Aggregates in the strength and gravity regime: Particles sizes in Saturn's rings

Particles in Saturn's main rings range in size from dust to kilometer-sized objects. Their size distribution is thought to be a result of competing accretion and fragmentation processes. While growth is naturally limited in tidal environments, frequent collisions among these objects may contribute to both accretion and fragmentation. As ring particles are primarily made of water ice attractive surface forces like adhesion could significantly influence these processes, finally determining the resulting size distribution. Here, we derive analytic expressions for the specific self-energy Q and related specific break-up energy Q(star) of aggregates. These expressions can be used for any aggregate type composed of monomeric constituents. We compare these expressions to numerical experiments where we create aggregates of various types including: regular packings like the face-centered cubic (fcc), Ballistic Particle Cluster Aggregates (BPCA), and modified BPCAs including e.g. different constituent size distributions. We show that accounting for attractive surface forces such as adhesion a simple approach is able to: (a) generally account for the size dependence of the specific break-up energy for fragmentation to occur reported in the literature, namely the division into "strength" and "gravity" regimes and (b) estimate the maximum aggregate size in a collisional ensemble to be on the order of a few tens of meters, consistent with the maximum particle size observed in Saturn's rings of about 10 m. (c) 2012 Elsevier B.V. All rights reserved.

Dynamical evolution of Saturn's F ring dust particles

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Orbital evolution of the mu and nu dust ring particles of Uranus

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Zuspan's Scheme Versus an Alternative Magnesium Sulfate Scheme: Randomized Clinical Trial of Magnesium Serum Concentrations

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

The volumetric fraction of inorganic particles and the flexural strength of composites for posterior teeth

Objective. To evaluate the content of inorganic particles and the flexural strength of new condensable composites for posterior teeth in comparison to hybrid conventional composites.Method. The determination of the content of inorganic particles was performed by mass weighing of a polymerized composite before and after the elimination of the organic phase. The volumetric particle content was determined by a practical method based on Archimedes' principle, which calculates the volume of the composite and their particles by differential mass measured in the air and in water. The flexural. strength of three points was evaluated according to the norm ISO 4049:1988.Results. The results showed the following filter content: Alert, 67.26%; Z-100, 65.27%; Filtek P 60, 62.34%; Ariston pHc, 64.07%; Tetric Ceram, 57.22%; Definite, 54.42%; Solitaire, 47.76%. In the flexural strength test, the materials presented the following decreasing order of resistance: Filtek P 60 (170.02 MPa) > Z-100 (151.34 MPa) > Tetric Ceram (126.14 MPa) = Alert (124.89 MPa) > Ariston pHc (102.00 MPa) = Definite (93.63 MPa) > Solitaire (56.71 MPa).Conclusion. New condensable composites for posterior teeth present a concentration of inorganic particles similar to those of hybrid composites but do not necessarily present higher flexural strength. (C) 2003 Elsevier B.V. Ltd. Alt rights reserved.

Influence of potassium nutrition and the nitrate ammonium ratio on the putrescine and spermidine contents in banana vitroplants

Potassium (K) is required in high doses by the banana (Musa sp.) plant and interacts with other nutrient elements in which banana tissues are maintained under in vitro condition as a consequence modifications in the plant metabolism take place mainly in nitrogen (N) compounds, such as proteins, amino acids, and secondary compounds. When K is present in concentrations lower than that required, diamines such as putrescine and poliamines are formed. This metabolic disorder can also be correlated with the presence of different inorganic N forms, such as nitrate (NO3) and ammonium (NH4), and the ratios between both ions as well. In order to follow the physiological performance of the interrelationships, K/putrescine and of the NO3/NH4 ratio in the tissue of banana vitroplantlets, shoot apex of two banana cvs. Nanica and Prata Ana were maintained in modified MS medium in the presence of six different doses of K: 5, 10, 15, 20, 25, and 30 mM. After the period of tissue proliferation the cultures were transferred to rooting media containing the same different K doses. Dry matter, K, putrescine, and spermidine contents and their accumulation were determined in the shoots and roots of the vitroplantlets and in the shoot apex of the explant donor cultivar as well as the corresponding values for the whole vitroplantlets calculated. The data were statistically analyzed. The contents and accumulations of putrescine and spermidine in banana tissues were enhanced as K concentration decreased in the medium: four times (0.19% of the dry matter) for cv. Nanica and eight times (0.25% of the dry matter) for cv. Prata Ana. This behavior was not only related to the K depletion but to the NO3/NH4 ratio as well.

Chelating agent effects in the synthesis of mesoscopic-size superconducting particles

Several routes and procedures have been used in these last years as an effort to achieve single-phase mesoscopic-size superconducting samples. In this paper, the effects of using citric acid (CA), tartaric acid (TA) and ethylenediaminetetraacetic acid (EDTA) as chelating agents and ethylene-glycol (EG) as polyhydroxy alcohol were studied in order to establish conditions to avoid the occurrence of BaCO(3) undesirable secondary phase in YBa(2)Cu(3)O(7-delta) (YBCO). Thermal evolution of intermediate compounds formed during the calcinations process by the use of different chelating agents was traced using thermogravimetric and spectroscopic methods. The obtained results indicated that the polymer breakdown of samples prepared using EDTA occurs at higher temperatures than others chelating agents and also reduces the occurrence of BaCO(3) secondary phase as studied by X-ray diffraction measurements. Furthermore, the magnetic response of the mesoscopic-size YBCO specimens obtained was verified showing that samples present different superconducting response.

Crystal structure of human PNP complexed with hypoxanthine and sulfate ion

Purine nucleoside phosphorylase (PNP) is a ubiquitous enzyme, which plays a key role in the purine salvage pathway, and PNP deficiency in humans leads to an impairment of T-cell function, usually with no apparent effects on B-cell function. Human PNP has been submitted to intensive structure-based design of inhibitors, most of them using low-resolution structures of human PNP. Here we report the crystal structure of human PNP in complex with hypoxanthine, refined to 2.6 Angstrom resolution. The intermolecular interaction between ligand and PNP is discussed. (C) 2004 Elsevier B.V. All rights reserved.