Activation volume of selected liquid crystals in the density scaling regime

In this paper, we demonstrate and thoroughly analyze the activation volumetric properties of selected liquid crystals in the nematic and crystalline E phases in comparison with those reported for glass-forming liquids. In the analysis, we have employed and evaluated two entropic models (based on either total or configurational entropies) to describe the longitudinal relaxation times of the liquid crystals in the density scaling regime. In this study, we have also exploited two equations of state: volumetric and activation volumetric ones. As a result, we have established that the activation volumetric properties of the selected liquid crystals are quite opposite to such typical properties of glass-forming materials, i.e., the activation volume decreases and the isothermal bulk modulus increases when a liquid crystal is isothermally compressed. Using the model based on the configurational entropy, we suggest that the increasing pressure dependences of the activation volume in isothermal conditions and the negative curvature of the pressure dependences of isothermal longitudinal relaxation times can be related to the formation of antiparallel doublets in the examined liquid crystals. A similar pressure effect on relaxation dynamics may be also observed for other material groups in case of systems, the molecules of which form some supramolecular structures

Direct vs. indirect optical recombination in Ge films grown on Si substrates

The optical emission spectra from Ge films on Si are markedly different from their bulk Ge counterparts. Whereas bulk Ge emission is dominated by the material's indirect gap, the photoluminescence signal from Ge films is mainly associated with its direct band gap. Using a new class of Ge-on-Si films grown by a recently introduced CVD approach, we study the direct and indirect photoluminescence from intrinsic and doped samples and we conclude that the origin of the discrepancy is the lack of self-absorption in thin Ge films combined with a deviation from quasi-equilibrium conditions in the conduction band. The latter is confirmed by a simple model suggesting that the deviation from quasi-equilibrium is caused by the much shorter recombination lifetime in the films relative to bulk Ge

Defining rational hospital catchments for non-urban areas based on travel-time

BACKGROUND: Cost containment typically involves rationalizing healthcare service delivery through centralization of services to achieve economies of scale. Hospitals are frequently the chosen site of cost containment and rationalization especially in rural areas. Socio-demographic and geographic characteristics make hospital service allocation more difficult in rural and remote regions. This research presents a methodology to model rational catchments or service areas around rural hospitals – based on travel time. RESULTS: This research employs a vector-based GIS network analysis to model catchments that better represent access to hospital-based healthcare services in British Columbia's rural and remote areas. The tool permits modelling of alternate scenarios in which access to different baskets of services (e.g. rural maternity care or ICU) are assessed. In addition, estimates of the percentage of population that is served – or not served -within specified travel times are calculated. CONCLUSION: The modelling tool described is useful for defining true geographical catchments around rural hospitals as well as modelling the percentage of the population served within certain time guidelines (e.g. one hour) for specific health services. It is potentially valuable to policy makers and health services allocation specialists

The mosaic of surface charge in contact electrification

When dielectric materials are brought into contact and then separated, they develop static electricity. For centuries, it has been assumed that such contact charging derives from the spatially homogeneous material properties (along the material's surface) and that within a given pair of materials, one charges uniformly positively and the other negatively. We demonstrate that this picture of contact charging is incorrect. Whereas each contact-electrified piece develops a net charge of either positive or negative polarity, each surface supports a random "mosaic" of oppositely charged regions of nanoscopic dimensions. These mosaics of surface charge have the same topological characteristics for different types of electrified dielectrics and accommodate significantly more charge per unit area than previously thought

Saturation of front propagation in a reaction-diffusion process describing plasma damage in porous low-k materials

We propose a three-component reaction-diffusion system yielding an asymptotic logarithmic time-dependence for a moving interface. This is naturally related to a Stefan-problem for which both one-sided Dirichlet-type and von Neumann-type boundary conditions are considered. We integrate the dependence of the interface motion on diffusion and reaction parameters and we observe a change from transport behavior and interface motion \sim t^1/2 to logarithmic behavior \sim ln t as a function of time. We apply our theoretical findings to the propagation of carbon depletion in porous dielectrics exposed to a low temperature plasma. This diffusion saturation is reached after about 1 minute in typical experimental situations of plasma damage in microelectronic fabrication. We predict the general dependencies on porosity and reaction rates.Comment: Accepted for publication in Phys. Rev.

Optical coherence tomography in patients with chronic migraine: Literature review and update

Migraine is a chronic disease characterized by unilateral, pulsating, and often moderate-to-severe recurrent episodes of headache with nausea and vomiting. It affects approximately 15% of the general population, yet the underlying pathophysiological mechanisms are not fully understood. Optical coherence tomography (OCT) is a safe and reproducible diagnostic technique that utilizes infrared wavelengths and has a sensitivity of 8-10 µm. It can be used to measure thinning of the retinal nerve fiber layer (RNFL) in some neurological disorders. Although ophthalmologists are often the first specialists to examine patients with migraine, few studies have addressed the involvement of the optic nerve and retino-choroidal structures in this group. We reviewed the literature on the etiological and pathological mechanisms of migraine and the relationship between recurrent constriction of cerebral and retrobulbar vessels and ischemic damage to the optic nerve, retina, and choroid. We also assessed the role of OCT for measuring peripapillary RNFL thickness and macular and choroidal changes in migraine patients. There is considerable evidence of cerebral and retrobulbar vascular involvement in the etiology of migraine. Transitory and recurrent constriction of the retinal and ciliary arteries may cause ischemic damage to the optic nerve, retina, and choroid in patients with migraine. OCT to assess the thickness of the peripapillary RNFL, macula, and choroid might increase our understanding of the pathophysiology of migraine and facilitate diagnosis of retino-choroidal compromise and follow-up of therapy in migraine patients. Future studies should determine the usefulness of OCT findings as a biomarker of migraine