Regeneration of thixotropic magnetic gels studied by mechanical spectroscopy : effect of pH
Résumé
Regeneration of thixotropic gels resulting from aggregation of positively charged magnetic nanoparticles is here investigated by mechanical spectroscopy. The frequency independence of the ratio ( and being respectively the viscous and the elastic moduli) at the gel point seen as a liquid–solid transition allows the determination of the gelation time tg and of the power law exponent on the angular frequency for both moduli (, ). This behaviour is similar to the one observed for classical chemical and physical gels. It implies a broad relaxation time spectrum which is compatible with previous dynamic magneto-optical birefringence measurements. The value of tg decreases exponentially with an increase of the pH that controls the interactions between particles through their surface density of charges. In contrast, the relaxation exponent is found to be nearly insensitive to the pH. A fractal dimension of 2.07 ± 0.06 is deduced from the power law exponent Δ at the gelation time.
