Bulk Displacements Research Articles

Modulated segregation of a bidisperse granular mixture due to recirculation currents

A bidisperse medium of large sand particles and small glass beads is mixed in a cylindrical vessel that is put in rotational motion around an eccentric static rod. The subsequent segregation occurring in the system is investigated primarily at the free-surface where the large particles concentration globally increases towards a limit value while oscillating. In order to gain deeper understanding of the intrinsic mechanisms causing the segregation, we also explore the dynamics in the bulk. Average residual bulk displacements fields both vertically and orthoradially could then be measured and show a secondary flow that counteracts the segregation process.

Comparison of single particle dynamics at the center and on the surface of equilibrium glassy films

Glasses prepared by vapor depositing molecules onto a properly prepared substrate can have enhanced kinetic stability when compared with glasses prepared by cooling from the liquid state. The enhanced stability is due to the high mobility of particles at the surface, which allows them to find lower energy configurations than for liquid cooled glasses. Here we use molecular dynamics simulations to examine the temperature dependence of the single particle dynamics in the bulk of the film and at the surface of the film. First, we examine the temperature dependence of the self-intermediate scattering functions for particles in the bulk and at the surface. We then examine the temperature dependence of the probability of the logarithm of single particle displacements for bulk and surface particles. Both bulk and surface particle displacements indicate populations of slow and fast particles, i.e., heterogeneous dynamics. We find that the temperature dependence of the surface dynamics mirrors the bulk despite being several orders of magnitude faster.

Microscale characterization of the viscoelastic properties of hydrogel biomaterials using dual-mode ultrasound elastography

Characterization of the microscale mechanical properties of biomaterials is a key challenge in the field of mechanobiology. Dual-mode ultrasound elastography (DUE) uses high frequency focused ultrasound to induce compression in a sample, combined with interleaved ultrasound imaging to measure the resulting deformation. This technique can be used to non-invasively perform creep testing on hydrogel biomaterials to characterize their viscoelastic properties. DUE was applied to a range of hydrogel constructs consisting of either hydroxyapatite (HA)-doped agarose, HA-collagen, HA-fibrin, or preosteoblast-seeded collagen constructs. DUE provided spatial and temporal mapping of local and bulk displacements and strains at high resolution. Hydrogel materials exhibited characteristic creep behavior, and the maximum strain and residual strain were both material- and concentration-dependent. Burger's viscoelastic model was used to extract characteristic parameters describing material behavior. Increased protein concentration resulted in greater stiffness and viscosity, but did not affect the viscoelastic time constant of acellular constructs. Collagen constructs exhibited significantly higher modulus and viscosity than fibrin constructs. Cell-seeded collagen constructs became stiffer with altered mechanical behavior as they developed over time. Importantly, DUE also provides insight into the spatial variation of viscoelastic properties at sub-millimeter resolution, allowing interrogation of the interior of constructs. DUE presents a novel technique for non-invasively characterizing hydrogel materials at the microscale, and therefore may have unique utility in the study of mechanobiology and the characterization of hydrogel biomaterials.

Improving reconstruction of the baryon acoustic peak: The effect of local environment

Precise measurements of the baryon acoustic oscillation (BAO) scale as a standard ruler in the clustering pattern of large-scale structure is a central goal of current and future galaxy surveys. The BAO peak may be sharpened using the technique of density-field reconstruction, in which the bulk displacements of galaxies are estimated using a Zel'dovitch approximation. We use numerical simulations to demonstrate how the accuracy of this approximation depends strongly on local environment, and how this information may be used to construct an improved BAO measurement through environmental re-weighting and using higher-order perturbation theory. We outline further applications of the displacement field for testing cosmological models.

Stress and Displacement Fields Around Misfit Dislocation in Anisotropic Dissimilar Materials With Interface Stress and Interface Elasticity

Interfaces frequently exist in polycrystalline and multiphase materials. In nanoscale joints, interface properties, such as interface stresses and interface elasticity, influence the stress and displacement field near the interface. Generally, a misfit dislocation exists in the interface due to the mismatch of lattice length in crystals composing the joints. In the present paper, a misfit dislocation is introduced to a coherent interface in order to calculate the stress and displacement distributions in an incoherent interface. A model with an interface zone transferring traction only in the zone from one region to the opposite region is proposed, because these regions slip against each other due to the misfit dislocation. The traction in the interface depends on the displacement and the interface properties. Stresses and displacements considering the interface properties are deduced using a three-dimensional Stroh’s formalism. Bulk stress and displacements around the misfit dislocation are shown to increase with increasing the values of the interface stress and the interface elastic moduli. The stresses and displacements obtained from the derived solutions are compared with those obtained through molecular dynamic (MD) analysis. It is shown that the proposed interface zone model can adequately express the displacement and stress near the misfit dislocation.

