Uniform Disk Research Articles

Modulation of Vesicle Adhesion and Spreading Kinetics by Hyaluronan Cushions

The adhesion of giant unilamellar phospholipid vesicles to planar substrates coated with extracellular matrix mimetic cushions of hyaluronan is studied using quantitative reflection interference contrast microscopy. The absolute height of the vesicle membrane at the vicinity of the substrate is measured by considering, for the first time, the refractive indices of the reflecting media. The thickness of the cushion is varied in the range of ∼50–100 nm, by designing various coupling strategies. On bare protein-coated substrates, the vesicles spread fast (0.5 s) and form a uniform adhesion disk, with the average membrane height ∼4 nm. On thick hyaluronan cushions (>80 nm), the membrane height is approximately the same as the thickness of the cushion, implying that the vesicle lies on top of the cushion. On a thin and inhomogeneous hyaluronan cushion, the adhesion is modified but not prevented. The spreading is slow (∼20 s) compared to the no-cushion case. The average membrane height is ∼10 nm and the adhesion disk is studded with blisterlike structures. Observations with fluorescent hyaluronan indicate that the polymer is compressed under, rather than expelled from, the adhesion disk. The adhesion energy density is approximately threefold higher in the no-cushion case (1.2 μJ/m 2) as compared to the thin-cushion case (0.54 μJ/m 2). In the thin-cushion case, the presence of short (∼4 nm) glyco-polymers on the vesicles results in a hitherto unreported stable partial adhesion state—the membrane height ranges from zero to ∼250 nm. The minimal model system presented here mimics in vitro the hyaluronan-modulated early stages of cell adhesion, and demonstrates that the presence of a polymer cushion influences both the final equilibrium adhesion-state and the spreading kinetics.

Generation of potential/surface density pairs in flat disks

We report a simple method to generate potential/surface density pairs in flat axially symmetric finite size disks. Potential/surface density pairs consist of a ``homogeneous'' pair (a closed form expression) corresponding to a uniform disk, and a ``residual'' pair. This residual component is converted into an infinite series of integrals over the radial extent of the disk. For a certain class of surface density distributions (like power laws of the radius), this series is fully analytical. The extraction of the homogeneous pair is equivalent to a convergence acceleration technique, in a matematical sense. In the case of power law distributions, the convergence rate of the residual series is shown to be cubic inside the source. As a consequence, very accurate potential values are obtained by low order truncation of the series. At zero order, relative errors on potential values do not exceed a few percent typically, and scale with the order N of truncation as 1/N**3. This method is superior to the classical multipole expansion whose very slow convergence is often critical for most practical applications.

Effects of texture and shape on perceived time to passage: Knowing “what” influences judging “when”

In the present study, we examined whether it is easier to judge when an object will pass one's head if the object's surface is textured. There are three reasons to suspect that this might be so: First, the additional (local) optic flow may help one judge the rate of expansion and the angular velocity more reliably. Second, local deformations related to the change in angle between the object and the observer could help track the object's position along its path. Third, more reliable judgments of the object's shape could help separate global expansioncaused by changes in distance from expansion due to changes in the angle between the object and the observer. We can distinguish among these three reasons by comparing performance for textured and uniform spheres and disks. Moving objects were displayed for 0.5-0.7 sec. Subjects had to decide whether the object would pass them before or after a beep that was presented 1 sec after the object started moving. Subjects were not more precise with textured objects. When the disk rotated in order to compensate for the orientation-related contraction that its image would otherwise undergo during its motion, it appeared to arrive later, despite the fact that this strategy increases the global rate of expansion. We argue that this is because the expected deformation of the object's image during its motion is considered when time to passage is judged. Therefore, the most important role for texture in everyday judgments of time to passage is probably that it helps one judge the object's shape and thereby estimate how its image will deform as it moves.

