Studying the aging of Laponite suspensions using extensional rheology.

The effect of aging on the break-up dynamics of Laponite suspensions was studied in an extensional geometry. It was found that samples of increased age undergo stronger necking at the midpoint. The thinning of samples, driven purely by motion of the plates, was compared with standard shear rheology to understand how the dynamics are related to the sample properties. The Laponite suspensions exhibit a growing stress overshoot with monotonically decreasing yield strain as they age. However, it is shown that the thinning curves in extension are only a good indicator of the sample's static yield stress, being insensitive to its yield strain. These measurements suggest that following an initial linear visco-elastic regime, samples accumulate significant plastic deformations prior to the complete yielding of the sample. The implications of this for the importance of assessing changes to the ductile-brittle nature of samples are also discussed.

Slip in adhesion tests of a Kaolin clay.

Adhesion tests were performed on concentrated suspensions of Kaolin clay. At low concentrations samples formed conical deposits on both the top and bottom plates with the central region narrowing to a filament before undergoing breakup. In contrast high concentration samples deformed as a cylinder before apparently fracturing into two pieces. As the concentration of the samples was increased the samples underwent quite different forms of slip which it is shown can be deduced from their respective force distance curves. The type of slip behaviour for a given concentration of clay could be modified with changes to surface roughness, the initial compressive load prior to an experiment and with the separation velocity of the plates. The different slip characteristics appear to arise from the concentration dependent way in which particles interact with the rough surface topography.

Collision-enhanced friction of a bouncing ball on a rough vibrating surface.

We describe experiments and simulations to investigate the dynamics of a ball bouncing on a rough vibrating surface. Directly measuring the impulse due to each bounce we find that the frictional interaction with the surface is strongly enhanced near to the side wall. The enhanced dissipation arises as a consequence of the coupling between the collision, rotation and surface friction. This dissipation, which for our experimental conditions was estimated to be up to three times larger than the more obvious inelastic collision, can result in an enhanced probability density near boundaries and particle-particle spatial correlations. Our findings imply that the effective particle collision properties cannot be considered independently of the surface's frictional properties.

Topographic Control of Order in Quasi-2D Granular Phase Transitions.

We experimentally investigate the nature of 2D phase transitions in a quasi-2D granular fluid. Using a surface decorated with periodically spaced dimples we observe interfacial tension between coexisting granular liquid and crystal phases. Measurements of the orientational and translational order parameters and associated susceptibilities indicate that the surface topography alters the order of the phase transition from a two-step continuous one to a first-order liquid-solid one. The interplay of boundary inelasticity and geometry, either order promoting or inhibiting, controls whether it is the granular crystal or the granular fluid which makes contact with the edge. This order induced wetting has important consequences, determining how coexisting phases separate spatially.

Boundary effects in a quasi-two-dimensional driven granular fluid.

The effect of a confining boundary on the spatial variations in granular temperature of a driven quasi-two-dimensional layer of particles is investigated experimentally. The radial drop in the relative granular temperature ΔT/T exhibits a maximum at intermediate particle numbers which coincides with a crossover from kinetic to collisional transport of energy. It is also found that at low particle numbers, the distributions of radial velocities are increasingly asymmetric as one approaches the boundary. The radial and tangential granular temperatures split, and in the tails of the radial velocity distribution there is a higher population of fast moving particles traveling away rather than towards the boundary.

Fracture of Jammed Colloidal Suspensions.

Concentrated colloidal suspensions display dramatic rises in viscosity, leading to jamming and granulation, with increasing shear rate. It has been proposed that these effects result from inter particle friction, as lubrication forces are overcome. This suggests the jamming of concentrated colloidal suspensions should exhibit some shared phenomenology with macroscopic granular systems where friction leads to two different types of jammed state. Here we show that transient rheological measurements can be used to probe the processes of granulation in concentrated colloidal suspensions. Our results support the idea that frictional contacts are created between jammed particles. The jamming behaviour displays two qualitatively different regimes separated by a critical strain rate with qualitatively different types of fracture/break up behaviour. In the lower strain rate regime, it is found that vibrations can be used to control jamming and granulation, resulting in a flowable fluid.

Origin of contact line forces during the retraction of dilute polymer solution drops.

