Experimental study: the impact of surfactant and nanoparticles concentration on the stability of alumina nanofluids

To address the instability of nanoparticles (NPs) in base fluids (BF) due to agglomeration and sedimentation, nanofluids (NFs) were developed to enhance heat transfer and thermal system efficiency. