Wetting of binary powder mixtures

The wetting process involved when a liquid droplet comes into contact with a mixture of particles is a complex phenomenon which is often understood by reference to Cassie-Baxter theory. However, various authors have applied the Cassie-Baxter theory for the prediction of contact angles on two-component mixtures without success. We hypothesise that the main difficulty in applying the Cassie-Baxter theory to mixtures is that if the particles differ in size, it is possible for the small particles to coat the large particles, so reducing the available surface area of the large particles. This leads to the view that bulk volume fractions are not good estimates of surface fractions of the components within the mixture. We argue that the Cassie-Baxter theory over represents the influence of large particles and that below a certain critical volume fraction they exert no influence. We present a simple geometrical model that relates the critical surface coverage volume fraction to the Sauter mean particle size of the binary mixture components. As a consequence, the wetting behaviour can be determined from the bulk volume fractions and the calculated critical surface coverage volume fraction, by means of a simple geometric model. We show that the simple model describes the five two-components systems reported here and a further four systems reported in the literature, irrespective of whether the larger or small particles are hydrophobic/hydrophilic. With this model, it is possible to predict the wetting behaviour of mixtures of particles that coat each other using very simple characterisation methods, so reducing the development time in the creation of formulations in the pharmaceutical industry

Insights into the high-energy γ-ray emission of Markarian 501 from extensive multifrequency observations in the Fermi era

We report on the γ-ray activity of the blazar Mrk 501 during the first 480 days of Fermi operation. We find that the average Large Area Telescope (LAT) γ-ray spectrum of Mrk 501 can be well described by a single power-law function with a photon index of 1.78 ± 0.03. While we observe relatively mild flux variations with the Fermi-LAT (within less than a factor of two), we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 ± 0.14, and the softest one is 2.51 ± 0.20. These unexpected spectral changes do not correlate with the measured flux variations above 0.3 GeV. In this paper, we also present the first results from the 4.5 month long multifrequency campaign (2009 March 15-August 1) on Mrk 501, which included the Very Long Baseline Array (VLBA), Swift, RXTE, MAGIC, and VERITAS, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign. The extensive radio to TeV data set from this campaign provides us with the most detailed spectral energy distribution yet collected for this source during its relatively low activity. The average spectral energy distribution of Mrk 501 is well described by the standard one-zone synchrotron self-Compton (SSC) model. In the framework of this model, we find that the dominant emission region is characterized by a size ≲0.1 pc (comparable within a factor of few to the size of the partially resolved VLBA core at 15-43 GHz), and that the total jet power (≃1044 erg s-1) constitutes only a small fraction (∼10-3) of the Eddington luminosity. The energy distribution of the freshly accelerated radiating electrons required to fit the time-averaged data has a broken power-law form in the energy range 0.3 GeV-10 TeV, with spectral indices 2.2 and 2.7 below and above the break energy of 20 GeV. We argue that such a form is consistent with a scenario in which the bulk of the energy dissipation within the dominant emission zone of Mrk 501 is due to relativistic, proton-mediated shocks. We find that the ultrarelativistic electrons and mildly relativistic protons within the blazar zone, if comparable in number, are in approximate energy equipartition, with their energy dominating the jet magnetic field energy by about two orders of magnitude. © 2011. The American Astronomical Society

Hydrophobic/hydrophilic powders : practical implications of screw element type on the reduction of fines in twin screw granulation