The forced dewetting of water and dilute poly(ethylene oxide) solution (PEO) drops is investigated for syringe-driven flow. Comparisons are made with the free dewetting observed during drop impact. We provide strong evidence that during droplet retraction, polymer deposited on the substrate results in a velocity-dependent force at the contact line. These findings are in stark contrast to previous studies which attributed dissipation to bulk viscoelastic effects or normal stress effects at the contact line.

Effects of substrate constraint on crack pattern formation in thin films of colloidal polystyrene particles.

Crack formation and the evolution of stress in drying films of colloidal particles were studied using optical microscopy and a modified cantilever deflection technique, respectively. Drying experiments were performed using polystyrene particles with diameters of 47 ± 10 nm, 100 ± 16 nm, and 274 ± 44 nm that were suspended in water. As the films dried, cracks with a well-defined spacing were observed to form. The crack spacing was found to be independent of the particle size used, but to increase with the film thickness. The characteristic crack spacing was found to vary between 20 and 300 µm for films with thickness values in the range 3-70 µm. Cantilever deflection measurements revealed that the stresses that develop in the film increase with decreasing film thickness (increasing surface-to-volume ratio). The latter observation was interpreted in terms of the effects of a substrate constraint which causes the build up of stresses in the films. This interpretation was confirmed by crack formation experiments that were performed on liquid mercury surfaces in which removal of the substrate constraint prevented crack formation. Experiments were also performed on compliant elastomer surfaces in which the level of constraint was varied by changing the substrate modulus. The cracking length scale was found to increase with decreasing substrate modulus. A simple theory was also developed to describe the substrate modulus dependence of the cracking length scale. These combined experiments and theory provide convincing evidence that substrate constraints are an important factor in driving crack formation in thin colloidal films.

Dilatancy in the flow and fracture of stretched colloidal suspensions.

Concentrated particulate suspensions, commonplace in the pharmaceutical, cosmetic and food industries, display intriguing rheology. In particular, the dramatic increase in viscosity with strain rate (shear thickening and jamming), which is often observed at high-volume fractions, is of practical and fundamental importance. Yet, manufacture of these products and their subsequent dispensing often involves flow geometries substantially different from that of simple shear flow experiments. In this study, we show that the elongation and breakage of a filament of a colloidal fluid under tensile loading is closely related to the jamming transition seen in its shear rheology. However, the modified flow geometry reveals important additional effects. Using a model system with nearly hard-core interactions, we provide evidence of surprisingly strong viscoelasticity in such a colloidal fluid under tension. With high-speed photography, we also directly observe dilatancy and granulation effects, which lead to fracture above a critical elongation rate.

Effect of polymer additives on the wetting of impacting droplets.

When a droplet of water impacts a hydrophobic surface, the drop is often observed to bounce. However, for about 10 years it has been known that the addition of very small quantities (approximately 100 ppm) of a flexible polymer such as poly-(ethylene oxide) can completely prevent rebound. This effect has for some time been explained in terms of the stretching of polymer chains by a velocity gradient in the fluid, resulting in a transient increase in the so-called "extensional viscosity." Here we show, by measuring the fluid velocity inside the impacting drop, that the extensional viscosity plays no role in the antirebound phenomenon. Using fluorescently labeled lambda DNA we demonstrate that the observed effect is due to the stretching of polymer molecules as the droplet edge sweeps the substrate, retarding the movement of the receding contact line.

Insulin fibril nucleation: the role of prefibrillar aggregates.

Dynamic light scattering and Fourier transform infrared spectroscopy were used to study the formation of prefibrillar aggregates and fibrils of bovine pancreatic insulin at 60 degrees C and at pH 1. The kinetics of disintegration of the prefibrillar aggregates were also studied using these techniques after a quench to 25 degrees C. These experiments reveal that formation of prefibrillar aggregates is reversible under the solution conditions studied and show that it is possible to significantly reduce the nucleation (lag) times associated with the onset of fibril growth in bovine pancreatic insulin solutions by increasing the concentration of prefibrillar aggregates in solution. These results provide convincing evidence that less structured prefibrillar aggregates can act as fibril-forming intermediates.

Nucleation and growth of insulin fibrils in bulk solution and at hydrophobic polystyrene surfaces.