On the terminal motion of sliding spinning disks with uniform Coulomb friction

We review previous investigations concerning the terminal motion of disks sliding and spinning with uniform dry friction across a horizontal plane. Previous analyses show that a thin circular ring or uniform circular disk of radius R always stops sliding and spinning at the same instant. Moreover, under arbitrary nonzero initial values of translational speed v and angular rotation rate ω , the terminal value of the speed ratio ϵ 0 = v / R ω is always 1.0 for the ring and 0.653 for the uniform disk. In the current study we show that an annular disk of radius ratio η = R 1 / R 2 stops sliding and spinning at the same time, but with a terminal speed ratio dependent on η . For a two-tier disk with lower tier of thickness H 1 and radius R 1 and upper tier of thickness H 2 and radius R 2 , the motion depends on both η and the thickness ratio λ = H 1 / H 2 . While translation and rotation stop simultaneously, their terminal ratio ϵ 0 either vanishes when k > 2 / 3 , is a nonzero constant when 1 / 2 < k < 2 / 3 , or diverges when k < 1 / 2 , where k is the normalized radius of gyration. These three regimes are in agreement with those found by Goyal et al. [S. Goyal, A. Ruina, J. Papadopoulos, Wear 143 (1991) 331] for generic axisymmetric bodies with varying radii of gyration using geometric methods. New experiments with PVC disks sliding on a nylon fabric stretched over a plexiglass plate only partially corroborate the three different types of terminal motions, suggesting more complexity in the description of friction.

WE‐C‐L100J‐07: Investigation of Fourier‐Based, Hypothesis‐Testing Detection and Discrimination Tasks in Dual‐Energy Imaging

Purpose: To relate theoretical descriptions of detectability index to imaging performance as assessed by real observers for a variety of detection and discrimination tasks. Dual‐energy (DE) imaging provided a foundation for the research, incorporating a variety of decomposition techniques that significantly affect the noise‐equivalent quanta (NEQ). Method and Materials: The NEQ was derived using cascaded systems analysis, extended to DE imaging with a flat‐panel detector. Three idealized task functions were considered: sphere detection, shape discrimination (sphere vs. disk), and texture discrimination (uniform vs. crenellated disk). Detectability index was derived by integrating the task function over the NEQ. These idealized tasks were physically emulated with a chest phantom. DE images were acquired over a range of conditions and noise‐reduction techniques, and the performance of real observers was assessed in rank‐order and alternative‐forced‐choice (AFC) tests. The degree to which theoretical detectability index correlated to human observer performance was investigated. Results: Decomposition technique profoundly affected the NEQ. Theoretical detectability index agreed qualitatively with human observers. In DE soft‐tissue images, noise reduction (e.g. anti‐correlated noise reduction) had a minor effect on sphere detection, but a significant effect on shape and texture discrimination tasks, consistent with a factor of ∼2 increase in detectability. In DE bone‐only images, noise reduction improved the detectability index for all tasks, particularly for detection of small spheres and texture discrimination. The results were consistent with phantom images where, for example, a 1.6 mm sphere became conspicuous (Az∼1.0) upon application of simple‐smoothing on the high‐kVp image. Conclusions: Analysis of DE imaging performance provided a useful probe of the complicated interplay between frequency‐dependent NEQ and various imaging tasks, evident in both theoretical and real observer studies. The results provide initial validation that theoretically derived detectability index represents a meaningful surrogate for observer performance and is a useful objective function for system optimization.

Resolving the B[e] star Hen 3-1191 at 10 $\mathsf{\mu}$m with VLTI/MIDI

We report spatially resolved, spectrally dispersed N-band observations of the B[e] star Hen 3-1191 with the MIDI instrument of the Very Large Telescope Interferometer. The object is resolved with a 40 m baseline and has an equivalent uniform disc diameter ranging from 24 mas at 8 ${\mu{\rm m}}$ to 36 mas at 13 ${\mu{\rm m}}$. The MIDI spectrum and visibilities show a curvature which can arise from a weak silicate feature in which the object appears ≈ 15% larger than in the continuum, but this could result from a change in the object's geometry within the band. We then model Hen's 3-1191 spectral energy distribution (.4-60 $\mu{\rm m}$) and N-band visibilities. Because of the unknown nature of the object, we use a wide variety of models for objects with IR excesses. We find the observations to be consistent with a disc featuring an unusually high mass accretion and a large central gap almost void of matter, an excretion disc, and a binary made of two IR sources. We are unable to find a circumstellar shell model consistent with the data. We review the different hypotheses concerning the physical nature of the star and conclude that it is neither a Be supergiant nor a symbiotic star. However, we could not discriminate between the scenario of a young stellar object featuring an unusually strong FU Orionis-like outburst of mass accretion ($4{-}250\times10^{-4}~\ensuremath{M_\odot}/\ensuremath{{\rm yr}}$) and that of a protoplanetary nebula with an equatorial mass excretion rate $\ga$$4 \times 10^{-5}\,\ensuremath{M_\odot}/\ensuremath{{\rm yr}}$. In both cases, taking the additional presence of an envelope or wind into account would result in lower mass flows.

