Grains are Great
Have you patiently observed an hourglass? Sand flows and soon or later the upper part empties. Then, once below it behaves as a solid. This is the main attractive of granular matter: sometimes is a complex fluid, sometimes solid or sometimes even a gas. Marvelous phenomena occur precisely due to this cause: critical events, beautiful patterns, cooperation, self-organized dynamics, jamming, counterintuitive segregations, dissipative behaviors, etc.
Many and very interesting scientific papers appear every day and we still don't understand the laws that rule granular matter.
This is a short list of problems studied by our group:
Lubrication
Spherical and light grains can be very good lubricants. We have found that the friction coefficient between a flat object and a monolayer of such grains is close to zero. As a matter of fact, we have even made a cheap hockey table (similar to those expensive tables where air levitates the pucks). See here
Crystallization
Pour marbles into a box and shake. Even if you do it for the rest of your life, the packing fraction will not surpass the value of 0.64. In other words, the marbles will never form an ordered structure. Add, however, a drop of oil (or water) and shake the box again. You will see a crystal. The trick is to add a bit of cohesion to introduce enthalpy. See here.
Segregation
When you buy in the supermarket a box of cereal with nuts, the nuts are always on the top. Why? Because heavy (or large) grains segregate from the lighter (or smaller) when someone shakes the container. Of course, we are assuming that when you buy the cereal, the box has been already inspected on the shelves (with a little shaking) by many customers. We have explained why.
Cratering
Type "crater Tycho" on Google. You will learn that this is a lunar crater that has a peak in the center. 99.99999% of the people (well, 100% except us) claim that the peak came from below (the meteorite that produced the crater long ago melted the lunar terrain and the peak was the splash). Read the review on Reports on Progress in Physics and you will understand why we disagree.
Penetration
Can an object penetrate indefinitely into a granular medium by its own weight? We did and experiment (in the longest silo ever used for penetration studies) and found that it can. We also discovered for the first time that the object is able to reach a terminal velocity! See here.
Nucleation and Annealing
Grains nucleate around a seed and form ordered structures. We did an experiment to find a probability distribution of clusters to evaluate the entropy and free energy landscapes. It was very hard to convince the referees that Shannon's entropy is superior to Boltzmann's. We are proud that the EPJE gave us the front page in May 2014.
Cooperation
A group of heavy projectiles penetrates a light granular medium like a flock of birds flies in the sky: cooperating. Read the story in Nature Communications.
Disintegration
Mars has more or less 500000 craters and our planet only 200. Why? Everybody believes that the reason is the atmosphere (Mars's atmosphere is very thin). We believe instead (and have carried out experiments to probe it) that the main reason is the composition of the meteorites. See here.
Braking with a Diamagnetic trick
Put a magnetic sphere close to copper and you see no interaction. Why? Because copper is not paramagnetic (it is the contrary: diamagnetic). But throw it to the copper surface and it will stop it. We have measured the amazing restitution coefficients of magnetic particles. See here.
Drawing in sand
There are artists that make wonderful drawings on sand, with details that almost violate the laws of physics. Could this be possible? See here.
Have you patiently observed an hourglass? Sand flows and soon or later the upper part empties. Then, once below it behaves as a solid. This is the main attractive of granular matter: sometimes is a complex fluid, sometimes solid or sometimes even a gas. Marvelous phenomena occur precisely due to this cause: critical events, beautiful patterns, cooperation, self-organized dynamics, jamming, counterintuitive segregations, dissipative behaviors, etc.
Many and very interesting scientific papers appear every day and we still don't understand the laws that rule granular matter.
This is a short list of problems studied by our group:
Lubrication
Spherical and light grains can be very good lubricants. We have found that the friction coefficient between a flat object and a monolayer of such grains is close to zero. As a matter of fact, we have even made a cheap hockey table (similar to those expensive tables where air levitates the pucks). See here
Crystallization
Pour marbles into a box and shake. Even if you do it for the rest of your life, the packing fraction will not surpass the value of 0.64. In other words, the marbles will never form an ordered structure. Add, however, a drop of oil (or water) and shake the box again. You will see a crystal. The trick is to add a bit of cohesion to introduce enthalpy. See here.
Segregation
When you buy in the supermarket a box of cereal with nuts, the nuts are always on the top. Why? Because heavy (or large) grains segregate from the lighter (or smaller) when someone shakes the container. Of course, we are assuming that when you buy the cereal, the box has been already inspected on the shelves (with a little shaking) by many customers. We have explained why.
Cratering
Type "crater Tycho" on Google. You will learn that this is a lunar crater that has a peak in the center. 99.99999% of the people (well, 100% except us) claim that the peak came from below (the meteorite that produced the crater long ago melted the lunar terrain and the peak was the splash). Read the review on Reports on Progress in Physics and you will understand why we disagree.
Penetration
Can an object penetrate indefinitely into a granular medium by its own weight? We did and experiment (in the longest silo ever used for penetration studies) and found that it can. We also discovered for the first time that the object is able to reach a terminal velocity! See here.
Nucleation and Annealing
Grains nucleate around a seed and form ordered structures. We did an experiment to find a probability distribution of clusters to evaluate the entropy and free energy landscapes. It was very hard to convince the referees that Shannon's entropy is superior to Boltzmann's. We are proud that the EPJE gave us the front page in May 2014.
Cooperation
A group of heavy projectiles penetrates a light granular medium like a flock of birds flies in the sky: cooperating. Read the story in Nature Communications.
Disintegration
Mars has more or less 500000 craters and our planet only 200. Why? Everybody believes that the reason is the atmosphere (Mars's atmosphere is very thin). We believe instead (and have carried out experiments to probe it) that the main reason is the composition of the meteorites. See here.
Braking with a Diamagnetic trick
Put a magnetic sphere close to copper and you see no interaction. Why? Because copper is not paramagnetic (it is the contrary: diamagnetic). But throw it to the copper surface and it will stop it. We have measured the amazing restitution coefficients of magnetic particles. See here.
Drawing in sand
There are artists that make wonderful drawings on sand, with details that almost violate the laws of physics. Could this be possible? See here.