Magnetic detection of nanoparticle sedimentation in magnetized ferrofluids

Colloidal stability in external magnetic field is crucial for applications of ferrofluids. Here, we introduce a magnetic analysis approach to monitor how rapidly magnetic nanoparticles are pulled out of the liquid in an external magnetic field gradient. The motion of the sedimentation front is deduced from the time-dependent field produced by a column of ferrofluid placed on a permanent magnet. Citrate-stabilized nanoparticles in a homemade aqueous ferrofluid are found to sediment at the rate expected of single nanoparticles. More rapid sedimentation occurs in two other types of ferrofluid, indicating that our magnetic sedimentation analysis method can differentiate ferrofluids with respect to their in-field colloidal stability. Our method is further validated by comparison with time-dependent X-ray transmission profiles.