Ferrofluids

History of Ferrofluids 

All magnetic materials known to man since their discovery were in solid forms, either as permanent magnets or soft magnetic materials. Although magnetic suspensions existed in the 1930s, it was the dream of man to create a durable, stable liquid magnet. The drive to create such a magnetic liquid came during the early years (1960s) of the NASA program. 

Steven Papell at NASA was tasked with controlling and directing liquid rocket fuel in outer space. The absence of gravity allowed the fuel to float in the holding tank and it was therefore a challenge to pump the fuel efficiently into the rocket engine. Papell envisioned converting the nonmagnetic rocket fuel into a fuel having magnetic properties so that it could be controlled under zero gravity by powerful magnets or pumped through switching magnetic fields. Papell is credited in literature with the preparation of the first magnetic fluid, based on kerosene. He obtained a US patent in 1965. 

This idea was never put into practice by the space agency, due to their preference for solid rocket fuel. By all modern standards, the magnetic fluid synthesized by Papell was crude and would not be practical for any present-day applications. However, it set the foundation for intense scientific research and development on magnetic fluid technology. The pioneering research funded by NASA under the technical leadership of Ron Rosenzweig at AVCO Corporation, Wilmington, MA led to the development of a wide variety of early magnetic liquids for commercial use. The magnetization and stability of the fluids were greatly improved. The fluid mechanics of magnetic fluid phenomenon was recognized as a new branch of science and was named Ferrohydrodynamic. 

Commercial use of ferrofluid began in 1968 with the founding of Ferrofluidic Corporation by R.E. Rosenzweig and R. Moskowitz. The official name for the product was given as ferrofluid. Since then, the company and the product have both changed a great deal, in both size and applications. To understand how ferrofluids are used in today’s world, let’s begin with an understanding of how ferrofluids are created. 

Ferrofluid Chemistry 

Ferrofluids are composed of three basic components, 

a) Sub-microscopic, magnetically permeable particles of iron oxide, usually magnetite, 

b) A surfactant or dispersant coating and 

c) A carrier fluid. 

A typical ferrofluid consists of about 5% of nano sized magnetite (Fe3O4) particles and surfactant ~ 10 % in a carrier fluid ~ 85%. 

The surfactants are soap-like materials that work to coat the particles and keep them from being attracted to each other. The carrier fluid may be water or oil, and will help to determine the over-all viscosity, or thickness of the ferrofluid. The concentration of particles in a ferrofluid also contributes to the viscosity, but since this value is typically low, the nature of the carrier fluid has the greatest influence. The particle concentration also determines the “Saturation Magnetization” (Ms) of the fluid, or the degree to which it is attracted to a magnet. Ferrofluids are not magnetically attractive in themselves; they must be in a magnetic field in order to behave as one magnet. 

Ferrofluid Behaviour 

Many of you may have seen commercials on television that show what looks like black oil taking on a life of its own, spiking up and dancing around. Perhaps you have seen videos on YouTube that look like magically dancing black fluid spiralling up and down and wondered what it was. These were most likely ferrofluids, exposed to magnetic fields to make them “perform”. 

Ferrofluid can be defined as a uniform mixture of magnetic particles, soap-like molecules and carrier fluid that will behave as a single unit in a magnetic field. The sharp spikes that appear are still fluid and are not solid points. The fluid behaves as one unit when the conditions are right, and magic apparently happens!! 

Ferrofluid also exhibits an apparent change in density when exposed to a magnetic field. One of the favourite tricks for science demonstrations is the “floating penny” in which a ferrofluid will lift small items, such as non-magnetic coins, on their surfaces when in a reasonably strong magnetic field. 

Typical Applications of Ferrofluids 

The typical applications of ferrofluids are listed below. 

(a) Liquid seals and bearings
(b) Better loudspeakers
(c) MRI contrast agent
(d) Cellular imaging

For more than 40 years, the primary application for ferrofluids has been in loudspeakers for cooling, cantering of the “voice coil” and smoothing of the sound produced from the electro-mechanical device that drives a speaker. The diagram below shows the speaker cut in half, so you can see where the fluid is applied. Just a tiny amount of ferrofluid makes a huge difference for the quality of sound and the length of life of over 500 million speakers produced every year! Today the tiny speakers inside your cell phones, laptop computers, tablets and headphones or earbuds may contain ferrofluid – in tiny amounts! 

Ferrofluid is also used inside mechanical devices called “vacuum rotary seals” and acts as a seal against moisture, dust and gases. The ferrofluid forms what we call a “liquid O-ring” and is exactly what it sounds like – a ring of fluid that seals a space and protects the moving parts of the device. These seals are often found in clean rooms and robotic devices that perform functions that require free motion and protection from contamination by the environment outside their working parts. These seals allow us to produce components such as computer chips and memory cards that are flawless. 

Ferrofluid for bio-medical applications is usually one of two types: 

(1) Magnetic Hyperthermia, where particles are injected into a tumour and then excited in a magnetic field to heat the tissue to a point where the tumour cells become weakened and vulnerable. 

(2) Targeted Drug Delivery, where drugs or antibodies and “targeting molecules” are attached to the nanoparticles and are injected into the blood stream, then guided by magnetic fields to the site of the tumour or lesion. Both methods cause little damage to the body, much less than some current standard therapies. 

Ferrofluids are also used for cellular imaging and as an MRI contrast agent. 

Ferrofluids are very popular in exhibitions pertaining to many concepts in science and art.

Magnetic behaviour of ferrofluids: 

Ferrofluids are superparamagnetic. Ferromagnetic materials at nanoscale behave like superparamagnetic. They respond only in the presence of a magnet/magnetic field. 

Upcoming Applications of Ferrofluids 

Self-pumping magnetic cooling 

Efficient thermal management and heat recovery devices are of high technological significance for innovative energy conservation solutions. A novel technique involving a self-pumping magnetic cooling device, which does not require external energy input, employing Mn–Zn ferrite nanoparticles suspended in water is proposed in literature. The device performance depends strongly on magnetic field strength, nanoparticle content in the fluid and heat load temperature. Cooling (ΔT) by ~20 °C and ~28 °C was achieved by the application of 0.3 T magnetic field when the initial temperature of the heat load was 64 °C and 87 °C, respectively. These experiments results were in good agreement with simulations performed with COMSOL Multiphysics. This system is a self-regulating device; as the heat load increases, the magnetization of the ferrofluid decreases; leading to an increase in the fluid velocity and consequently, faster heat transfer from the heat source to the heat sink. 

How and why Ferrofluids behave like as they do 

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 its 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. 

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 an enough 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 Ferrofluid. Why does it rise and fall (The Inspiration)? Initially, the Ferrofluid is denser than the clear liquid and sits at the bottom. But, the Ferrofluid changes its density when it absorbs heat from the lamp at the bottom and becomes 'lighter'. This allows it to float to the top. Then, it begins to cool and its density changes once again. As it cools it gets 'heavier' and falls back down. Then the process repeats itself.

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