Magnetic Resonance Imaging Assessment of Spinal Cord and Cauda Equina Motion in Supine Patients With Spinal Metastases Planned for Spine Stereotactic Body Radiation Therapy

To assess motion of the spinal cord and cauda equina, which are critical neural tissues (CNT), which is important when evaluating the planning organ-at-risk margin required for stereotactic body radiation therapy. We analyzed CNT motion in 65 patients with spinal metastases (11 cervical, 39 thoracic, and 24 lumbar spinal segments) in the supine position using dynamic axial and sagittal magnetic resonance imaging (dMRI, 3T Verio, Siemens) over a 137-second interval. Motion was segregated according to physiologic cardiorespiratory oscillatory motion (characterized by the average root mean square deviation) and random bulk shifts associated with gross patient motion (characterized by the range). Displacement was evaluated in the anteroposterior (AP), lateral (LR), and superior-inferior (SI) directions by use of a correlation coefficient template matching algorithm, with quantification of random motion measure error over 3 separate trials. Statistical significance was defined according to P<.05. In the AP, LR, and SI directions, significant oscillatory motion was observed in 39.2%, 35.1%, and 10.8% of spinal segments, respectively, and significant bulk motions in all cases. The median oscillatory CNT motions in the AP, LR, and SI directions were 0.16mm, 0.17mm, and 0.44mm, respectively, and the maximal statistically significant oscillatory motions were 0.39mm, 0.41mm, and 0.77mm, respectively. The median bulk displacements in the AP, LR, and SI directions were 0.51mm, 0.59mm, and 0.66mm, and the maximal statistically significant displacements were 2.21mm, 2.87mm, and 3.90mm, respectively. In the AP, LR, and SI directions, bulk displacements were greater than 1.5mm in 5.4%, 9.0%, and 14.9% of spinal segments, respectively. No significant differences in axial motion were observed according to cord level or cauda equina. Oscillatory CNT motion was observed to be relatively minor. Our results support the importance of controlling bulk patient motion and the practice of applying a planning organ-at-risk margin.

Rigid frictionless indentation on elastic half space with influence of surface stresses

This paper proposes an application of continuum-based concepts in the analysis of an axisymmetric rigid frictionless indentor acting on an isotropic, linearly elastic half-space accounted for surface energy effects. The influence of surface stresses is considered by employing a complete Gurtin–Murdoch continuum model for surface elasticity. With use of standard Love’s representation and Hankel integral transform, such boundary value problem is reduced to a set of dual integral equations that can be further transformed into an equivalent Fredholm integral equation of the second kind. Selected numerical procedures based on the solution discretization and standard collocation technique are then implemented to construct its solution numerically. Obtained numerical results for elastic fields within the bulk are shown and compared for indentors of different profiles and contact radii at various depths. It is found that the influence of surface free energy on bulk stresses and displacements and the size-dependency of solutions become more apparent in a region very near the free surface. The significant contribution of the residual surface tension on predicted responses is obviously observed in comparison with existing results. The proposed mathematical model not only offers an alternative for specifically studying both mechanical properties and elastic fields for indentors of arbitrary axisymmetric profiles but also provides, in general, a crucial basis for further investigations in the area of nano-mechanics.

Abstract 246: Evaluation of 2-Dimensional Aortic Arch Motion at the Origin of the Left Subclavian Artery: Importance of Respiratory Motion

