Effects of hydrophilicity/lipophilicity of nano-TiO2 on surface tension of TiO2-water nanofluids

Highlights: • The surface adsorption of hydrophilic lipophilic nano-TiO₂ is positive adsorption. • The surface adsorption of hydrophilic nano-TiO₂ is negative adsorption. • The hydrophilic nano-TiO₂ increases the surface tension of the base fluid. • The hydrophilic lipophilic nano-TiO₂ decreases surface tension of the base fluid. • Surface tension changes linearly with an increase in the concentration of nanofluids. Platinum plate method based on Wilhelmy plate was used to measure the surface tension of hydrophilic nano-titanium dioxide-deionized water (nano-TiO₂- DIW) fluids and hydrophilic lipophilic nano-TiO₂-DIW fluids. Experimental results show that the surface adsorption of hydrophilic lipophilic nano-TiO₂ is positive adsorption, and hydrophilic nano-TiO₂ is negative adsorption. The hydrophilic nano-TiO₂ in deionized water increases the surface tension of base fluid, and hydrophilic lipophilic nano-TiO₂ in deionized water decreases. The surface tension of hydrophilic TiO₂-DIW nanofluids increases, and hydrophilic lipophilic TiO₂-DIW nanofluids decreases linearly with an increase in the natural logarithm of the molar concentration of nanofluids.