Growth Mechanism of GaAs Microdisk Structures by Area-Selective Epitaxy Using Migration-Enhanced Epitaxy

We have carried out area-selective epitaxy of GaAs microdisk structures on a SiO2-masked GaAs substrate by migration-enhanced epitaxy. We have successfully grown “damage-free” disks at 590 °C. It is found that, for the structures grown on (111)B substrates, the facets surrounding the disks are controlled by the V/III beam equivalent pressure ratio. This characteristic is caused by the reduced growth rate in the [111]B direction under high V/III ratios. Using an optimized growth condition, uniform hexagonal GaAs disks surrounded by six vertical {110} facets and a flat (111)B top are grown. The microdisks grown on (001) substrates, however, show no flat-top structures, regardless of the V/III ratio, probably due to the fact that the growth rate in the (001) direction is not sensitive to the V/III ratio. In the latter case, square-pyramidal structures surrounded by four inclined {011} facets are obtained.

Spatial frequency difference between textures interferes with brightness perception

Abrupt changes in luminance trigger and restrict brightness filling-in. If brightness was actively filled-in and mediated by cells signaling both luminance borders and surface brightness, then brightness spreading could also get disrupted by changes in texture. We measured psychophysically the brightness of a uniform luminance disk, which was segmented into two parts by different textures. The brightness of the central part of the disk was substantially reduced, and the reduction depended on spatial frequency, but not on the orientation difference between the textures. The results show that texture borders are able to block brightness filling-in. The bandwidth of brightness spreading was estimated to be ∼1.5 octaves. This suggests that brightness information spreads only between neurons of similar spatial frequency characteristics.

Validation of the Coupled-Laplace-Finite-Element Method for the Loading Problem of a Laterally Heterogeneous Spherical Earth

Three numerical cases are considered for the simulation of loading responses of stratified viscoelastic earth to second degree harmonic load, uniform disc ice load, and last-glacial realistic ice load respectively. The surface displacements, geoid perturbation, relative sea level and the present-day velocities of these quantities are computed with the newly developed Coupled-Laplace-Finite-Element (CLFE) method and compared with the result of the traditional spectral method. The agreements between the results from two methods are found to be generally good. Therefore the CLFE method is proved to be correct, effective and can be applied to study the effects of lateral heterogeneity in the mantle. The new method is valuable since it is not limited to small perturbations in lateral heterogeneity

Thermal buckling of a thin uniform circular disk: a comparison of predictions

AbstractThe conditions for thermally-induced buckling of an unloaded thin, circular disk are compared from two well-known but unconnected studies: an approximate solution by Freund for a constant thickness disk, which must neglect the free edge condition, and an exact solution by Mansfield but only for a disk whose thickness tapers to zero in a particular manner. It is shown that buckling occurs at slightly higher values compared to a finite element analysis of a constant thickness disk but that the case of variable thickness seems to offer a closer result, which suggests that it better models the boundary layer behaviour near the free edge.

Heating of a uniform wafer disk

The closed-form heating response of a thin uniform circular wafer is obtained, in view of a new processing method for semiconductor materials. A strain energy formulation is obtained expeditiously using Gaussian curvature and associated structural concepts. The method is developed for generally curved wafers, which accounts for flat, spherical, cylindrical and twisted shapes. Solutions for the first two types become available in closed form, and the deformation can exhibit a sudden change in axi-symmetrical response or a snap-through buckling, or both: for the latter two types, a numerical solution points to progressive deformation in both without buckling. All results are shown to compare rather well with finite element analysis.

