Magnetic sculptures

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.

(Click the image to enlarge)

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.

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.

La Fête de la Science

Bermouli makes waves!
The blue liquid (water saturated with salt) is denser than the other liquids (oil silicone). When the tube is tilted, the blue liquid “goes down” and the transparent liquid “goes up”. This relative movement of the two fluids is accompanied by the appearance of small waves to the interface. When two fluids (liquids of gas) in parallel move one with the other at different speeds, the interface which separates them can be unstable.

It is a consequence of the Bernoulli effect; when a fluid moves, the greater the speed, the weaker the pressure.

Let us imagine wind blowing on the surface of a lake. If the surface of the water incidentally grows hollow somewhere, then the speed of the wind in the trough is weaker as the pressure is greater.

The air presses thus more on the water at the bottom of the trough than at the top of the bumps and the initial deformation is accentuated.

Fortunately, there are mechanisms which moderate this instability and prohibit that the waves do not become infinite… In particular, the interfacial tension prevents that the surface of separation of the two fluids increases too much and its force of gravity supports the horizontality of the interface.

The sky is blue!

What to do?
Put the lamp in front of the small tube, then in front of the big tube, and observe the change of the colour. Do it again and look now through the extremity of each tube. What is the colour of the lamp?

What we learn?
The light that comes from the Sun is a mix of colours: blue, yellow, green, red… the atmosphere of the Earth doesn’t entirely transmit all those colours and radiations. During daytime, part of the radiations is absorbed and diffused again: the blue colour is the most diffuxed one. It is the case in the whole atmosphere, and that is why the sky is blue.

In the evening, on the horizon, the light crosses a thicker layer of atmosphere. The blue colour does not reach us anymore, and the red one is still diffused. This sometimes happens also during the day, when the air is full of industrial dust in suspension which accentuates the phenomenon of diffusion.