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Wetting dynamics and contact angles of powders studied through capillary rise experiments
Full text linkWettability is an important property involved in the industrial use of granular solids and powders. It is commonly described with the contact angle and an experimental method for its determination in dynamic conditions is proposed in this work. The method is based on the capillary rise of the wetting liquid into a packed bed of the material under analysis. Differently from the classical Washburn method, the packed bed is closed to the atmosphere and the air pressure increase is measured allowing to evaluate the powder contact angle through a dynamic balance of the pressure forces. In the expression of such forces a new equivalent capillary radius for the powder bed is used based on an alternative definition of the particle equivalent diameter. This diameter is closely related to the length of the three phase line which divide the wet portion of the bed from the dry one and mirrors the physics of wetting process better than the classical Sauter diameter. A way to determine it with optical microscopy is given. Also the measure of the packed bed porosity (entering in the equivalent capillary radius definition) has been improved by using the effective porosity concept [Hapgood et al., J. Coll. Interface Sci. 253 (2002) 353-366] and by modifying the way of estimating it. The proposed experimental technique, coupled to the theoretical model for the packed bed, can describe accurately the packed bed geometry and the wetting dynamics by following the changes of the contact angle from its initial maximum value up to the final equilibrium one
Modellazione numerica di flussi granulari
No full textIl flusso denso di solidi granulari è un fenomeno, comune in molte applicazioni industriali e fenomeni naturali, che presenta notevoli difficoltà nella sua descrizione matematica e modellazione. Questo contributo vuole presentare un modello idrodinamico in grado di simulare questa tipologia di flussi. Esso si basa su un approccio continuo e utilizza le equazioni di bilancio (conservazione di massa e quantità di moto) integrate da una equazione di conservazione dell’energia cinetica fluttuante che utilizza il concetto di ‘temperatura granulare’. La temperatura granulare è usata per definire delle relazioni costitutive in grado di replicare la fenomenologia dei materiali granulari in
moto. Il modello è qui applicato ad una geometria semplice (canale verticale più tramoggia convergente), ma è applicabile nella sua formulazione più generale a qualsiasi altra configurazione geometrica o di flusso. Il modello prevede comportamenti di flusso realistici, che comunque richiedono la validazione quantitativa con misure dettagliate. Questo lavoro si concentra sulla previsione di alcuni aspetti peculiari legati al flusso dei solidi granulari quali i profili di sforzo normale a parete (a cui corrispondono strettamente le
correlazioni di Jannsen e Walker), la previsione del picco di sforzo in presenza di una variazione di sezione di flusso, la portata costante nel tempo tipica delle clessidre, la distinzione tra diversi regimi di scarico (funnel e mass flow)
Transition to movement in granular chute flows
No full textThis 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 identifcation 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 one-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 ÿlming the flowing granular bed
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