Ferrofluid has iron particles that are on the nanoscopic scale and are suspended in an oil or acid. Ferrofluid it alters its shape to match the geometry of any magnetic field lines that pass through it. The stronger the magnetic field, the denser the grouping of field lines, the more that ferrofluid will follow those lines. Regardless of time, the iron particles are small enough, and the surfactants in the fluid strong enough, that brownian motion is enough to prevent the particles from separating from the oil. But no matter how strong the magnetic field gets, the viscosity of ferrofluid never changes for practical purposes.
Magnetorheological fluid – “MR fluid” for short – is a ‘smart’ magnetic fluid that is a close cousin of ferrofluid. While chemically similar, the two fluids behave extremely differently thanks to the different scale of their magnetic particle. Both MR fluid and ferrofluid are comprised of iron particles, an oil or acid, and a surfactant, but the similarities end there.
MR fluid has particles that measure on the microscopic scale. It is a slightly thicker fluid, but instead of altering shape as a function of electromagnetism, it alters viscosity. As the magnetic field strength increases, so does the viscosity of the MR fluid – up until the fluid reaches a magnetic saturation point that varies from fluid to fluid. MR fluid will never change shape to match the fields it is exposed to. Because the particles are an order of magnitude larger, the forces from the surfactants are not strong enough for brownian motion to keep the iron particles suspended in the oil. MR fluid will inevitably separate if unused, and regardless of use, it will wear out to the point that it needs to be replaced.
Robots all require very precise control, and the easier it is to eliminate the percentage overshoot from the mechanical systems themselves, the easier an electrical and software system will be able control the robot. To obtain a more organic motion, a small overshoot and rapid response time are both necessary. Current feedback systems are good at prioritizing one, or the other. To accomplish organic mechanical motion, novel mechanical systems will be necessary.