Circumstellar material in the Vega inner system revealed by CHARA/FLUOR

Only a handful of debris disks have been imaged up to now. Due to the need for high dynamic range and high angular resolution, very little is known about the inner planetary region, where small amounts of warm dust are expected to be found. We investigate the close neighbourhood of Vega with the help of infrared stellar interferometry and estimate the integrated K-band flux originating from the central 8 AU of the debris disk. We performed precise visibility measurements at both short (~30 m) and long (~150 m) baselines with the FLUOR beam-combiner installed at the CHARA Array (Mt Wilson, California) in order to separately resolve the emissions from the extended debris disk (short baselines) and from the stellar photosphere (long baselines). After revising Vega's K-band angular diameter (3.202+/-0.005 mas), we show that a significant deficit in squared visibility (1.88+/-0.34%) is detected at short baselines with respect to the best-fit uniform disk stellar model. This deficit can be either attributed to the presence of a low-mass stellar companion around Vega, or as the signature of the thermal and scattered emissions from the debris disk. We show that the presence of a close companion is highly unlikely, as well as other possible perturbations (stellar morphology, calibration), and deduce that we have most probably detected the presence of dust in the close neighbourhood of Vega. The resulting flux ratio between the stellar photosphere and the debris disk amounts to 1.29+/-0.19% within the FLUOR field-of-view (~7.8 AU). Finally, we complement our K-band study with archival photometric and interferometric data in order to evaluate the main physical properties of the inner dust disk. The inferred properties suggest that the Vega system could be currently undergoing major dynamical perturbations.

Femtosecond Laser Preparation of Donor Tissue From the Endothelial Side

The femtosecond laser (Intralase) may provide advantages for dissecting a thin, uniform thickness posterior lamellar disk of donor tissue to be used for endothelial transplantation. We investigated the use of the Intralase to dissect the donor cornea from the posterior side to better obtain a thin and uniform lamellar disk. We investigated the use of a viscoelastic "cushion" to protect the endothelium during applanation and laser delivery. Human eye bank donor buttons were placed endothelial side up, covered with a thin coat of viscoelastic, and brought into contact with the Intralase applanation lens. A 7-mm diameter, 100-microm lamellar disk was cut from the endothelial side. The endothelial viability after these procedures was determined using a live cell/dead cell assay. Controls were designed to assess the endothelial viability after applanation and laser application using only a balanced salt solution (BSS) cushion instead of viscoelastic material. Additionally, applanation without lasering using either BSS or a viscoelastic cushion was studied. The average endothelial cell loss in the laser experiment sets were 10% (n = 5, range of 4-17%, Sodium Hyaluronate), 14% (n = 5, range of 7-19%, Sodium Hyaluronate-Sodium Chondroitin) and 6% (n = 5, range of 3-11%, Hydroxypropylmethyl-cellulose). In the controls, laser and applanation with BSS resulted in an average endothelial loss of 18% (n = 5, range of 14-26%). Applanation alone without laser dissection resulted in cell loss of 9% (n = 5, range of 7-12%) using BSS and 9% (n = 6, range 1-42%) Hydroxypropylmethyl-cellulose. The technique of using a viscoelastic "cushion" to protect endothelial cells from damage during posterior laser dissection prior to transplantation is promising. Viscoelastic coating protects the endothelial layer from damage from the coupling lens better than a layer of BSS. The lasering process, however, causes damage in addition to applanation with the laser lens. Further studies are warranted to optimize reproducibility of endothelial cell survival and evaluate the smoothness of stromal dissections in the posterior cornea.

Constraining Disk Parameters of Be Stars using Narrowband Hα Interferometry with the Navy Prototype Optical Interferometer

Abstract : Interferometric observations of two well-known Be stars, gamma Cas and phi Per, were collected and analyzed to determine the spatial characteristics of their circumstellar regions. The observations were obtained using the Navy Prototype Optical Interferometer equipped with custom-made narrowband filters. The filters isolate the H(alpha) emission line from the nearby continuum radiation, which results in an increased contrast between the interferometric signature due to the H(alpha)-emitting circumstellar region and the central star. Because the narrowband filters do not significantly attenuate the continuum radiation at wavelengths 50 nm or more away from the line, the interferometric signal in the H(alpha) channel is calibrated with respect to the continuum channels. The observations used in this study represent the highest spatial resolution measurements of the H -emitting regions of Be stars obtained to date. These observations allow us to demonstrate for the first time that the intensity distribution in the circumstellar region of a Be star cannot be represented by uniform disk or ringlike structures, whereas a Gaussian intensity distribution appears to be fully consistent with our observations.

Regimes of Terminal Motion of Sliding Spinning Disks

Analysis of the frictional motion of a uniform circular disk of radius sliding and spinning on a horizontal table reported by Farkas et al. [Phys. Rev. Lett. 90, 248302 2003] shows that the disk always stops sliding and spinning at the same instant with a terminal speed ratio epsilon = v/Romega = 0.653. We show that different terminal behaviors can be found when one considers the motion of a two-tier disk with lower section thickness H(1) and radius R(1), and upper section thickness H(2) and radius H(3). The terminal motion may be analyzed in terms of the normalized radius of gyration k. It is found that while translation and rotation cease simultaneously, their terminal ratio epsilon(0) either vanishes when k > sq.root(2/3), is a nonzero constant when k < 1/2 < k < sq.rt (2/3), or diverges when k < 1/2. Experiments performed with plastic disks sliding on a nylon fabric stretched over a horizontal plate qualitatively corroborate the three different types of terminal motion.

