Almost every science-related account on Instagram or Tumblr has a video clip or a gif showing a ferrofluid in action. And why not? Who doesn’t like shiny metallic things that make epic shapes in the presence of a magnet? But beyond their obvious beauty, ferrofluids are perfect to understand a few important concepts of physics and chemistry. So let’s break em down and really understand what’s going on here.
What is a ferrofluid?
“Ferrofluid” is a word made from the combination of the words “ferromagnets” and “fluids”. Ferromagnets are really strong magnets that display permanent magnetism. Combine a ferromagnetic substance with a liquid and that is essentially what a ferrofluid is.
But we need to dive deeper. A ferrofluid is a mixture of finely grounded ferromagnets (containing iron) coated with a surfactant (prevents the magnetic particles from clumping) and mixed in an organic solvent or water. Ferrofluids have a lustrous spiky shape that morphs into different sizes and patterns in the presence of magnets.
How does a ferrofluid work?
A ferrofluid is a colloidal suspension. Which means that in a properly concocted ferrofluid of high quality, the ferromagnetic substances in the solution do not settle down. They are suspended within the solution. With the ferrofluid solution, you might expect the magnetic particles to eventually settle down due to gravity. But they don’t because of something called Brownian Motion. According to which, the molecular movement within a solution keeps the particles afloat.
When this solution is brought into the presence of a magnetic field, the ferromagnetic powder, naturally, experiences force along the magnetic field lines and acts accordingly. But wait, there are other forces than just the magnetic field at play here.
The surface tension of the ferrofluid surfactant is another force at play here. All liquids have this property where the outermost layer has slightly higher energy than the remaining molecules of the liquid. And according to physics 101, all physical systems tend to move towards a stable energy level. So in response to the fact that they have higher energy, the top surface of the liquid exert a force on the molecules within them. This is called surface tension. Consequently, this results in the liquid assuming the lowest possible surface area, since that way, lesser molecules are in the high energy region. And according to mathematics, the lowest surface area for any given volume is a sphere. Hence water droplets always appear spherical.
Or if that was too complex to understand, you can envision molecules of water with attractive forces all around. The molecules on the top will have more attractive forces between themselves, leading to a higher forces in this layer, which is basically surface tension.
The third force in the above scenario is good old gravity. It just wants the liquid to stay flat.
So to summarize
- Magnetic field – Makes the ferrofluid move in the direction of the field lines
- Gravity – Makes it stay flat
- Surface tension – makes the spikes look a bit curvy around the base
The net result from the interplay of all these forces is what sets up the visual marvel of a ferrofluid and leads to the formation of those beautiful spikes.
What are the uses of a ferrofluid?
Ferrofluids find applications in things ranging from consumer products to testing of theoretical propulsion thrusters. Let’s take a look at a few interesting examples.
Ferrofluids in speakers
You might have seen speakers blaring audios at loud volumes. No really, imagine a speaker. There is a circular (usually) part of a good quality speaker that moves in and out depending upon the beats of the music. This part is called the cone of the speaker. It moves because it is attached to the frame using a flexible material. The flexibility of this material adjusts according to the frequencies that the speaker is trying to produce. Some of the really expensive brands of speakers on the market use ferrofluid coating in the right spots to allow the cone to be flexible at right times.
The ferrofluid in speakers also acts as a cooling agent and help cool down the speaker via a process known as thermomagnetic convection. The heat produced by a voice coil is absorbed by the ferrofluid, which heats it up and consequently lowers its magnetic susceptibility. The magnetic field around the voice coil thus attracts the cooler ferrofluid more resulting in an outward motion of the hotter ferrofluid, which naturally eliminates heat from the system.
Ferrofluids in the aviation and semiconductor industries
Ferrofluids are also used to make semi-active dampers to absorb vibrations in machines and aircraft. They also find applications in the semiconductor industry as a seal to keep silicon wafers safe from contamination.
Ferrofluids in Medicine
It is also being researched for use in medicine for magnetic drug targeting. Cancer sucks. A big reason behind that statement is because chemotherapy is way underdeveloped in terms of proper targeting. According to a study at the University of Maryland, less than 0.1% of chemotherapy drugs reach the tumor, the remaining are taken up by healthy cells. So in an effort to improve drug targetability, scientists are working on methods where the medicine can be combined with a magnetic carrier (a ferrofluid). This would result in a manual targeting of the drug to specific locations using magnetic fields.
How to make ferrofluid?
It’s actually really easy. Get some laser printer toner fluid in a beaker, say 50ml. Add a little corn oil to it. The spikes depend on the quantity of oil so start with very little. Mix them using a stirrer. Bring a small neodymium magnet near the beaker and you should be able to see spikes.
To make a cheaper one without laser printer toner check out this tutorial.
Also, check out this brilliant video to get answers to all of your questions about what it would be like to play with some ferrofluid.
- Magnetic Nanomaterials for Hyperthermia-based Therapy and Controlled Drug Delivery – Faruq Mohammad and Challa S. S. R. Kumar.
- Magnetic Drug Targeting: Developing the basics – A dissertation by Aleksandar Nelson Nacev
- Magnetic Fluids Deliver Better Speaker Sound Quality – NASA spinoff technologies