Introduction: Interactions between components of branched thoracic main stent grafts (MSG) and branch stent grafts (BSG) in the arch have not yet been evaluated. As the chest is subject to both cardiac and respiratory motion, the purpose of this study is to evaluate the cardiac and respiratory effects on interactions between components in the aortic arch. Methods: Two swine had a BSG alone placed in the LSA and two had an MSG/BSG system placed in the LSA. Frames representing the extreme positions of the BSG due to cardiac imposed motion during apnea and through the respiratory cycle were identified in 2-D cines. These images were overlaid as layers, using external reference markers common to each image for accurate overlay positioning. The BSG’s were traced and magnitudes of horizontal and vertical bulk displacements were quantified to evaluate cardiac (CM) and respiratory (RM) [deep inspiration (30 mmHg)]. Relative motion was extracted from bulk motion by comparing displacements at the top and bottom of the BSG. Results: RM resulted in significant difference in bulk horizontal motion compared to CM for the BSG [5.65 + / - 0.07 versus 3.15 + / - 0.63mm, (p=0.002)] and was nearly significant for the MSG /BSG system [4.25 + / - 0.07 versus 2.3 + / - 0.42mm, (p=0.061)]. RM resulted in significant difference in bulk vertical movement compared to CM for the BSG [2.2 + / - 0.28 versus 0.85 + / - 0.35mm, (p=0.026)] and the MSG/BSG system [2.05 + / - 0.07 versus 0.6 + / - 0 (p=0.000001)]. The MSG presence significantly dampened bulk horizontal movement for RM [4.25 + / - 0.07 versus 5.65 + / - 0.07mm, (p=0.0013)] but had no effect on vertical movement [2.05 + / - 0.07 versus 2.2 + / - 0.28mm, (p=0.15)]. RM resulted in significant difference in relative horizontal motion compared to CM for the BSG [4.25 + / - 0.49 versus 2.7 + / - 0.28mm, (p=0.014)] and the MSG/BSG system [1.85 + / - 0.07 versus 0.95 + / - 0.35mm, (p=0.0048)]. RM resulted in significant difference in relative vertical movement compared to CM for the BSG [1.25 + / - 0.07 versus 0.3 + / - 0.28mm, (p=0.002)] and MSG/BSG system [0.7 + / - 0.14 versus 0.25 + / - 0.21mm, (p=0.03)]. The MSG presence significantly dampened relative horizontal and vertical movement for RM [1.85 + / - 0.07 versus 4.75 + / - 0.49mm, (p=0.0004)] and [0.7 + / - 0.14 versus1.25 + / - 0.07mm, (p=0.02)], respectively. Conclusions: RM adds considerably to bulk and relative motion, with horizontal motion dominating. Relative motion accounts for high percentage of bulk motion. The presence of the MSG dampens motion considerably. This is the first study to account for the effects of respiratory motion on branched endografts in the aortic arch. This movement should be taken into account when developing a MSG/BSG for the therapy of aortic arch aneurysms.

To B or not to B: a question of resolution?

In choosing and refining any crystallographic structural model, there is tension between the desire to extract the most detailed information possible and the necessity to describe no more than what is justified by the observed data. A more complex model is not necessarily a better model. Thus, it is important to validate the choice of parameters as well as validating their refined values. One recurring task is to choose the best model for describing the displacement of each atom about its mean position. At atomic resolution one has the option of devoting six model parameters (a 'thermal ellipsoid') to describe the displacement of each atom. At medium resolution one typically devotes at most one model parameter per atom to describe the same thing (a 'B factor'). At very low resolution one cannot justify the use of even one parameter per atom. Furthermore, this aspect of the structure may be described better by an explicit model of bulk displacements, the most common of which is the translation/libration/screw (TLS) formalism, rather than by assigning some number of parameters to each atom individually. One can sidestep this choice between atomic displacement parameters and TLS descriptions by including both treatments in the same model, but this is not always statistically justifiable. The choice of which treatment is best for a particular structure refinement at a particular resolution can be guided by general considerations of the ratio of model parameters to the number of observations and by specific statistics such as the Hamilton R-factor ratio test.

Elastic–Plastic Spherical Contact Modeling Including Roughness Effects

The effect of material properties and surface roughness on the contribution of asperities and sphere bulk displacements to the total displacement of a rough spherical contact is investigated. A dimensionless transition load, above which the contribution of the bulk displacement exceeds the contribution of the asperities displacement, is found as a function of the plasticity index and dimensionless critical interference of the sphere bulk. A criterion is proposed for evaluating the importance of surface roughness in calculating the displacement of a rough spherical contact. Some experimental results with a spherical micro-contact are presented to verify the model.

