What is ferrofluid?
As you can see, ferrofluid is a magnetic liquid.
This is something special because all known magnetic materials lose their magnetization when they get too hot. And this always happens long before the material reaches it's melting point. The trick is to suspend tiny pieces of solid magnetic material in the liquid.
Normally, solid particles mixed in a liquid will eventually settle to the bottom because of gravity. Much like mixing sand in water. But, if the particles are small enough (say 10 nm) they won't be pulled down by gravity. This is because of a force called Brownian Motion.
Brownian Motion is the tendency of liquid molecules to be constantly moving around. As a result, they bombard other particles in the liquid. And when the particles are very small in size this force is strong enough to counteract the force of gravity.
It's a lot like a balloon being bounced around by a crowd of people. The sea of waving hands will keep bouncing the balloon back into the air.
Why does it spike?
The spiking is the result of a few different forces competing with each-other. On a fundamental level these would be the van der Waals force (attractive/repulsive forces between molecules), gravity and the magnetic force.
In this case, the first two forces (van der Waals and gravity) manifest themselves as surface tension and play a very big role in how the ferrofluid spikes.
Obviously, the magnetic force plays a major role in forming the spikes because we only see the spikes when we apply a sufficient magnetic field. What's really happening is we are attracting the magnetic nanoparticles with the magnetic field and this creates an uneven distribution of particles, or gradient, within the ferrofluid. This gradient follows the magnetic field lines and rearranges the nanoparticles in the ferrofuid.
Other ferrofluid applications?
Cooling in loudspeakers for better sound
One of the first economical applications for ferrofluid was in voice coils for loudspeakers. It increases the quality of the sound as well as the lifetime of the device with a very tiny amount of ferrofluid by efficiently cooling the system through a process called thermomagnetic convection. In this system, the voice coil produces heat as well as a magnetic field. The process is the result of the fact that magnetic materials in general tend to lower their magnetic susceptibility (how strongly they are attracted to a magnetic field) as they get hotter. This causes the colder regions of the ferrofluid to be more attracted to the voice coil than the hotter regions of the ferrofluid directly near it. This movement of the colder fluid towards the coil displaces the warmer fluid away from it. The whole process then repeats itself in a cyclical manner.
Liquid O-rings for making computer chips
Really, liquid o-rings can be used for more than just making computer chips. People love regurgitating that they are used in computer hard drives. The truth is the first serious application of ferrofluid was for vacuum rotary seals in the semiconductor industry (but the same principles apply). When computer chips are made, they are processed in the form of a 'wafer' that sits in a process chamber that must be under vacuum. It is very useful to be able to rotate the wafer in the chamber while it is being processed. The only way to rotate the wafer in a stationary chamber and still keep the system under vacuum without creating friction and introducing particles is with ferrofluid. The ferrofluid acts both as a seal and a lubricant. A magnetic field is used to keep the liquid in place around the drive shaft as a permanent seal for the system.
Magnetic hyperthermia for better chemotherapy
Magnetic particles can be made to vibrate under the control of a magnetic field. These vibrations give way to heat. In this way, they can be injected into tumor cells to weaken them.
Targeted drug delivery for cancer treatment
Magnetic nanoparticles can be surface modified to carry drugs as well as chemicals that act like sensors. These 'sensors' are designed to have an affinity for unhealthy cells. Then the magnetic nanoparticles are injected into the bloodstream and a magnetic field is used to guide them area in need of treatment. When the chemical sensor interacts with the unhealthy cells, it releases the drugs to kill the unhealthy cells. This is a bit like using a rifle as opposed to a shotgun when compared to traditional chemotherapy, which kills all cells indiscriminately regardless if they are healthy or not.
Shape shifting mirrors
There is research into using the shape shifting properties of ferrofluid to make tunable mirrors capable of filtering different wavelengths of light. This could be useful for applications with tunable mirrors so that filters don't have to be replaced.
Space craft propulsion
It has been re-iterated on the internet many times that NASA invented ferrofuid in 1963 in an attempt to use it as rocket fuel that could be manipulated in zero gravity by a magnetic field. However, they threw this idea out the window. However, in a somewhat ironic twist, ferrofluid is now being researched once again for space propulsion but in a much different way. Researchers interested in creating tiny satellites (about 10 cm) are aware that ion propulsion is would be effective. This is the use of a stream of ionizing gas to create thrust. This stream requires a very small needle like point. When ferrofluid is made out of an ionic liquid (special liquid salt material) it can be made to self assemble it's spikes into the required needle like points. These points then emit a stream ions that could possibly one day act as thrusters for satellites.
Keeping you curious
Most people don't realize it, but curiosity is a very important human emotion. It's the curious mind that figures out how things work and how to improve those things. All innovation stems from curiosity. Therefore, things that make you curious are very practical. No great achievement would be made or even pursued if it wasn't for a deep desire to better understand. Our curiosity is a good thing and we should embrace it.