Temperature-Induced Gelation in Dilute Nanofluids

We report a temperature-induced gelation in dilute nanofluids containing surfactant capped iron oxide and alumina particles of average diameter similar to 10 nm. We observe a dramatic enhancement in the elastic modulus, viscous modulus, and viscosity, by 3-6 orders of magnitude for a volume fraction (phi) less than 0.035, above a critical shear rate ((gamma)over dot(c)) and temperature (T(c)). The T(c) follows a weak power law scaling with phi as T(c) similar to phi(beta), where the scaling exponent beta is found to be -0.24. The observed gel-like transition at elevated temperature is attributed to strong van der Waals attractions on the kT energy scale due to poor solvent conditions, which is reminiscent of the phase behavior reported in polymer-coated colloids.