Sliding Dynamics of Parallel Graphene Sheets: Effect of Geometry and Van Der Waals Interactions on Nano-Spring Behavior

Graphene and carbon nanotubes are promising materials for nanoelectromechanical systems. Among other aspects, a proper understanding of the sliding dynamics of parallel graphene sheets or concentric nanotubes is of crucial importance for the design of nano-springs. Here, we analytically investigate the sliding dynamics between two parallel, rigid graphene sheets. In particular, the analysis focuses on configurations in which the distance between the sheets is kept constant and lower than the equilibrium interlayer spacing of graphite (unstable configurations). The aim is to understand how the interlayer force due to van der Waals interactions along the sliding direction changes with the geometrical characteristics of the configuration, namely size and interlayer spacing. Results show metastable equilibrium positions with completely faced sheets, namely a null force along the sliding direction, whereas net negative/positive forces arise when the sheets are approaching/leaving each other. This behavior resembles a molecular spring, being able to convert kinetic into potential energy (van der Waals potential), and viceversa. The amplitude of both storable energy and entrance/exit forces is found to be proportional to the sheet size, and inversely proportional to their interlayer spacing. This model could also be generalized to describe the behavior of configurations made of concentric carbon nanotubes, therefore allowing a rational design of some elements of carbon-based nanoelectromechanical systems

An overview on the use of additives and preparation procedure in phase change materials for thermal energy storage with a focus on long term applications

In this review we aim at providing an up-to-date and comprehensive overview on the use of additives within selected Phase Change Materials (PCMs) from both an experimental and more theoretical perspective. Traditionally, mostly focusing on short-term thermal energy storage applications, the addition of (nano)fillers has been extensively studied to enhance unsatisfactory thermo-physical properties in PCMs, in order to overcome limiting aspects such as low thermal conductivity possibly leading to unacceptable long charging and/or discharging periods and inefficient heat-storage systems. On the other hand, here we focus on the most important PCMs for long-term thermal energy storage (i.e. spanning from classical solid-to-liquid to more recent solid-to-solid PCMs) and make an effort in shedding light on the role played not only by additives but also (and importantly) by additivation protocols on the resulting thermo-physical and stability properties. While introducing and connecting to general advantages related to additivation in classical PCMs for thermal energy storage, we discuss specifically the use of additives in sugar alcohols and sodium acetate trihydrate, as well as in novel emerging classes of PCMs capable of undergoing solid-to-solid transitions and showing promising features for long-term heat storage materials. We highlight outstanding issues in the use of additives for property enhancement in PCMs and expect that the present work can contribute to expand the current understanding and field of application of the less mature PCMs for thermal energy storage, especially as far as long term applications are concerned

Bright light emission and waveguiding in conjugated polymer nanofibers electrospun from organic-salt added solutions

Light emitting electrospun nanofibers of poly-[(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N'-diphenyl)-N,N'-di(p-butyl-oxy-phenyl)-1,4-diaminobenzene)] (PFO-PBAB) are produced by electrospinning under different experimental conditions. In particular, uniform fibers with average diameter of 180 nm are obtained by adding an organic salt to the electrospinning solution. The spectroscopic investigation assesses that the presence of the organic salt does not alter the optical properties of the active material, therefore providing an alternative approach for the fabrication of highly emissive conjugated polymer nanofibers. The produced nanofibers display self-waveguiding of light, and polarized photoluminescence, which is especially promising for embedding active electrospun fibers in sensing and nanophotonic devices.Comment: 31 pages, 9 figures, 1 table. Macromolecules (2013

Thermally triggered nanorocket from double-walled carbon nanotube in water

In this work, we propose and investigate the use of double-walled carbon nanotubes (DWCNTs) as nanosized rockets. The nanotubes are immersed in water, and the propulsion of inner nanotube is achieved by heating the water encapsulated within the DWCNT. Considering a setup made of (5,5)(8,8) DWCNT, molecular dynamics simulations for different water temperatures show that the trajectory can be divided into four phases: trigger, expulsion, damping and final equilibrium. After analysing the dynamics and the involved forces, we find out that the inner nanotube expulsion is mainly controlled by van der Waals interactions between the nanotubes; whereas, the damping role is predominantly played by the external aqueous environment. Based on these results, we propose an analytical model able to predict both the triggering time for a given water temperature and the whole dynamics of nanorocket. The validity of such dynamical model can be extended also to a broader variety of DWCNT configurations, once the different forces acting on the inner nanotube are provided. The proposed model may contribute to assist the design of nanorockets in several nanotechnology applications, such as triggered drug delivery, cell membrane piercing, or colloids with thermophoretic properties