This study focuses on the role that various screw elements play in the reduction of fines in twin screw wet granulation (TSG) when using increasing amounts of hydrophobic component in the formulation. The elements used are conveying-type elements (CE-type), kneading elements (KE-type) and tooth mixing-type elements (TME-type)). Fines are defined as <150 μm granules. Further, this work attempts to rationalize the positioning of the various screw elements along the barrel by studying the granule size, granule porosity, liquid binder distribution and residence time distribution results. The length of the screw elements was kept constant to ensure constant and comparable mixing quality. It was found that CE-type elements promoted poor mixing through concentration of activity in the intermeshing region of the twin screw, which resulted in large agglomerates being formed, together with large amounts of <150 μm granules. At increased formulation hydrophobicity, the distribution of the liquid binder droplets using the CE-type elements became unachievable, forming liquid marbles - ‘solid particles spreading around the liquid droplet’. The order of superior cross-sectional mixing behavior across the barrel channel width was observed to be CE < KE < TME, particularly with increase in formulation hydrophobicity. This was due to progressively improved liquid distribution with increase in material residence time, which resulted in less production of <150 μm granules and greater extent of granule formation with that order of elements. The axial cuttings through the flights of the CE-type elements are necessary to reduce the internal free volume of the intermeshing gap and cross-sectional area which promote material compression and higher degree of fill before the mixing zone in TSG. Whilst, the increase in internal free volume of the mixing elements provide more cross-sectional free volume for granules to be compacted

Hydrophobic/hydrophilic static powder beds : competing horizontal spreading and vertical imbibition mechanisms of a single droplet

There are two competing mechanisms of a single liquid droplet impacting on static powder beds. The dh spreading describes the length the liquid droplet travels horizontally across the powder bed, whereas the dv imbibition is the length the liquid droplet travels vertically into the powder bed. In this work, results are presented from an experimental study of single liquid droplets of varying viscosities impacting static hydrophilic and hydrophobic powder beds. By using a high-speed video camera, the dynamics of the droplet spreading behaviour on static powder bed compacts was captured. The resultant nuclei were then analysed. Results show that the measured maximum dh spreading diameter is less than the diameter of the resultant nuclei. This indicates that the dh spreading of the liquid droplet continues to spread after complete imbibition in a late-diffusive intra-spreading process. This process refers to liquid movement driven by capillary forces in the powder beds and showed a higher degree of dependency on the liquid binder viscosity. The dv imbibition length was found to be dependent on the interfacial tension between the solid and the liquid droplet. This transition in the liquid spreading and imbibition behaviour is explained based on the transportation of a liquid droplet occurring increasingly via site percolation theory with increase in powder bed hydrophobicity. The liquid droplet imbibition rates are described in terms of Darcy's law scaling behaviour. Understanding of single droplet spreading mechanisms provides a basis to select the optimal binder for granulation to form more uniform and robust granules when using hydrophilic and hydrophobic formulations

Assessing particle segregation using near-infrared chemical imaging in twin screw granulation

In the present study the application of near-infrared chemical imaging (NIR-CI) for assessing particle segregation in granules from continuous twin screw granulation (TSG) granules, were the complex attributes of the machinery configuration in relation to particle segregation is not well understood was investigated. Experiments were performed along the compartmental length of the TSG barrel channel by varying the screw element type and liquid binder viscosity. Examination of the data showed a direct correlation between dispersion due to shear force and de-mixing of particles, which allowed for identification of fundamental granule segregation mechanisms affecting content uniformity in TSG. Particle segregation behavior was linked to dispersion due to shear force through a proposed regime mapping approach which links de-mixing potential to controlling granule formation mechanisms with a new dimensionless mixing number. This was carried out in order to provide a general guideline of how particles segregate along the length of the TSG barrel channel

Improving the performance of the single-dish Cherenkov telescope MAGIC through the use of signal timing.

The Cherenkov light flashes produced by Extensive Air Showers are very short in time. A high bandwidth and fast digitizing readout, therefore, can minimize the influence of the background from the light of the night sky, and improve the performance in Cherenkov telescopes. The time structure of the Cherenkov image can further be used in single-dish Cherenkov telescopes as an additional parameter to reduce the background from unwanted hadronic showers. A description of an analysis method which makes use of the time information and the subsequent improvement on the performance of the MAGIC telescope (especially after the upgrade with an ultra fast 2 GSamples/s digitization system in February 2007) will be presented. The use of timing information in the analysis of the new MAGIC data reduces the background by a factor two, which in turn results in an enhancement of about a factor 1.4 of the flux sensitivity to point-like sources, as tested on observations of the Crab Nebula.Comment: 27 pages, 11 figures, accepted by Astroparticle Physic