A technique was developed for studying the nucleation and growth of fibrillar protein aggregates. Fourier transform infrared and attenuated total reflection spectroscopy were used to measure changes in the intermolecular beta-sheet content of bovine pancreatic insulin in bulk solution and on model polystyrene (PS) surfaces at pH 1. The kinetics of beta-sheet formation were shown to evolve in two stages. Combined Fourier transform infrared, dynamic light scattering, atomic force microscopy, and thioflavin-T fluorescence measurements confirmed that the first stage in the kinetics was related to the formation of nonfibrillar aggregates that have a radius of 13 +/- 1 nm. The second stage was found to be associated with the growth of insulin fibrils. The beta-sheet kinetics in this second stage were used to determine the nucleation and growth rates of fibrils over a range of temperatures between 60 degrees C and 80 degrees C. The nucleation and growth rates were shown to display Arrhenius kinetics, and the associated energy barriers were extracted for fibrils formed in bulk solution and at PS surfaces. These experiments showed that fibrils are nucleated more quickly in the presence of hydrophobic PS surfaces but that the corresponding fibril growth rates decrease. These observations are interpreted in terms of the differences in the attempt frequencies and energy barriers associated with the nucleation and growth of fibrils. They are also discussed in the context of differences in protein concentration, mobility, and conformational and colloidal stability that exist between insulin molecules in bulk solution and those that are localized at hydrophobic PS interfaces.

Endoplasmic reticulum stress-induced apoptosis requires bax for commitment and Apaf-1 for execution in primary neurons.

Apoptosis triggered by endoplasmic reticulum (ER) stress is associated with various pathophysiological conditions including neurodegenerative diseases and ischemia. However, the mechanism by which ER stress induces neuronal apoptosis remains controversial. Here we identify the pathway of apoptosis carried out in sympathetic neurons triggered to die by ER stress-inducing agent tunicamycin. We find that ER stress induces a neuronal apoptotic pathway which upregulates BH3-only genes DP5 and Puma. Importantly, we show that ER stress commits neurons to die before cytochrome c release and this commitment requires Bax activation and c-jun N-terminal kinase signaling. Furthermore, ER stress engages the mitochondrial pathway of death as neurons release cytochrome c and Apaf-1 deficiency is sufficient to block apoptosis. Our findings identify a critical function of Bax in committing neurons to ER stress-induced apoptosis and clarify the importance of the apoptosome as the non-redundant caspase activation pathway to execute neuronal apoptosis in response to ER stress.

Spinodal wrinkling in thin-film poly(ethylene oxide)/polystyrene bilayers.

Optical microscopy and atomic force microscopy were used to study a novel roughness-induced wrinkling instability in thin-film bilayers of poly(ethylene oxide) (PEO) and polystyrene (PS). The observed wrinkling morphology is manifested as a periodic undulation at the surface of the samples and occurs when the bilayers are heated above the melting temperature of the semi crystalline PEO (T(m) = 63 Celsius) layer. During the wrinkling of the glassy PS capping layers the system selects a characteristic wavelength that has the largest amplitude growth rate. This initial wavelength is shown to increase monotonically with increasing thickness of the PEO layer. We also show that for a given PEO film thickness, the wavelength can be varied independently by changing the thickness of the PS capping layers. A model based upon a simple linear stability analysis was developed to analyse the data collected for the PS and PEO film thickness dependences of the fastest growing wavelength in the system. The predictions of this theory are that the strain induced in the PS layer caused by changes in the area of the PEO/PS interface during the melting of the PEO are sufficient to drive the wrinkling instability. A consideration of the mechanical response of the PEO and PS layers to the deformations caused by wrinkling then allows us to use this simple theory to predict the fastest growing wavelength in the system.

Evidence implicating a role for orexin-1 receptor modulation of paradoxical sleep in the rat.

The neuropeptide orexin-A modulates the sleep-wake cycle such that central administration to rats increases arousal, reduces slow-wave-sleep (SWS) and paradoxical sleep (PS) and delays PS onset. The contribution of orexin-1 and -2 receptor (OXR) activation to this orexin-A response is still unknown. Using the OX(1)R antagonist SB-334867-A we investigated the role of this receptor in orexin-A-induced PS alteration. Male rats prepared for frontal-occipital electroencephalograph, nuchal muscle electromyograph recording and lateral ventricle cannulae received vehicle or orexin-A (10 microg icv) at lights on in combination with vehicle or SB-334867-A (10 or 30 mg/kg ip) 30 min pre-icv injection. The amount of arousal, SWS 1, SWS 2 and PS was determined during the 1st h post icv administration along with the latency to onset of the first> or =10 s epoch of PS. Orexin-A administration reduced the amount and increased the latency to onset of PS. SB-334867-A reversed this effect of orexin-A. The present study demonstrates that the OX(1)R also has a role in orexinergic sleep modulation.

