Characterizing anomalous diffusion in crowded polymer solutions and gels over five decades in time with variable-lengthscale fluorescence correlation spectroscopy.

The diffusion of macromolecules in cells and in complex fluids is often found to deviate from simple Fickian diffusion. One explanation offered for this behavior is that molecular crowding renders diffusion anomalous, where the mean-squared displacement of the particles scales as 〈r(2)〉∝t(α) with α < 1. Unfortunately, methods such as fluorescence correlation spectroscopy (FCS) or fluorescence recovery after photobleaching (FRAP) probe diffusion only over a narrow range of lengthscales and cannot directly test the dependence of the mean-squared displacement (MSD) on time. Here we show that variable-lengthscale FCS (VLS-FCS), where the volume of observation is varied over several orders of magnitude, combined with a numerical inversion procedure of the correlation data, allows retrieving the MSD for up to five decades in time, bridging the gap between diffusion experiments performed at different lengthscales. In addition, we show that VLS-FCS provides a way to assess whether the propagator associated with the diffusion is Gaussian or non-Gaussian. We used VLS-FCS to investigate two systems where anomalous diffusion had been previously reported. In the case of dense cross-linked agarose gels, the measured MSD confirmed that the diffusion of small beads was anomalous at short lengthscales, with a cross-over to simple diffusion around ≈1 µm, consistent with a caged diffusion process. On the other hand, for solutions crowded with marginally entangled dextran molecules, we uncovered an apparent discrepancy between the MSD, found to be linear, and the propagators at short lengthscales, found to be non-Gaussian. These contradicting features call to mind the "anomalous, yet Brownian" diffusion observed in several biological systems, and the recently proposed "diffusing diffusivity" model.

Hierarchical, self-similar structure in native squid pen.

The structure of native squid pen (gladius) was investigated in two different species on different length scales. By combining microscopy, atomic force microscopy (AFM), and X-ray diffraction, the experiments probed length scales from millimetres down to nanometres. The gladii showed a hierarchical, self-similar structure in the optical experiments with fibres of different size oriented along the long axis of the gladius. The fibre-like structure was reproduced at the nanoscale in AFM measurements and fibres with diameters of 500 µm, 100 µm, 10 µm, 2 µm and 0.2 µm were observed. Their molecular structure was determined using X-ray diffraction. In the squid gladius, the chitin molecules are known to form nano-crystallites of monoclinic lattice symmetry wrapped in a protein layer, resulting in ß-chitin nano-fibrils. Signals corresponding to the α-coil protein phase and ß-chitin crystallites were observed in the X-ray experiments and their orientation with respect to the fibre-axis was determined. The size of a nano-fibril was estimated from the X-ray experiments to be about 150 × 300 Å. About 100 of these nano-fibrils are needed to form a 0.2 µm thick micro-fibre. We found that the molecular structure is highly anisotropic with ∼90% of the α-coils and ß-chitin crystallites oriented along the fibre-axis, indicating a strong correlation between the macroscale structure and molecular orientation.

Strain rate effects on symmetric diblock copolymer liquid bridges: order-induced stability of polymer fibres.

Optical microscopy is used to study the effect of lamellar order on the evolution of polymer-melt bridges. Measurements are performed on symmetric diblock copolymers and linear homopolymers in the melt state. Diblock copolymer bridges measured in the disordered phase are shown to exhibit the same strain rate response as their homopolymer counterparts: shear thinning at low strain rates and shear thickening at high strain rates. However, when measured in the ordered phase, copolymer-melt bridges demonstrate an increased effective viscosity due to the lamellar order and a shear thinning response over the entire range of strain rates probed. The increased viscosity demonstrates an enhanced stability in lamellae forming diblock liquid bridges, presumed to be caused by the isotropic orientational order of lamellar domains that provide energy barriers to flow within the bridge. The shear thinning can be understood as an alignment of lamellae along the axis of the bridge due to flow, facilitating unimpeded diffusion of polymer out of the liquid bridge along lamellar boundaries.

Prospective and retrospective high order eddy current mitigation for diffusion weighted echo planar imaging.

PURPOSE: The proposed method is aimed at reducing eddy current (EC) induced distortion in diffusion weighted echo planar imaging, without the need to perform further image coregistration between diffusion weighted and T2 images. These ECs typically have significant high order spatial components that cannot be compensated by preemphasis. THEORY AND METHODS: High order ECs are first calibrated at the system level in a protocol independent fashion. The resulting amplitudes and time constants of high order ECs can then be used to calculate imaging protocol specific corrections. A combined prospective and retrospective approach is proposed to apply correction during data acquisition and image reconstruction. RESULTS: Various phantom, brain, body, and whole body diffusion weighted images with and without the proposed method are acquired. Significantly reduced image distortion and misregistration are consistently seen in images with the proposed method compared with images without. CONCLUSION: The proposed method is a powerful (e.g., effective at 48 cm field of view and 30 cm slice coverage) and flexible (e.g., compatible with other image enhancements and arbitrary scan plane) technique to correct high order ECs induced distortion and misregistration for various diffusion weighted echo planar imaging applications, without the need for further image post processing, protocol dependent prescan, or sacrifice in signal-to-noise ratio.

Film thickness dependent ordering dynamics of lamellar forming diblock copolymer thin films.