GASP XIII. Star formation in gas outside galaxies

Based on MUSE data from the GASP survey, we study the Halpha-emitting extraplanar tails of 16 cluster galaxies at z~0.05 undergoing ram pressure stripping. We demonstrate that the dominating ionization mechanism of this gas (between 64% and 94% of the Halpha emission in the tails depending on the diagnostic diagram used) is photoionization by young massive stars due to ongoing star formation (SF) taking place in the stripped tails. This SF occurs in dynamically quite cold HII clumps with a median Halpha velocity dispersion sigma = 27 km s^-1. We study the characteristics of over 500 star-forming clumps in the tails and find median values of Halpha luminosity L_{Halpha} = 4 X 10^38 erg s^-1, dust extinction A_V=0.5 mag, star formation rate SFR=0.003 M_sun yr^-1, ionized gas density n_e =52 cm^-3, ionized gas mass M_gas = 4 X 10^4 Msun, and stellar mass M_{*} = 3 X 10^6 Msun. The tail clumps follow scaling relations (M_gas-M_{*}, L_{Halpha} -sigma, SFR-M_gas) similar to disk clumps, and their stellar masses are comparable to Ultra Compact Dwarfs and Globular Clusters.The diffuse gas component in the tails is ionized by a combination of SF and composite/LINER-like emission likely due to thermal conduction or turbulence. The stellar photoionization component of the diffuse gas can be due either to leakage of ionizing photons from the HII clumps with an average escape fraction of 18%, or lower luminosity HII regions that we cannot individually identify.Comment: accepted for publication in MNRA

Superdense galaxies and the mass-size relation at low redshift

We search for massive and compact galaxies (superdense galaxies, hereafter SDGs) at z=0.03-0.11 in the Padova-Millennium Galaxy and Group Catalogue, a spectroscopically complete sample representative of the local Universe general field population. We find that compact galaxies with radii and mass densities comparable to high-z massive and passive galaxies represent 4.4% of all galaxies with stellar masses above 3 X 10^10 M_sun, yielding a number density of 4.3 X 10^-4 h^3 Mpc^-3. Most of them are S0s (70%) or ellipticals (23%), are red and have intermediate-to-old stellar populations, with a median luminosity-weighted age of 5.4 Gyr and a median mass-weighted age of 9.2 Gyr. Their velocity dispersions and dynamical masses are consistent with the small radii and high stellar mass estimates. Comparing with the WINGS sample of cluster galaxies at similar redshifts, the fraction of superdense galaxies is three times smaller in the field than in clusters, and cluster SDGs are on average 4 Gyr older than field SDGs. We confirm the existence of a universal trend of smaller radii for older luminosity-weighted ages at fixed galaxy mass. On top of the well known dependence of stellar age on galaxy mass, the luminosity-weighted age of galaxies depends on galaxy compactness at fixed mass, and, for a fixed mass and radius, on environment. This effect needs to be taken into account in order not to overestimate the evolution of galaxy sizes from high- to low-z. Our results and hierarchical simulations suggest that a significant fraction of the massive compact galaxies at high-z have evolved into compact galaxies in galaxy clusters today. When stellar age and environmental effects are taken into account, the average amount of size evolution of individual galaxies between high- and low-z is mild, a factor ~1.6. (abridged)Comment: ApJ, in pres

The evolution of galaxy sizes

We present a study of galaxy sizes in the local Universe as a function of galaxy environment, comparing clusters and the general field. Galaxies with radii and masses comparable to high-z massive and compact galaxies represent 4.4% of all galaxies more massive than 3 X 10^{10} M_sun in the field. Such galaxies are 3 times more frequent in clusters than in the field. Most of them are early-type galaxies with intermediate to old stellar populations. There is a trend of smaller radii for older luminosity-weighted ages at fixed galaxy mass. We show the relation between size and luminosity-weighted age for galaxies of different stellar masses and in different environments. We compare with high-z data to quantify the evolution of galaxy sizes. We find that, once the progenitor bias due to the relation between galaxy size and stellar age is removed, the average amount of size evolution of individual galaxies between high- and low-z is mild, of the order of a factor 1.6.Comment: to appear in the proceedings of the IAU S295: The intriguing life of massive galaxies, editors D. Thomas, A. Pasquali & I. Ferrera

