Ferrofluid

Ferrofluid emerged in 1963 from a peculiar problem: how do you pump liquid fuel in zero gravity? At NASA's Lewis Research Center in Cleveland, mechanical engineer Solomon 'Steve' Papell was working on liquid hydrogen engines when he confronted the challenge. In space, without gravity to settle fuel at the bottom of tanks, liquid propellants slosh unpredictably. Engines that need to restart after a coast phase might draw vapor instead of liquid.

Papell's solution was elegant: add magnetic dust to rocket fuel so it could be drawn toward pumps with a magnet. He ground iron oxide particles small enough to remain suspended indefinitely in the carrier fluid, coating each particle with surfactant to prevent clumping. The result was the first true ferrofluid—a stable colloidal suspension of nanometric ferromagnetic particles that responded to magnetic fields while flowing like a liquid.

The physics were fascinating. Ferrofluid particles are small enough (typically 10 nanometers) that Brownian motion—random collisions with solvent molecules—keeps them suspended against gravity. The surfactant coating prevents the particles from aggregating into larger clumps that would settle out. When a magnetic field is applied, the fluid as a whole moves toward the field source, forming the distinctive spiky structures that have made ferrofluid a favorite of science demonstrations.

Papell received a patent in 1965, but his approach never flew in actual rockets. Adding iron oxide changed combustion efficiency—the particles don't burn cleanly with the fuel. 'You have non-ideal combustion because of these particles,' as one researcher explained. The specific problem Papell set out to solve was eventually addressed through other means.

But ferrofluid found unexpected applications elsewhere—a classic case of exaptation. By the mid-1970s, the material was cooling loudspeaker voice coils (the fluid conducts heat while the magnet keeps it in place) and sealing rotating shafts in computer hard drives (creating a magnetic liquid bearing with no mechanical contact). Medical researchers explored using ferrofluids to target cancer drugs to tumors using external magnets.

In 1982, NASA's Inventions and Contributions Board awarded Papell $15,000—their highest monetary award at the time—for inventing ferrofluids. He retired in 1983 after 33 years of government service and passed away in 2015 at age 97, having seen his failed rocket fuel become a material with applications he never imagined.