intervertebral rotations (L1 to L6) and nucleus pressure in the topmost cranial instrumented disc. The model was used to compare the biomechanical effect of anchor types. FindingsGood agreement was observed between the model and validation experiments. For upper transverse hooks construct, intervertebral rotations increased at the upper instrumented vertebra and decreased at the adjacent level. Additionally, nucleus pressures and stress on the annulus decreased in the adjacent disc and increased in the upper instrumented disc. The pull-out forces predicted for both anchor configurations were significantly lower than the pull-out strength found in the literature. InterpretationThese numerical observations suggest that upper transverse process hooks constructs reduce the mobility gradient and cause less stress in the adjacent disc, which could potentially reduce adjacent segment disease and proximal junction kyphosis incidence without increasing the risk of fixation failure. Future work needs to assess the long-term effect of such constructs on clinical and functional outcomes.

Influence of chemistry, interfacial width, and non-isothermal conditions on spatially heterogeneous activated relaxation and elasticity in glass-forming free standing films.

We employ the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory of activated relaxation to study several questions in free standing thin films of glass-forming molecular and polymer liquids. The influence of non-universal chemical aspects on dynamical confinement effects is found to be relatively weak, but with the caveat that for the systems examined, the bulk ECNLE polymer theory does not predict widely varying fragilities. Allowing the film model to have a realistic vapor interfacial width significantly enhances the reduction of the film-averaged glass transition temperature, Tg, in a manner that depends on whether a dynamic or pseudo-thermodynamic averaging of the spatial mobility gradient is adopted. The nature of film thickness effects on the spatial profiles of the alpha relaxation time and elastic modulus is studied under non-isothermal conditions and contrasted with the corresponding isothermal behavior. Modest differences are found if a film-thickness dependent Tg is defined in a dynamical manner. However, adopting a pseudo-thermodynamic measure of Tg leads to a qualitatively new form of the alpha relaxation time gradient where highly mobile layers near the film surface coexist with strongly vitrified regions in the film interior. As a consequence, the film-averaged shear modulus can increase with decreasing film thickness, despite the Tg reduction and presence of a mobile surface layer. Such a behavior stands in qualitative contrast to the predicted mechanical softening under isothermal conditions. Spatial gradients of the elastic modulus are studied as a function of temperature, film thickness, probing frequency, and experimental protocol, and a rich behavior is found.

EPR Structural Dynamics of Calmodulin-RyR Peptide Complex

We have used a combination of site-directed spin labeling and EPR spectroscopy to probe the interaction of calmodulin (CaM) with a peptide (RyRp) corresponding to the CaM-binding site on the ryanodine receptor, the muscle calcium release channel. A TOAC spin label, which is bifunctionally and stereospecifically coupled to the alpha-carbon and directly reports peptide backbone dynamics, was used to label RyRp at positions 5, 12, 18, or 25. CW-EPR showed that RyRp alone exhibits a gradient of rotational dynamics (mobility), with the N-terminal region having greater mobility than the C-terminal region. Addition of CaM restricted mobility at all four positions, while preserving the mobility gradient. In the presence of saturating Ca, the immobilization of TOAC-RyRp by CaM is further increased. A bifunctional spin label (BSL) attached stereospecifically at Cys sites i and i+4 was used to label CaM at positions 34 and 38 in the N-lobe and 106 and 110 in the C-lobe. Interspin distances measured by DEER indicated that CaM, in the absence of peptide and Ca, exists in a closed conformation, while Ca induces a shift in equilibrium toward two additional conformations, one more open and another more closed (“compact”). Addition of RyRp caused the two lobes of CaM to collapse toward each other (compact conformation), while exhibiting a broad distribution, indicating high flexibility. Removal of Ca from the complex caused increased distance between the spin labels, indicating greater lobe separation. Distances measured between BSL (on CaM) and TOAC (on RyRp) confirmed the compact but flexible conformation of the complex. These results provide insight into the structural dynamics of the Ca-dependent regulation of RyR by CaM.

Spatial Rearrangement and Mobility Heterogeneity of an Anionic Lipid Monolayer Induced by the Anchoring of Cationic Semiflexible Polymer Chains.

