Effects of surfactant on the stability and thermal conductivity of Al2O3/de-ionized water nanofluids

• Non-ionic surfactant PVP is more effective than anionic surfactant SDS in the dispersion of Al2O3 nano-particles.
• High SDS concentration would deteriorate the stability of Al2O3/de-ionized nanofluids.
• Thermal conductivities of nanofluids have a great relationship with the concentration ratio of surfactant and particles.
• Nanofluids with larger particle size need higher concentration of surfactant to get the highest thermal conductivity.

Al2O3/de-ionized water nanofluids of different volume fractions with different surfactant mass fractions are prepared by two-step method. Thermal conductivities of the suspensions are measured by a Hot Disk thermal constant analyzer. The uncertainty of liquid thermal conductivity measurement is within 1.5% at room temperature according to the accuracy verification test of de-ionized water. Effects of two kinds of surfactants — sodium dodecyl sulphate (SDS) and polyvinylpyrrolidone (PVP) — on the stability and thermal conductivity of Al2O3/de-ionized water nanofluids are analyzed respectively. Results show that surfactant plays an important role in dispersing the nanoparticles into the base fluid and improving the stability of Al2O3/de-ionized water nanofluids. Non-ionic surfactant PVP shows better positive effects than anionic surfactant SDS on the dispersion and stability of the nanofluids. The highest thermal conductivity occurred at an optimal concentration ratio of surfactant mass fraction and particle volume fraction, where the ratio is partly associated with the particle size and decreases with the increase of particle volume fraction.