Monitoring gelation using magnetic nanoparticles

The temperature dependent response of magnetic nanoparticles in oscillating magnetic fields was used to monitor gelation of the surrounding fluid at the size scale of the nanoparticles. The dynamic susceptibility of PEG-coated cobalt ferrite nanoparticles suspended in an aqueous gelatin solution had sharp transitions which correlated with gelation and melting observed using macroscale rheological measurements. The reported method requires small sample volumes and provides information on network formation at the scale of the probe nanoparticles.