Using Dynamic Light Scattering in heterodyne mode, we measure velocity profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to exhibit both shear-banding and stress plateau behavior. Our data provide evidence for the simplest shear-banding scenario, according to which the effective viscosity drop in the system is due to the nucleation and growth of a highly sheared band in the gap, whose thickness linearly increases with the imposed shear rate. We discuss various details of the velocity profiles in all the regions of the flow curve and emphasize on the complex, non-Newtonian nature of the flow in the highly sheared band.PACS numbers: 83.80.Qr, 47.50.+d, 83.85.Ei Understanding the correlation between mechanical and structural response in non-Newtonian fluids submitted to high deformation rates is crucial on both fondamental and technological grounds [1]. Among the variety of complex fluids investigated in recent years, a wide class exhibits flow-structure coupling that leads to a strong shear-thinning behavior: along the steady-state flow curve (shear stress σ vs. shear rateγ), a drop of up to three orders of magnitude in the effective viscosity η = σ/γ is observed in a very narrow stress range leading to a stress plateau (for a review, see for instance Refs. [1,2]). In correlation with this stress plateau, bands of different micro-structures and normal to the velocity gradient appear. Such bands correspond to a new shearinduced structure (SIS), whose low viscosity is in general supposed to be responsible for the shear-thinning. This so-called shear-banding behavior has been observed in both ordered mesophases (lamellar, hexagonal, cubic) [3] and transient gels [4].A particularly well-documented example is the group of wormlike micellar systems of self-assembled surfactant molecules [5,6]. They consist of very long cylindrical aggregates whose configurations mimic polymer solutions. However their dynamics is strongly modified by the equilibrium character of the chains, which enables them to break and recombine [7]. Generically, one starts from an isotropic viscoelastic solution of these micelles above the semidilute regime, which behaves like a Maxwell fluid at low shear rates. Upon increasingγ and entering the nonlinear regime, the onset of the stress plateau for a critical shear rateγ 1 is associated with the nucleation and growth of highly birefringent bands, suggesting strong alignment of the micelles along the velocity direction [5,6]. As the shear rate is further increased aboveγ 1 , the new organization progressively fills the gap at almost constant stress, up to a second critical shear rateγ 2 . Aboveγ 2 , the system enters a second regime of apparently homogeneous structure, with a second branch of increasing stress. The flow curve of Fig. 1 is typical of micellar systems like that investigated in the present work. Such a stress plateau has been reported for concentrations close to the equilibrium isotropic-nematic (I-N) transition, where coupling between the order parameter an...
To maintain homeostasis, hypothalamic neurons in the arcuate nucleus must dynamically sense and integrate a multitude of peripheral signals. Blood-borne molecules must therefore be able to circumvent the tightly sealed vasculature of the blood-brain barrier to rapidly access their target neurons. However, how information encoded by circulating appetite-modifying hormones is conveyed to central hypothalamic neurons remains largely unexplored. Using in vivo multiphoton microscopy together with fluorescently labeled ligands, we demonstrate that circulating ghrelin, a versatile regulator of energy expenditure and feeding behavior, rapidly binds neurons in the vicinity of fenestrated capillaries, and that the number of labeled cell bodies varies with feeding status. Thus, by virtue of its vascular connections, the hypothalamus is able to directly sense peripheral signals, modifying energy status accordingly.hormone diffusion | in vivo imaging | median eminence | metabolism C ontinuous integration of peripheral signals by neurons belonging to the arcuate nucleus of the hypothalamus (ARH) is critical for central regulation of energy balance and neuroendocrine function (1). To dynamically report alterations to homeostasis and ensure an appropriate neuronal response, blood-borne factors such as hormones must rapidly access the central nervous system (CNS). This is particularly evident in the case of food intake, which is regulated by a plethora of circulating satiety signals (2) whose levels fluctuate in an ultradian manner. Despite this, it remains unclear how key energy status-signaling hormones such as ghrelin can be rapidly sensed by target neurons to alter feeding responses (3). Elucidation of the mechanisms underlying molecule entry into the brain is important for understanding not only normal maintenance of homeostasis but also how this is perturbed during common pathologies such as obesity and diabetes (4, 5).Although molecule transport mechanisms within the ARH are poorly characterized, they likely assume one of two forms. First, chronic feedback may be accomplished by uptake of circulating molecules into the ARH via saturable receptor-mediated transport at the level of the choroid plexus and/or bloodbrain barrier (BBB) (6-9). Second, the ARH is morphologically located in close apposition to the median eminence (ME), a circumventricular organ composed of fenestrated capillaries. Because these vessels project toward the ventromedial ARH (vmARH), they could represent a direct vascular input for passive diffusion of peripheral molecules into the hypothalamus (10-13). So far, study of the functional importance of fenestrated capillaries in molecule entry into the metabolic brain has been impeded by lack of appropriate tools.To evaluate the role of fenestrated ME/ARH capillaries in rapid detection of peripheral signals by the hypothalamus, we used a recently developed in vivo imaging approach to visualize in real time the extravasation of fluorescent molecules (14). Ghrelin was chosen as a candidate hormone because i...
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