In the tube, the surface of the ferrofluid is free and in contact with air. It is roughcast when the magnet approaches and forms a labyrinthal structure. Ferrofluid is a homogeneous material made of minute particles of some millionth of millimeter dispersed in a liquid. These particles are small magnets sensitive to the magnetic field. Which tend to be directed in its direction (like a compass in the terrestrial magnetic field). However, they are so small that the magnetic field succeeds in overcoming their Brownian agitation and to order them completely. In the case of the ferrofluid on free face, it is gravity and its surface stress with the air which limits its rise and gives it the appearance of many peaks.
When the ferrofluid is caught between two plates, it cannot obviously rise, it is divided then into layers.Ferrofluids are used in very varied fields: from medical application to the seals of computer hard disks. The majority of the application are based on the capacity of handling and moving these liquids using a magnetic field.
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A factory with dunes
What to do?
Flatten sand with the spatula. Give a slow and regular back and forth movement with the container. Small dunes will appear.
What we learn?
It is difficult to reproduce dunes of small size with a blower. This is a problem of scale. On the other hand, while placing a sand bed under water, one can obtain regular wrinkles like those which can be observe under the sea (thanks to the movement of the waves) or those seen on sand beaches and sand dunes (thanks to wind transport).
Combined with the palette of colours of various sands which one observes on the surface of the dunes (thanks to the segregation), the different types of sand give this incomparable aesthetics to the desert which fascinated Theodore Monod.
What to do?
Flatten sand with the spatula. Give a slow and regular back and forth movement with the container. Small dunes will appear.
What we learn?
It is difficult to reproduce dunes of small size with a blower. This is a problem of scale. On the other hand, while placing a sand bed under water, one can obtain regular wrinkles like those which can be observe under the sea (thanks to the movement of the waves) or those seen on sand beaches and sand dunes (thanks to wind transport).
Combined with the palette of colours of various sands which one observes on the surface of the dunes (thanks to the segregation), the different types of sand give this incomparable aesthetics to the desert which fascinated Theodore Monod.
Turn, turn!
What to do?
Make a ball turn, then 5 or 6 in the container and observe. Repeat the experiment by filling the bottom of the container. What happens? Why this phenomenon?
What we learn?
You may concentrate you observation and improve your deductive skills by gradually increasing the number of balls.
For a better understanding of the phenomenon, put the disc with an arrow on it in the container and make it turn. Observe again.
The disc is involved, by the centrifugal force, in the direction of rotation of the container. But it also whirls in the other direction. That is due to the friction of the disc (and the balls) against the edge of the container.
Notice that the container turns in opposite direction!
For the balls, in small number, they are pulled by friction towards the bottom of the container. In greater number, they follow the force of friction – therefore more important onto the vertical wall.
What to do?
Make a ball turn, then 5 or 6 in the container and observe. Repeat the experiment by filling the bottom of the container. What happens? Why this phenomenon?
What we learn?
You may concentrate you observation and improve your deductive skills by gradually increasing the number of balls.
For a better understanding of the phenomenon, put the disc with an arrow on it in the container and make it turn. Observe again.
The disc is involved, by the centrifugal force, in the direction of rotation of the container. But it also whirls in the other direction. That is due to the friction of the disc (and the balls) against the edge of the container.
Notice that the container turns in opposite direction!
For the balls, in small number, they are pulled by friction towards the bottom of the container. In greater number, they follow the force of friction – therefore more important onto the vertical wall.
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