Effect of SB-243213, a selective 5-HT(2C) receptor antagonist, on the rat sleep profile: a comparison to paroxetine.

5-HT(2) receptor antagonists promote slow wave sleep (SWS) in humans and rats, conversely 5-HT(2) agonists inhibit SWS in rats. These alterations are thought to be predominantly mediated via the 5-HT(2C) receptor subtype. It is evident that 5-HT(2) receptor function also plays an important role in depression. Here, we examine the acute effect of the selective 5-HT(2C) receptor antagonist 5-methyl-1-[[-2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-6-triflouromethylindoline hydrochloride (SB-243213-A) on rat sleep in comparison to the selective serotonin reuptake inhibitor (SSRI) paroxetine. Both SB-243213-A (10 mg/kg po) and paroxetine (3 mg/kg po) significantly increased deep SWS (SWS2) quantity (27% and 24%, respectively) and reduced paradoxical sleep (PS) quantity (35%) during the sleep period. Following SB-243213-A, SWS2 occurrence frequency was reduced (24.1%); however, elevated quantity of SWS2 can be attributed to an increase in occurrence duration (81%). Reduced PS quantity results from a decrease in occurrence frequency (46%). In comparison, paroxetine increased SWS2 occurrence frequency (50%), with decreased frequency (27%) and duration (21%) of PS. The data for SB-243213-A in the present study is consistent with that following ritanserin supporting 5-HT(2C) receptor subtype mediation of this response. The similar effect of SB-243213-A to paroxetine with regard to PS quantity provides further evidence that 5-HT(2C) receptor antagonists maybe beneficial in the treatment of depression/anxiety.

Diffusion-weighted MRI used to detect in vivo modulation of cortical spreading depression: comparison of sumatriptan and tonabersat.

Spreading cortical depolarization and depression of electroencephalographic activity (SD) may underlie the aura and spreading neurovascular events of migraine. Cortical depolarization may also precipitate the progressive development of cerebral pathology following ischemia. However, data on SD in the human brain are sparse, most likely reflecting the technical difficulties involved in performing such clinical studies. We have previously shown that the transient cerebral water disturbances during SD can be quantitatively investigated in the gyrencephalic brain using repetitive diffusion-weighted magnetic resonance imaging (DWI). To investigate whether DWI could detect modulation of the spatiotemporal properties of SD in vivo, the effects of the antimigraine drug sumatriptan (0.3 mg/kg iv) and the novel anticonvulsant tonabersat (10 mg/kg ip) were evaluated in the cat brain. Supporting previous findings, sumatriptan did not affect the numbers of events (range, 4-8), the duration of SD activity (39.8 +/- 4.4 min, mean +/- SEM), and event velocity (2.2 +/- 0.4 mm min(-1)); tonabersat significantly reduced SD event initiation (range, 0-3) and duration (13.2 +/- 5.0 min) and increased primary event velocity (5.4 +/- 0.7 mm min(-1)). However, both drugs significantly decreased, by >50%, the spatial extent of the first KCl-evoked SD event, and sumatriptan significantly increased event propagation across the suprasylvian sulcus (5.5 +/- 0.6 vs 2.4 +/- 0.4 events in controls). These results demonstrate (1) the feasibility of using DWI to evaluate therapeutic effects on SD, and (2) that sumatriptan may directly modulate the spatial distribution of SD activity in the gyrencephalic brain.

Investigation of feline brain anatomy for the detection of cortical spreading depression with magnetic resonance imaging.

