Andrea C. Santomaso

Powder flowability and density ratios: the impact of granules packing

Abstract The propensity of powders to flow under given circumstances (flowability) affects a large number of industrial applications. A single, reliable and widely applicable flowability test does not exist, because of the variety of both granular materials and influence of handling on the measurements results. Here we critically examined the results provided by Hausners method, based on apparent densities ratio, with several granular materials. Major limitations appeared to be the achievement and measurement of a dense packing condition, provided by the tapped density in the Hausners ratio. After a detailed discussion of standard and modified techniques to measure bulk density, we eventually suggest a new flowability criterion based on a novel technique to determine a high packing density. The proposed criterion is more sensitive to differences in flowability, as quantified by the repose angle. In order to investigate also the domain of cohesive powders, we developed a novel procedure to measure the repose angle of such powders. Eventually, the new criterion was able to account consistently for free-flowing and cohesive powders. It also stimulates the discussion on subtle issues involved in the determination and use of elementary powders properties.

Investigation of the Granular Behaviour in a Rotating Drum Operated over a Wide Range of Rotational Speed

Rotating drums are extensively used in the chemical and process industries as mixers, dryers, granulators and reactors for processing granular materials. As a result, granular behaviour in rotating drums has attracted numerous research efforts from both engineering and physics communities over the past few decades. Most of these studies have been focused on drums operated in or close to the rolling mode. However, there are many industrial cases where drums are operated in other modes, e.g. the cascading and cataracting modes, which forms the main motivation for this work. Comprehensive experiments have been carried out to investigate granular behaviour in a drum operated over a wide range of rotational speed with solids motion across the rolling, cascading and cataracting modes. A digital recording device was used to capture images of the transverse plane of the material bed. Analyses of the images were carried out to extract the bed behaviour as a function of rotational speed, drum fill level and particle size. This has led to three relationships between the surface shape expressed in terms of three characteristic lengths, operating conditions, as well as the friction properties of both particles and drum wall. These relations are found to apply approximately to the whole range of rotational speed used in this work. The generality of these relationships and possible application of them for drum scaling are discussed.

Transition to movement in granular chute flows

Abstract This experimental investigation deals with the observation of the behaviour that dense granular materials present when they flow in steady regime on a rough chute, focusing the attention on the transition to movement of the bed and on quantities involved as the internal friction angle. An important aspect of the study is the identification of parameters that distinguish granular from fluid flows, aiming to verify the possibility to describe a granular bed as it was a pseudo-fluid having a particular rheological behaviour. In the experiments we have not used idealised particles (spheres, rods or disks) but sieved powders of ethylenediaminetetraacetic acid (EDTA), constituted of non-spherical particles with polydisperse size distribution and surface roughness. A static and a flowing (dynamic) layer are clearly identified. The thickness of the observed layers (static and dynamic) along the chute has been measured for different inclination, finding out that they collapse into a single curve when considered in non-dimensional scale. On the ground of the experimental data we propose a direct way of measuring the dynamic friction angle from chute observations and a simple constitutive law for granular materials in the frictional regime of motion. The law has been tested using velocity profiles obtained by filming the flowing granular bed.

Effective boundary conditions for dense granular flows

We derive an effective boundary condition for dense granular flow taking into account the effect of the heterogeneity of the force network on sliding friction dynamics. This yields an intermediate boundary condition which lies in the limit between no slip and Coulomb friction; two simple functions relating wall stress, velocity, and velocity variance are found from numerical simulations. Moreover, we show that this effective boundary condition corresponds to Navier slip condition when the model of G. D. R. Midi [Eur. Phys. J. E 14, 341 (2004)] is assumed to be valid, and that the slip length depends on the length scale that characterizes the system, viz. the particle diameter.

Development and characterization of a new thief sampling device for cohesive powders

New sampling probes and methods for investigating cohesive powders are conceived, designed and characterized. Probes are made of two metallic shells (a slide and a cover) which need to be inserted sequentially into the bed of powder in order to extract representative samples. The thin profile of the shells, combined with a particular insertion procedure, is intended to minimize stresses on the powder; thereby reducing both the invasiveness and the dragging of material through the bed. Probes of similar design with different shape and size have been tested on stratified beds of cohesive powders of different colors. Sampling performances are quantitatively compared among different probes (for size and shape) and also with literature data. The comparison has indicated that the new sampling devices effectively improved sampling efficiency, reliability and possibilities. The simple construction and use suggest they can be viable and effective alternatives to traditional probes for cohesive mixtures.

