Magnetic liquids are taking off, Hayley Bennett reports, but not as their inventor once imagined
Rocket scientists at the Nasa Lewis Research Center in Cleveland, US, were working day and night. It was 1963 and they were testing hundreds of different combustion chamber designs and injectors for liquid hydrogen engines, trying to iron out problems with combustion stability that could thwart efforts to put astronauts on the moon.
Solomon ‘Steve’ Papell, an ex-Army Air Corps navigator turned mechanical engineer, was at Lewis at that time, working on a problem particular to liquid propellants. Back then, it wasn’t clear how liquid fuel sloshing about in zero gravity could be guided to the combustion chamber when the engine needed to be restarted. Papell attempted to solve the problem by adding magnetic dust to rocket fuel, proposing that the fuel could be drawn to the chamber using a strong magnet.
Papell’s approach worked but it never took off, so to speak. His solution meant adding ground-up iron oxide particles to the fuel and that didn’t sit well with the rocket scientists as it changed the efficiency of their rockets, explains Manfred Ehresmann, a researcher at the Institute of Space Systems at the University of Stuttgart in Germany.
‘You have non-ideal combustion because of these particles,’ says Ehresmann. ‘That’s one of the reasons it was never applied.’ However, Papell did receive a 1965 patent for his magnetic fluid, which turned out to have some intriguing properties – and not just in zero gravity.
Outside a magnetic field, a so-called ‘ferrofluid’ behaves as you might expect any liquid to behave, but if a container of it is held next to a strong magnet, its surface appears to become solid, with jagged spikes pointing along the magnetic field lines. This phenomenon is referred to as Rosensweig instability after Ronald Rosensweig, the now-retired American chemist who devoted much of his career to understanding the behaviour of ferro- and magnetic fluids. Ferrofluids can also be poured onto a magnet being levitated by a superconductor to become suspended in mid-air.
All this weirdness makes ferrofluids well-suited to showy science demonstrations and art installations, even if they didn’t make it as rocket fuels – as Ehresmann points out, most existing applications tend to be non-combustible. One widespread use in the last 40 years has been in liquid seals for rotating parts, such as in computer hard disks, where they are kept in place by magnets and give smooth, high-speed operation with very little vibration or noise. ‘
A drop of fluid makes these devices possible; without that strategic drop the devices would not function,’ wrote Rosensweig in his 1985 book Ferrohydrodynamics. However, the last decade has seen scientific interest in ferrofluids rising again, with emerging uses in biomedicine, liquid robotics, plastic recycling and environmental remediation.
A true ferrofluid is a magnetic colloid; a stable suspension of magnetic particles in an oil- or water-based carrier liquid made by milling iron oxide in a surfactant, or by chemical synthesis.
The magnetic particles are small enough, usually between 3–15nm, to move by Brownian motion outside of a magnetic field and do not clump together or settle out. When drawn by a magnet, as in Papell’s proposition for pulling fuel to a combustion chamber, these tiny particles move their carrier liquid around with them, so that the suspension flows as a liquid. However, as the particles are paramagnets, the magnet does not give them long-term ordering and they go back to floating around like dust motes in air as soon as the field is removed.
The fact that these liquids can be controlled in remote fashion, using magnets, has seen them gain use in biomedical settings, explains Neil Telling, an expert in biomedical nanophysics at Keele University in Staffordshire, UK. Telling works on a much-talked-about technique called magnetic nanoparticle hyperthermia, which has long promised to lay waste to tumours deep within in the body by remotely heating up magnetic nanoparticles that then destroy the cancerous cells.
‘You supply the energy by the magnetic field,’ he says. ‘[The nanoparticles] can’t store that energy so it ends up getting released as heat.’ It’s thought cancer cells are more susceptible than healthy cells to heat, with researchers generally shooting for a few degrees above body temperature at around 40–42ºC.
One therapeutic ferrofluid called Nanotherm, developed by Berlin-based MagForce, was approved EU-wide for hyperthermia treatment of brain tumour patients in the EU in 2011, although it is currently only available in five clinics. Patients lie in an alternating magnetic field generator that focuses energy in the region of the brain where the ferrofluid is applied. A US trial in prostate cancer is also ongoing, with positive results announced in 10 patients in February this year.
The technique isn’t just proposed as a cancer treatment though and, in 2019, Telling’s team successfully used it to kill the parasite that causes the disfiguring lesions in the tropical disease leishmaniasis for the first time. Most recently, in 2021, Kent State University researchers in Ohio reported clearing the amyloid plaques associated with Alzheimer’s in human brain cells.
There are, however, still issues with getting the magnetic nanoparticles to heat in the same way in real tissues as in lab tests. ‘When you actually try and use [them] in a tumour, you get a different response because of the local biological environment,’ Telling says. Part of it, he explains, is that the magnetic nanoparticles in the ferrofluid need to be tailor-made to the application – many of those tested so far have been approved for applications in magnetic resonance imaging (MRI) but are not particularly well-suited to hyperthermia.
Potential biomedical applications also rely on ferrofluids’ liquid qualities to deliver drugs and squeeze through tight spaces into biological tissues. In 2020, Hui Xie’s team at the Harbin Institute of Technology in Harbin, China, published a study showcasing liquid microrobots made from ferrofluid droplets. Their droplet robots are oil-based because they envisage applications in the watery fluids of the body, with which the droplets do not mix. They are stabilised by surface tension but reconfigured using magnetic force.
By clever programming of small electromagnet arrays (or by movements of permanent magnets), the team get their ferrofluid microrobots to split, merge, morph shape and corral delicate hydrogel balls like cattle. These skills mean they can carry liquid cargoes to demonstrate drug delivery and navigate through 4mm-wide tubes representing bile ducts or catheters. ‘
A major advantage of a [ferrofluid droplet robot] is that it can squeeze through narrow spaces and enter enclosed areas with minimal damage to the surrounding environments that previous soft robots with a comparable overall size cannot reach,’ they write. However, they employ off-the-shelf ferrofluids and the distance between their robots and electromagnetic arrays is just 10mm – stronger magnets would be needed for control over greater distances.