Download il "ferrofluido" ha quelle caratteristiche di comportamento

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In our experiment we will explore how to make a
ferrofluid, a liquid apparently without anything
special, but that turns out to be amazing as soon
as you approach a magnetic field! It consists of
very small magnetic particles, hundreds of
thousands of times smaller than a millimeter, a
few nanometers, immersed in mineral oil, vegetable
oil or engine oil. The iron filings will not work
well, they are too big. Good sources of magnetic
particles are:
 toner of laser printers;
 magnetic powder control (sold in shops for
welding);
 •particles from burned steel wool, filtered ;
 •particles obtained by scraping the surface of
VHS-film or alternatively by burning it,
separating the residue with a magnet.
We have chosen to use the steel wool.
.
MATERIALS:
- steel wool, 1 package
- tights (to filter dust)
- engine oil, about 30 mL
- liquid soap (or other surfactants), a
few drops
TOOLS :
- beakers and capsules
- watch glass
- glass chopsticks
- aluminum container
-lighter
- magnets
PROCEDURE :
First you take your steel wool, which can be easily found in
household goods shops or iron mongers, put it in an aluminum
container and set it on fire. The "wool " burns out quickly,
with a nice effect but without dangerous sparks, and as soon
as it cools you can continue.
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What remains after combustion is a great base for the
ferrofluid, but it needs to be pulverized before. To do this
rub it with your hands very carefully, picking up the gray
powder that comes off the steel wool.
This powder is still too heterogeneous for the experiment,
it's necessary to filter it. So, collect the filings in a glass or
in a beaker, cover the opening with tights which act as a
filter. By turning the glass upside down and shaking it you will
get a fine smooth gray powder
For best results you can still crush the filtrate with pestle and mortar, or repeate
the filtration.
In a capsule add engine or brake oil to iron powder,
in a 1:1 ratio, and with a chopstick stir until
obtaining a homogeneous mixture, adding gradually
drops of liquid soap
We made several attempts, using seed oil , engine , brakes ,
etc. .
Our ferrofluid is ready!
To observe its behavior under the influence of a magnetic field you just have to pour
it on a capsule or on a watch glass and bring one or more magnets. The ferrofluid will
lose its "puddle" shape and gather around the poles of the magnet, creating beautiful
three dimensional forms. These vary from magnet to magnet and disappear as soon as
the magnetic field moves away. The greater the
strength of the magnet the more noticeable is the
effect obtained.
To find a very strong magnet we enjoyed ourselves opening an
old hard-disk.
The following pictures have been taken in our lab with a
ferrofluid bought on the web for a better effect.
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THEORETICAL CONTENTS
The ferrofluid is composed of
particles of a ferromagnetic
element dispersed in a fluid, in
our case oil. These particles tend
to form agglomerates and
structures
of
greater
dimensions, but the action of a
surfactant, such as soap, avoids
it. The soap molecules possess, in
fact, a hydrophilic hydrophobic
part that allows them to bind simultaneously with the particle of iron
and with the molecule of fat (oil), practically by coating and isolating the individual
particles. High temperatures also counteract the agglomeration, and are exploited to
achieve commercial ferrofluids.
To explain the behavior of ferrofluid under the action of a magnetic field you must
imagine each particle as a small magnet that can move freely within the oil. The
ferrofluid, however, does
not behave like a "liquid
magnet",
in
fact,
it
doesn't attract iron, but
if we approach an external
magnetic field, these tiny
magnets leave their mess
and get arranged parallel
to the lines of force field.
For example, if you move a
A sinistra i dipoli che si orientano secondo il campo magnetico
"north" pole to the right
(semplificato in linee parallele.)
of the ferrofluid, all the
particles that are dispersed in the oil in an extreme disorder will rotate turning their
pole "south" to the right. In addition, each magnetic field force line has
characteristics that can be observed with a few iron filings and a white sheet of
paper. Just as the filings on the sheet of paper,
the particles of ferrofluid are arranged following
the complex geometric patterns of the force
lines, which vary according to the frequency and
the amplitude of the magnetic field.
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To explain the behavior of a material subjected to a magnetic field, you need to
further analyze its atomic
structure. You've got to
remember that all the
coils
crossed
by
an
electric current generate
a vertical magnetic field in
the centre, behaving just
like a compass needle.
Every single electron that revolves around a core represent a microscopic electric
current, so it is a magnetic dipole. The arrangement of the dipoles is disorganized,
and the thermal agitation (the vibration of the molecules due to heat) prevents the
wheel from being tidy. A ferrofluid doesn’t keep the polarization, obtained for
orientation, when you remove the external magnetic field. Therefore it doesn’t work
as a ferromagnet but as a super-paramagnet.
Albert Einstein
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