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
APEX takes a glimpse into the heart of darkness
25.05.2018 | Max-Planck-Institut für Radioastronomie
First chip-scale broadband optical system that can sense molecules in the mid-IR
24.05.2018 | Columbia University School of Engineering and Applied Science
The more electronics steer, accelerate and brake cars, the more important it is to protect them against cyber-attacks. That is why 15 partners from industry and academia will work together over the next three years on new approaches to IT security in self-driving cars. The joint project goes by the name Security For Connected, Autonomous Cars (SecForCARs) and has funding of €7.2 million from the German Federal Ministry of Education and Research. Infineon is leading the project.
Vehicles already offer diverse communication interfaces and more and more automated functions, such as distance and lane-keeping assist systems. At the same...
A research team led by physicists at the Technical University of Munich (TUM) has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.
The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative...
At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.
At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...
There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?
At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...
So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...
25.05.2018 | Event News
02.05.2018 | Event News
13.04.2018 | Event News
25.05.2018 | Event News
25.05.2018 | Machine Engineering
25.05.2018 | Life Sciences