pH Effect on an Ionic Ferrofluid: Evidence of a Thixotropic Magnetic Phase

We investigate the stability of aqueous colloidal dispersions of ionic magnetic nanoparticules as a function of pH. The charge of the γ-Fe 2 O 3 particles is modified, and as a consequence, the interparticle interactions may be tuned through pH variations. Scanning the whole pH scale from acidic to alkaline medium, different states are observed for the dispersion: sol, thixotropic gel, and floc (around the point of zero charge). Using the dynamic magnetooptical properties of the magnetic grains, we locally probe the sols and the thixotropic gels. A steep increase of the characteristic time of birefringence relaxation, in the sol phase close to pH Gel, marks a divergence of the macroscopic viscosity at the gel point, compatible with a percolation theory. A temporal study of the regeneration of shaken gels reveals that, in these thixotropic systems, the process is dominated by a cluster aggregation on a sample-spanning one, with an energy barrier to overcome, compatible with chemical measurements.