Single Particle Tracking Across Sequences of Microscopical Images: Application to Platelet Adhesion Under Flow

A versatile and automated image processing technique and data extraction procedure from videomicroscopic data is presented. The motivation is a detailed quantification of blood platelet adhesion from laminar flow onto a surface. The characteristics of the system under observation (type of cells, their speed of movement, and the quality of the optical image to analyze) provided the criteria for developing a new procedure enabling tracking for long image sequences. Specific features of the novel method include: automatic segmentation methodology which removes operator bias; platelet recognition across the series of images based on a probability density function (two-dimensional, Gaussian-like) tailored to the physics of platelet motion on the surface; options to automatically tune the procedure parameters to explore different applications; integrated analysis of the results (platelet trajectories) to obtain relevant information, such as deposition and removal rates, displacement distributions, pause times and rolling velocities. Synthetic images, providing known reference conditions, are used to test the method. The algorithm operation is illustrated by application to images obtained by fluorescence microscopy of the interaction between platelets and von Willebrand factor-coated surfaces in parallel-plate flow chambers. Potentials and limits are discussed, together with evaluation of errors resulting from an inaccurate tracking.

Scaling laws for the slip velocity in dense granular flows.

In this Letter, the two-dimensional dense flow of polygonal particles on an incline with a flat frictional inferior boundary is analyzed by means of contact dynamics discrete element simulations, in order to develop boundary conditions for continuum models of dense granular flows. We show the evidence that the global slip phenomenon deviates significantly from simple sliding: a finite slip velocity is generally found for shear forces lower than the sliding threshold for particle-wall contacts. We determined simple scaling laws for the dependence of the slip velocity on shear rate, normal and shear stresses, and material parameters. The importance of a correct determination of the slip at the base of the incline, which is crucial for the calculation of flow rates, is discussed in relation to natural flows.

Combining formulation and process aspects for optimizing the high-shear wet granulation of common drugs

The purpose of this research was to determine the effects of some important drug properties (such as particle size distribution, hygroscopicity and solubility) and process variables on the granule growth behaviour and final drug distribution in high shear wet granulation. Results have been analyzed in the light of widely accepted theories and some recently developed approaches. A mixture composed of drug, some excipients and a dry binder was processed using a lab-scale high-shear mixer. Three common active pharmaceutical ingredients (paracetamol, caffeine and acetylsalicylic acid) were used within the initial formulation. Drug load was 50% (on weight basis). Influences of drug particle properties (e.g. particle size and shape, hygroscopicity) on the granule growth behaviour were evaluated. Particle size distribution (PSD) and granule morphology were monitored during the entire process through sieve analysis and scanning electron microscope (SEM) image analysis. Resistance of the wet mass to mixing was furthermore measured using the impeller torque monitoring technique. The observed differences in the granule growth behaviour as well as the discrepancies between the actual and the ideal drug content in the final granules have been interpreted in terms of dimensionless quantity (spray flux number, bed penetration time) and related to torque measurements. Analysis highlighted the role of liquid distribution on the process. It was demonstrated that where the liquid penetration time was higher (e.g. paracetamol-based formulations), the liquid distribution was poorer leading to retarded granule growth and selective agglomeration. On the other hand where penetration time was lower (e.g. acetylsalicylic acid-based formulations), the growth was much faster but uniformity content problem arose because of the onset of crushing and layering phenomena.

Radial segregation driven by axial convection

We experimentally study the mixing of binary granular systems in a horizontal rotating cylinder. When materials have the same size and differ by dynamic angle of repose only, we observe an axial transport of matter that generates transient radial segregation. The system then evolves towards homogeneity. If materials differ by density also radial segregation becomes steady. A mechanism is suggested where radial segregation is promoted by axial differences of dynamic angle of repose. This differs from the free surface segregation suggested so far to explain radial segregation.

Texture analysis as a tool to study the kinetics of wet agglomeration processes

In this work wet granulation experiments were carried out in a planetary mixer with the aim to develop a novel analytical tool based on surface texture analysis. The evolution of a simple formulation (300g of microcrystalline cellulose with a solid binders pre-dispersed in water) was monitored from the very beginning up to the end point and information on the kinetics of granulation as well as on the effect of liquid binder amount were collected. Agreement between texture analysis and granules particle size distribution obtained by sieving analysis was always found. The method proved to be robust enough to easily monitor the process and its use for more refined analyses on the different rate processes occurring during granulation is also suggested.