Salt, as with rice, cement, sand, are finely divided solids which we call granular media. They are groups of similar particles that can behave as a liquid, flowing through orifices; or as a solid, given that they can maintain a constant shape and volume (see Fig A, sand sculptures). This is why the behaviour of granular media is curious and, on many occasions, problematic.
Figures A, B, C and D
One of the intrinsic phenomena on handling granular media is the spontaneous formation of arches as shown in figure B. Arches are structures as used in architecture in order to construct bridges and aqueducts (figure C). These arches are referred to as being vaulted when formed in three dimensions and have the common feature that the particles forming them stabilize each other. That is, if one of the arcs making up the vaulted arch is eliminated, the whole structure will collapse.
When a granular medium flows through an orifice, the formation of the vaulting can cause a blockage. The flow of particles stops and the arch supports the weight of all the material on top of it, in the same way that the arches of a bridge support the weight of the vehicles crossing over it. The blockages in the flow of a granular medium cause serious problems in certain industrial processes. The plastics, cement and pharmaceutical industries are example of where granular media are the main players.
The blockages in discharging silos or dosifiers have physical properties in common with other kinds of hold-ups. Who has not been in a traffic jam, with the start of their holiday delayed? An example closer to home – and certainly more dangerous – is that of the bull running in the fiestas of San Fermín in Pamplona (figure D). When the street narrows and the runners run into each other, a spectacular accumulation of bodies occurs.
Over the past decade numerous scientists have been trying to understand the properties of obstructions of this nature, as well as the factors that are most influential in their formation. Nevertheless, there are many questions left unanswered. In this thesis Iker Zuriguel has investigated the simplest example of blockages that can be studied in the laboratory: a small silo full of spherical particles and with a circular orifice at its base. The thesis was presented at the University of Navarra.
Despite the apparent simplicity of the phenomenon, the unresolved questions are many. For example, what controls the phenomenon of blockages? The particle size? The size of the orifice? This thesis shows that the really important factor is the relationship between the radius of the orifice and that of the particle. Another important question: with the same size of particle and orifice, is it always the same number of grains that fall in an avalanche before the system blocks up? This research is a resounding proof that this is not so. For the same experimental conditions, we can find avalanches of 10 to 10,000 spheres!
The most important result of this thesis was the discovery that, in a 3-dimensional silo, when spherical particles are used, it suffices for the radius of the orifice to be five times greater than that of the particles in order that obstruction does not occur. And, as the saltcellar has holes less than this size, it is necessary to shake it in order to break up the arches formed and that impede the salt to free-flow on to our food.
Irati Kortabitarte | alfa
SF State astronomer searches for signs of life on Wolf 1061 exoplanet
20.01.2017 | San Francisco State University
Molecule flash mob
19.01.2017 | Technische Universität Wien
An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by...
Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...
Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.
While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...
Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales
Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
19.01.2017 | Event News
10.01.2017 | Event News
09.01.2017 | Event News
20.01.2017 | Awards Funding
20.01.2017 | Materials Sciences
20.01.2017 | Life Sciences