We use Monte Carlo simulations to investigate the interactions between cationic semiflexible polymer chains and a model fluid lipid monolayer composed of charge-neutral phosphatidyl-choline (PC), tetravalent anionic phosphatidylinositol 4,5-bisphosphate (PIP2), and univalent anionic phosphatidylserine (PS) lipids. In particular, we explore how chain rigidity and polymer concentration influence the spatial rearrangement and mobility heterogeneity of the monolayer under the conditions where the cationic polymers anchor on the monolayer. We find that the anchored cationic polymers only sequester the tetravalent PIP2 lipids at low polymer concentrations, where the interaction strength between the polymers and the monolayer exhibits a non-monotonic dependence on the degree of chain rigidity. Specifically, maximal anchoring occurs at low polymer concentrations, when the polymer chains have an intermediate degree of rigidity, for which the PIP2 clustering becomes most enhanced and the mobility of the polymer/PIP2 complexes becomes most reduced. On the other hand, at sufficiently high polymer concentrations, the anchoring strength decreases monotonically as the chains stiffen—a result that arises from the pronounced competitions among polymer chains. In this case, the flexible polymers can confine all PIP2 lipids and further sequester the univalent PS lipids, whereas the stiffer polymers tend to partially dissociate from the monolayer and only sequester smaller PIP2 clusters with greater mobilities. We further illustrate that the mobility gradient of the single PIP2 lipids in the sequestered clusters is sensitively modulated by the cooperative effects between anchored segments of the polymers with different rigidities. Our work thus demonstrates that the rigidity and concentration of anchored polymers are both important parameters for tuning the regulation of anionic lipids.

Structure of the Interfacial Region in Adsorbed Poly(vinyl acetate) on Silica

We performed a combined calorimetric and molecular modeling investigation of poly(vinyl acetate) (PVAc) on silica to characterize the intermolecular interactions and the behavior of the adsorbed polymer. From temperature-modulated differential scanning calorimetry experiments, different regions of thermal activity suggested a gradient of mobility in the adsorbed polymer. Polymer segments in more direct contact with silica (tightly bound) showed a significantly elevated and broadened glass transition relative to the bulk polymer, while polymer further away (loosely bound) showed only a slightly elevated transition relative to the bulk polymer. A thermal transition for PVAc at the air interface (more-mobile) was also observed and was at lower temperatures than the bulk polymer. Density profiles from molecular dynamics studies suggested a structure of the adsorbed polymer similar to that experimentally observed. These studies were consistent with the presence of a motional gradient in the polymer segments, a...

Theory of activated glassy relaxation, mobility gradients, surface diffusion, and vitrification in free standing thin films.

We have constructed a quantitative, force level, statistical mechanical theory for how confinement in free standing thin films introduces a spatial mobility gradient of the alpha relaxation time as a function of temperature, film thickness, and location in the film. The crucial idea is that relaxation speeds up due to the reduction of both near-surface barriers associated with the loss of neighbors in the local cage and the spatial cutoff and dynamical softening near the vapor interface of the spatially longer range collective elasticity cost for large amplitude hopping. These two effects are fundamentally coupled. Quantitative predictions are made for how an apparent glass temperature depends on the film thickness and experimental probe technique, the emergence of a two-step decay and mobile layers in time domain measurements, signatures of confinement in frequency-domain dielectric loss experiments, the dependence of film-averaged relaxation times and dynamic fragility on temperature and film thickness, surface diffusion, and the relationship between kinetic experiments and pseudo-thermodynamic measurements such as ellipsometry.

Revealing spatially heterogeneous relaxation in a model nanocomposite.

The detailed nature of spatially heterogeneous dynamics of glycerol-silica nanocomposites is unraveled by combining dielectric spectroscopy with atomistic simulation and statistical mechanical theory. Analysis of the spatial mobility gradient shows no "glassy" layer, but the α-relaxation time near the nanoparticle grows with cooling faster than the α-relaxation time in the bulk and is ∼20 times longer at low temperatures. The interfacial layer thickness increases from ∼1.8 nm at higher temperatures to ∼3.5 nm upon cooling to near bulk Tg. A real space microscopic description of the mobility gradient is constructed by synergistically combining high temperature atomistic simulation with theory. Our analysis suggests that the interfacial slowing down arises mainly due to an increase of the local cage scale barrier for activated hopping induced by enhanced packing and densification near the nanoparticle surface. The theory is employed to predict how local surface densification can be manipulated to control layer dynamics and shear rigidity over a wide temperature range.