Criteria for three‐fluid configurations including layers in a pore with nonuniform wettability

Recently, a considerable effort has been made to determine the precise displacement criteria for three‐fluid configurations in pores of angular cross section. These configurations may contain thick conducting fluid layers, such as oil layers residing between gas in the center and water in the corners of the pore. For pores of uniform, but arbitrary, wettability and in the absence of contact angle hysteresis, a precise thermodynamic criterion for the existence of such layers has been established. In this paper we derive similar criteria for layers in pores of nonuniform wettability, where additional and more complicated layer configurations arise. The criteria for formation and removal of layers are consistent with the capillary entry conditions for the accompanying three‐phase bulk displacements, which is essential for accurate pore‐scale modeling of three‐phase flow. We consider the particular case of three‐phase gas invasion in a star‐shaped pore with a specific choice of interfacial tensions and contact angles. For this case all possible fluid configurations arise, but only if the water‐wet surface in the pore corners is small.

A cohesive zone model with a large displacement formulation accounting for interfacial fibrilation

Interfacial fibrilation is a typical mechanism that frequently occurs during delamination of a polymer coating from a steel substrate. It involves large displacements at the interface as well as large deformations in the bulk material. Classical small displacement cohesive zone formulations fail to describe such large deformations correctly. Therefore, a generic cohesive zone model is introduced that is suitable to describe both uniform and non-uniform fracture at an interface with large deformations in the delaminating bulk and large displacements at the crack tip.

Mixed matrix formulation for the analysis of laser-generated acoustic waves by a thermoelastic line source

An analytical model has been developed for the generation of bulk and surface acoustic waves at the surface of an isotropic solid by an infinite thermoelastic laser line source. This model is based on the mixed matrix formulation. It leads to expressions in closed form for the longitudinal and transverse bulk displacements and also for the Rayleigh wave components. A comparison with other models shows that the mechanical effect of the free surface reduces the strength of the two dipoles of force equivalent to the point thermoelastic source. For Rayleigh waves, quantitative agreement has been found with a numerical model and with experiments previously carried out on a duraluminum plate.

Sputtering and the formation of nanometre voids and holes in aluminium in a scanning transmission electron microscope

Abstract Nanometre voids and holes have been produced in aluminium films up to 220nm thick by the stationary focused 100 keV high-current-density electron probe in a dedicated scanning transmission electron microscope. The electron energy is below the threshold for bulk displacements in aluminium but is sufficient to cause sputtering of aluminium atoms from the electron-exit surface. The sputtering leads to the formation of a pit with a diameter determined by the electron probe size at the electron-exit surface. As the pit aspect ratio increases, atoms are sputtered from the pit base onto the pit side walls where they experience a much reduced electron intensity, rather than being sputtered directly out of the pit. Eventually the pit seals to leave a void, separated from the end of the small pit that remains at the electron-exit surface. By repeatedly interrupting the irradiation so as to image a projection of the irradiated volume, it is shown that the void then moves from near the electron-exit surface ...

Optical fiber interferometry for photoacoustic spectroscopy in liquids

Optical fiber interferometry was used for measurement of the subnanometer surface and bulk displacements associated with photoacoustic pressure waves in methanol and water. The measurement system is both broadband and noncontacting, giving a pressure sensitivity of 0.1 Pa/ radicalHz.

Partitioning of ion-induced surface and bulk displacements

An analytical method is described to determine the number and partitioning of displacements of surface and bulk atoms by low energy ions incident on a solid. Results are presented for Ge, Si and C (diamond and graphite). For Ge the results are compared with Monte Carlo calculations using the computer code TRIMRC. Good agreement is found in this comparison, thereby verifying the adequacy of several approximations introduced into the formalism. Displacement threshold and surface-to-bulk displacement ratio contours are presented as a function of ion mass and energy based on our analytic calculations. Comparison of these contours with recent experimental results for Si suggests that this approach provides a global framework for understanding and optimizing the effects of low-energy ions on semiconductor expitaxial growth.

Elastic distortion of a conducting surface in the vicinity of a point charge

The elastic distortion of a conducting surface when a point charge is brought near the suface is investigated theoretically. The displacement field is calculated with the aid of a Green's function in terms of known functions. General results for surface and bulk displacements are displayed graphically. Numerical results for tungsten and aluminum are presented. Modifications of the image potential due to the distortion are discussed.

Elastic continuum theory of bulk and surface-atom mean square displacements of hexagonal crystals

The mean square displacements of particles in a semi-infinite hexagonal elastic continuum with a free (0001) surface are calculated using a Green's function method in the high temperature limit.