Cortical spreading depression (CSD) and peri-infarct depolarisation (PID) are related phenomena that have been associated with the human clinical syndromes of migraine (CSD), head injury and stroke (PID). Nevertheless the existence of CSD in man remains controversial, despite the detection of this phenomenon in the brains of most, if not all, other animal species investigated. This failure to unambiguously detect CSD clinically may be at least partly due to the anatomically complex, gyrencephalic structure of the human brain. This study was designed to establish conditions for the study of CSD in the brain of a gyrencephalic species using the noninvasive technique of magnetic resonance imaging (MRI). The 3-dimensional (3D) gyrencephalic anatomy of the cat brain was examined to determine the imaging conditions necessary to detect CSD events. Orthogonal transverse, sagittal and horizontal T1-weighted image slices showed that the marginal and suprasylvian gyri were the most appropriate cortical structures to study CSD. This was in view of (1) their simple geometry: (2) their lengthy extent of grey matter orientated rostrocaudally in the cortex: (3) their separation by a sulcus across which CSD spread could be studied and (4) the discontinuity in the grey matter in these regions between the right and left hemispheres dorsal to the corpus callosum. The structure suggested by the T1-weighted images was corroborated by systematic diffusion tensor imaging to map the fractional anisotropy and diffusion trace. Thus a single horizontal image plane could visualise the neighbouring suprasylvian and marginal gyri of both cerebral hemispheres, whereas its complex shape and position ruled out the ectosylvian gyrus for CSD studies. With the horizontal imaging plane, CSD events were reproducibly detected by animating successive diffusion-weighted MR images following local KCl stimulation of the cortical surface. In single image frames, CSD detection and characterisation required image subtraction or statistical mapping methods that, nevertheless, yielded concordant results. In repeat experiments, CSD events were qualitatively similar in appearance whether elicited by sustained or transient KCl applications. Our experimental approach thus successfully describes cat brain anatomy in vivo, and elucidates the necessary conditions for the application of MRI methods to detect CSD propagation.

Repetitive cortical spreading depression in a gyrencephalic feline brain: inhibition by the novel benzoylamino-benzopyran SB-220453.

Transient cortical depolarization is implicated in the pathology of migraine. SB-220453 is a potent anti-convulsant which inhibits neurogenic inflammation and cortical spreading depression (SD)-evoked nitric oxide release via a novel but unknown mechanism. This study further investigates the effects of SB-220453 on generation and propagation of repetitive SD in the anaesthetized cat. Vehicle or SB-220453 1, 3 or 10 mg/kg was administered intraperitoneally 90 min prior to induction of SD in the suprasylvian gyrus (SG). Changes in d.c. potential were recorded in the SG and the adjacent marginal gyrus (MG). In vehicle-treated animals (n = 7), a brief exposure (6 min) to KCl induced a median (25-75% range) number of five (four to six) and three (two to four) depolarizations over a duration of 55 min (32-59 min) and 51 min (34-58 min) in the SG and MG, respectively. SB-220453 produced dose-related inhibition of the number of events and period of repetitive SD activity. SB-220453 also reduced SD-induced repetitive pial vasodilatation but had no effect on resting haemodynamics. However, when SD events were observed in the presence of SB-220453, it had no effect on metabolic coupling. These results show that SB-220453 produces marked inhibition of repetitive SD in the anaesthetized cat. SB-220453 may therefore have therapeutic potential in treatment of SD-like activity in migraine.

A quantitative analysis of cortical spreading depression events in the feline brain characterized with diffusion-weighted MRI.

Cortical spreading depression (CSD) in the gyrencephalic cat brain was detected with diffusion-weighted echoplanar (DWEP) magnetic resonance imaging (4-8/min for 1-2 hours) using a horizontal imaging plane through the suprasylvian (SG) and marginal gyri. A t-statistic mapping technique allowed a quantitative characterization of the passage of events through single-image pixels (0.15 mm(2)), thus providing a resolution unavailable to previous studies in which time-dependent changes instead were derived from averaging data over relatively large ROIs. Using the enhanced analysis, CSD events initiated by KCl could be quantified for the first time as primary or secondary according to their spatial and temporal features. Primary events covered 26.2 +/- 9.9 mm(2)of cortical surface (mean +/- SD, n = 7 experiments) and propagated rapidly (3.5 +/- 0.65 mm * min(-1)) with a hemispherical geometry. In contrast, the subsequent secondary events were multiple, spatially restricted (covering 7.6 +/- 4.6 mm(2), P < 0.005), slower in propagation (2.6 +/- 0.41 mm * min(-1), P < 0.012), and often confined to the originating gyrus (26 out of 59 events). However, both event types were associated with significantly reduced apparent diffusion coefficients (ADCs; from 800 to approximately 660 x 10(-6) mm(2)* s(-1), P < 0.05) that were similar for both primary (21 +/- 5.1%) and secondary waves (18 +/- 7. 7%) and that had similar durations (full width at half-maximal height: 86 +/- 17 vs. 79 +/- 20 seconds, respectively). These findings associate CSD for the first time with two categories of ADC disturbance that are similar in amplitude and duration but that differ in spatial extent, velocity, and extensiveness of spread.