(686g) Producing Reversible Gels to Control the State of Aggregation in Nanofluids

The presence of nanoparticles in thermal fluids (nanofluids) has been shown to improve the heat transfer properties of the fluid but the magnitude of the enhancement and the physical mechanism by which this enhancement occurs are open to debate. The standard classical model is Maxwell's mean field theory, which is borrowed from electricity and offers a prediction for the conductivity of the fluid as a function of the conductivity of particles and fluid. The standard theory produces two limiting bounds depending on whether the dispersed phase is the solid or the fluid. Since colloidal particles invariably suffer from aggregation, aggregate networks are thought to increase conductivity somewhere between the two bounds of the Maxwell theory. Direct experimental tests of this hypothesis have not been made because controlling the degree of aggregation in typical colloids in a systematic way is not possible. In this work we develop a colloidal system with the ability to undergo \textit{reversible} aggregation/deaggregation and produce stable aggregates of desired size, all the way from fully dispersed nanoparticles (30 nm in diameter) to fully gelled systems (infinite size). We accomplish this by treating Ludox silica with N-[3-(Trimethoxysilyl)propyl]ethylenediamine to produce a monolayer of the amine that is chemically bonded onto the particle. At basic pH the amino groups are positively charged and allow the full dispersion of the nanoparticles. At lower pH the amino groups are in neutral form such that the state of the suspension is dominated by the interaction between the hydrophobic chains and the surrounding fluid (water) causing the colloid to aggregate. The degree of aggregation depends on the degree of ionization of the amino groups and this in turn is tuned via pH. Since the surface of silica is covered, these aggregates are not permanent clusters but can be either grown to larger size, by decreasing the pH, or dissolved, by increasing the pH. This allows us to produce stable nanofluids ranging from 30 to 500 nm. We provide colloidal characterizations of this system and report the thermal conductivity of the nanofluids.