Early-Type Galaxies in the Hubble Deep Field: The Star Formation History

We have investigated the properties of a complete K-band selected sample of 35 elliptical and S0 galaxies brighter than K=20.15 in the Hubble Deep Field, as representative of the field galaxy population. This sample has been derived from deep K-band image by the KPNO-IRIM camera, by applying a rigorous morphological classification scheme based on quantitative analyses of the surface brightness profiles. The broad-band spectra of the sample galaxies allow us to date their dominant stellar populations. The majority of bright early-types in this field are found at redshifts z<1.3 to display colors indicative of a fairly wide range of ages (typically 1.5 to 3 Gyrs). We find that the major episodes of star-formation building up typical M_star galaxies have taken place during a wide redshift interval 11.3, which should be detectable during the luminous star-formation phase expected to happen at these redshifts. Obvious solutions are a) that the merging events imply perturbed morphologies which prevent selecting them by our morphological classification filter, or b) that a dust-polluted ISM obscures the (either continuous or episodic) events of star-formation. We conclude that the likely solution is a combination thereof, i.e. a set of dust-enshrouded merging-driven starbursts occurring during the first few Gyrs of the galaxy's lifetime

Dysbiosis and zonulin upregulation alter gut epithelial and vascular barriers in patients with ankylosing spondylitis

Background: Dysbiosis has been recently demonstrated in patients with ankylosing spondylitis (AS) but its implications in the modulation of intestinal immune responses have never been studied. The aim of this study was to investigate the role of ileal bacteria in modulating local and systemic immune responses in AS. Methods: Ileal biopsies were obtained from 50 HLA-B27+ patients with AS and 20 normal subjects. Silver stain was used to visualise bacteria. Ileal expression of tight and adherens junction proteins was investigated by TaqMan real-time (RT)-PCR and immunohistochemistry. Serum levels of lipopolysaccharide (LPS), LPS-binding protein (LPS-BP), intestinal fatty acid-BP (iFABP) and zonulin were assayed by ELISA. Monocyte immunological functions were studied in in vitro experiments. In addition the effects of antibiotics on tight junctions in human leukocyte antigen (HLA)-B27 transgenic (TG) rats were assessed. Results: Adherent and invasive bacteria were observed in the gut of patients with AS with the bacterial scores significantly correlated with gut inflammation. Impairment of the gut vascular barrier (GVB) was also present in AS, accompanied by significant upregulation of zonulin, and associated with high serum levels of LPS, LPS-BP, iFABP and zonulin. In in vitro studies zonulin altered endothelial tight junctions while its epithelial release was modulated by isolated AS ileal bacteria. AS circulating monocytes displayed an anergic phenotype partially restored by ex vivo stimulation with LPS+sCD14 and their stimulation with recombinant zonulin induced a clear M2 phenotype. Antibiotics restored tight junction function in HLA-B27 TG rats. Conclusions: Bacterial ileitis, increased zonulin expression and damaged intestinal mucosal barrier and GVB, characterises the gut of patients with AS and are associated with increased blood levels of zonulin, and bacterial products. Bacterial products and zonulin influence monocyte behaviour

Synergistic freshwater and electricity production using passive membrane distillation and waste heat recovered from camouflaged photovoltaic modules

A sustainable supply of both freshwater and energy is key for modern societies. In this work, we investigate a synergistic way to address both these issues, producing freshwater while reducing greenhouse gas emissions due to electricity generation. To this, we propose a coupling between a photovoltaic (PV) device and an innovative desalination technique based on passive multi-stage membrane distillation. The passive distillation device is driven by low-temperature heat and does not need any mechanical or electrical devices to operate. The required heat is recovered from the back side of the PV device that, for the first time, mitigates the aesthetic and environmental impact thanks to an innovative surface texture. The aim is to demonstrate the feasibility to generate PV electricity from the sun and, simultaneously, freshwater from the waste heat. The solution is studied by numerical simulations and experiments at the same time, achieving a good accordance between these two approaches. The device is able to produce up to 2 L m-2 h-1 of freshwater under one sun irradiance. Furthermore, a relative photovoltaic efficiency gain of 4.5% is obtained, since the temperature of the PV module is reduced by 9 °C when coupled with the desalination technology. This work paves the way to compact installations made of such passive units, which may easily provide energy and safe water with low environmental and visual impact, especially in off-grid areas and emergency conditions