Ellipsometry is used in a novel way to study the ordering dynamics of symmetric poly(styrene-methyl methacrylate) diblock copolymer thin films. Ordered thin films form lamellae parallel to the substrate which can form islands or holes at the free surface to ensure commensurability of the layers. The sensitivity of ellipsometry provides the unique ability to probe morphological changes during the ordering process before the ultimate formation of islands or holes at the free surface. We observe three distinct stages in the ordering process: i) an ordering into an intermediate state, ii) an incubation time where the film structure remains constant and iii) the nucleation of islands or holes to achieve equilibrium lamellar morphology. The time-resolved measurement of an incubation period and initial ordering stage provides a means for studying the effect of thickness on the ordering kinetics. The dependence of incubation time on the commensurability of the initial film height is explained using strong segregation theory.

Broadband phase and intensity compensation with a deformable mirror for an interferometric nuller.

Nulling interferometry has been proposed for the direct detection of Earth-like planets. Deep stable nulls require careful control of the relative intensity and phase of the beams that are being combined. We present a novel compensator, the Adaptive Nuller, that corrects the intensity and phase as a function of wavelength from 8 to 12 microm using a deformable mirror. This compensator has been used to produce rejection ratios of 82,000:1 over a bandwidth of 3.2 microm centered around 10 microm.

Interobserver and Test-Retest Reproducibility of T1ρ and T2 Measurements of Lumbar Intervertebral Discs by 3T Magnetic Resonance Imaging.

OBJECTIVE: To investigate the interobserver and test-retest reproducibility of T1ρ and T2 measurements of lumbar intervertebral discs using 3T magnetic resonance imaging (MRI). MATERIALS AND METHODS: This study included a total of 51 volunteers (female, 26; male, 25; mean age, 54 ± 16.3 years) who underwent lumbar spine MRI with a 3.0 T scanner. Amongst these subjects, 40 underwent repeat T1ρ and T2 measurement acquisitions with identical image protocol. Two observers independently performed the region of interest measurements in the nuclei pulposi of the discs from L1-2 through L5-S1 levels. Statistical analysis was performed using intraclass correlation coefficient (ICC) with a two-way random model of absolute agreement. Comparison of the ICC values was done after acquisition of ICC values using Z test. Statistical significance was defined as p value < 0.05. RESULTS: The ICCs of interobserver reproducibility were 0.951 and 0.672 for T1ρ and T2 mapping, respectively. The ICCs of test-retest reproducibility (40 subjects) for T1ρ and T2 measurements were 0.922 and 0.617 for observer A and 0.914 and 0.628 for observer B, respectively. In the comparison of the aforementioned ICCs, ICCs of interobserver and test-retest reproducibility for T1ρ mapping were significantly higher than T2 mapping (p < 0.001). CONCLUSION: The interobserver and test-retest reproducibility of T1ρ mapping were significantly higher than those of T2 mapping for the quantitative assessment of nuclei pulposi of lumbar intervertebral discs.

Oculomotor responses during partial and total sleep deprivation.

INTRODUCTION: Oculomotor responses related to the pupil light reflex (PLR) and saccadic velocity may be sensitive to the effects of sleepiness and therefore could be used to evaluate an individual's fitness for duty. METHODS: There were 12 normal subjects who completed an 8-d study. They were allowed 8 h in bed on the first three nights, 4 h in bed on the fourth night, and then were sleep deprived for the following 64 h. Approximately every 3 h, subjects performed a battery of tests which included a 45-s automated oculomotor test and a 40-min PC-based driving simulator task. Sleepiness was evaluated with a self-assessment instrument. Subjects were allowed 10 h of recovery sleep following sleep deprivation. RESULTS: Oculomotor results for nine subjects showed a significant increase in latency to pupil constriction and a significant decrease in saccadic velocity with total, but not partial, sleep deprivation. The most robust changes during sleep deprivation occurred for saccadic velocity. A night of recovery sleep reversed the effects of total sleep deprivation on latency to pupil constriction and saccadic velocity. Subjective sleepiness and off-road accidents were found to significantly increase over the sleep deprivation period. A significant positive correlation between increasing latency to pupil constriction and increasing sleepiness and driving accidents, and a significant negative correlation between decreasing saccadic velocity and increasing sleepiness and driving accidents during sleep deprivation were found. CONCLUSION: These findings suggest that oculomotor functions, particularly saccadic velocity, are feasible for assessing neurophysiological changes associated with and predictive of sleep deprivation-induced operational performance degradation.

Morphology Induced Spinodal Decomposition at the Surface of Symmetric Diblock Copolymer Films.

Atomic force microscopy is used to study the ordering dynamics of symmetric diblock copolymer films. The films order to form a lamellar structure which results in a frustration when the film thickness is incommensurate with the lamellae. By probing the morphology of incommensurate films in the early ordering stages, we discover an intermediate phase of lamellae arranged perpendicular to the film surface. This morphology is accompanied by a continuous growth in amplitude of the film surface topography with a characteristic wavelength, indicative of a spinodal process. Using self-consistent field theory, we show that the observation of perpendicular lamellae suggests an intermediate state with parallel lamellae at the substrate and perpendicular lamellae at the free surface. The calculations confirm that the intermediate state is unstable to thickness fluctuations, thereby driving the spinodal growth of surface structures.