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.

In this work, we present the evaluation of the thermotropic gelling ability and steady-state rheology of aqueous sodium alginate (NaAlg) solutions. NaAlg is been considered due to its gelling capabilities and to determine its suitability as a film-forming agent with the potential capacity for immobilization of very potent, solid, and hydrophobic drugs. The existence of a thermally induced gelation of 1.0 to 2.5 wt.% sodium alginate solutions between 13°C to 22°C was demonstrated through three independent rheological tests: the constant-stress temperature-ramp viscosity, dynamic viscoelastic moduli, and thixotropy tests. Oscillatory dynamic tests also showed the thermoreversible nature of the NaAlg gels, but a large hysteresis between the gel formation and the gel melting was observed. The steady-state viscosity was experimentally determined as a function of shear rate for NaAlg concentrations between 1.0 to 2.5 wt.% and for temperatures from 20°C (or above T gel ) to 90°C. A viscosity master curve was constructed by applying time-temperature superposition and a semi-empirical shift factor for concentration. A power-law fluid modeladjusted to the data from 20°C to 60°C and found to provide good agreement with fitting parameters k=4.81 and n=−0.449±0.003.

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