Electrokinetic Properties and Their Effect on Thermal Conductivity of Nanofluids

This paper presents the effects of pH value, surfactant, and electrolyte concentration on zeta potential and effective thermal conductivity of nanofluids. The zeta potential of TiO₂ (15 nm) nanoparticles in deionized water of different pH values was measured and the iso-electrical point was found to be between 4.9 and 5.2. The results show that the higher the electrolyte concentration, the smaller the particle zeta potential, which results in increased agglomeration of particles. The enhanced thermal conductivity of TiO₂/deionized water-based nanofluids was found to decrease with increasing pH value or electrolyte concentration. For example, for 0.2 volume %, the thermal conductivity of this nanofluid was found to decrease from 5.5% to 2.5% when the pH value was increased from 3.4 to 9. A similar decrease (≈ 2%) in thermal conductivity was observed when NaCl concentration was increased from 0.01 mM to 10 mM. The presence of a surfactant showed considerably larger enhancement of the effective thermal conductivity compared to that of nanofluids without surfactant.