Correcting spurious resolution

Spurious resolution is a byproduct of optical defocus whereby sinusoidal image components are reversed in sign: light stripes become dark and vice-versa. Such contrast-reversal effects are readily apparent in defocused projected images, and in subjective visual experience while viewing targets closer than the near point of the eye. Mathematically, spurious resolution is due to negative zones in the optical transfer function. Such zones appear in the transfer functions that correspond to blurring by convolution with a uniform disk (as in geometrical optics), and in those implied by physical optics for a defocused eye limited only by diffraction. What happens if these spurious negative zones are re-inverted, restoring the original contrast signs? The answer for both geometrical and physical optics is that the pointspread function is greatly re-sharpened. Since sign correction is purely a phase operation, this means that substantial deblurring can be produced by a simple phase-correcting filter that entails no amplitude restoration at all. In principle such a filter could be inserted at any point between a target (e.g., printed text) and its retinal image, by convolving the image at that stage (e.g., on the page) with the inverse Fourier transform of the sign of the blur transfer function.

Establishing Visible Interferometer System Responses: Resolved and Unresolved Calibrators

ABSTRACTThis investigation examines the propagation of errors through the uniform disk visibility function. The implications of those errors on measurements of absolute visibility through optical and near‐infrared interferometers are considered within the context of using calibration stars to establish system visibilities for these instruments. We suggest a simple ratio test to establish empirically whether or not the measured visibilities produced by such an instrument are relative (errors dominated by calibrator angular size prediction error) or absolute (errors dominated by measurement error).

Size Is Everything: Universal Features of Quasar Microlensing with Extended Sources

We examine the effect that the shape of the source brightness profile has on the magnitude fluctuations of images in quasar lens systems due to microlensing. We do this by convolving a variety of accretion disk models (including Gaussian disks, uniform disks, cones, and a Shakura-Sunyaev thermal model) with two magnification maps in the source plane, one with convergence κ = 0.4 and shear γ = 0.4 (positive parity) and the other with κ = γ = 0.6 (negative parity). By looking at magnification histograms of the convolutions and using χ2 tests to determine the number of observations that would be necessary to distinguish histograms associated with different disk models, we find that, for circular disk models, the microlensing fluctuations are relatively insensitive to all properties of the models except the half-light radius of the disk. Shakura-Sunyaev models are sufficiently well constrained by observed quasar properties that we can estimate the half-light radius at optical wavelengths for a typical quasar. If Shakura-Sunyaev models are appropriate, the half-light radii are very much smaller than the Einstein rings of intervening stars, and the quasar can be reasonably taken to be a point source except in the immediate vicinity of caustic-crossing events.

Analytically derived TOA-DOA distributions of uplink/downlink wireless-cellular multipaths arisen from scatterers with an inverted-parabolic spatial distribution around the mobile

This letter presents the joint/marginal distributions of the uplink (downlink) multipaths' azimuth angles of arrival and times of arrival at the cellular base station (mobile). These closed-form explicit densities are rigorously derived based on a new "geometrical model" wherein omnidirectional scatterers are modeled as spatially distributed at an inverted parabolic spatial distribution on a two-dimensional disc centered at the mobile. This "inverted parabolic" density, in contrast to the uniform disc density or the rim density, indirectly accounts for scattering power loss. This new model better least-squares-fit some empirical data than its competing models can.

Critical speeds and the response of a spinning disk to a stationary load using Mindlin plate theory

The critical speeds of a moderately thick circular spinning disk are determined using the Mindlin plate theory, which includes shear deformation and rotational inertia. A combination of analytical and numerical methods is used to calculate the four lowest critical speeds for a centrally clamped uniform circular disk and for different thicknesses and clamping ratios. Comparisons between the critical speeds and the corresponding critical speeds for the classical theory, which neglects shear deformation and rotational inertia, are made. The displacement response at the position of the applied force is also determined for different thickness and clamping ratios and compared with the corresponding results from the classical case.