Experimental and DLVO theory study on pH effects regarding the stability and thermal performance of TiO2/H2O nanofluids

TiO 2 /H 2 O NFs exhibit the highest stability at pH = 2 and pH = 12. • Near the IEP, electrostatic repulsion weakens, leading to particle aggregation. • Combined experimental and DLVO theory to analyze TiO 2 /H 2 O NFs stability. • Temperatures above 318 K and proximity to the IEP degrade TiO 2 /H 2 O NFs thermal performance. Nanofluids (NFs) attract attention in heat transfer due to high thermal conductivity, but nanoparticle aggregation limits their performance. Herein, 0.1 wt% TiO 2 /H 2 O NFs were prepared by a two-step method to investigate the effects of pH (2−12) and temperature (293–353 K) on their stability and thermal performance. Stability was highest at pH = 2 and 12, and the isoelectric point (IEP) was located at pH = 6.5. Near the IEP, weakened electrostatic repulsion promoted aggregation, as confirmed by particle size, Brownian-to-settling velocity ratio, and SEM observations. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory showed that the reduced interaction energy barrier near the IEP increased aggregation, which raised relative viscosity, lowered thermal conductivity, and deteriorated the performance enhancement ratio (PER). The best PER values were obtained at pH = 2 and 12, consistent with the enhanced stability away from the IEP. This study provides a basis for optimizing NF stability and thermal performance via pH control for reliable thermal management. Stability and thermal properties of TiO 2 /H 2 O nanofluids at different pH values. Download: Download high-res image (294KB) Download: Download full-size image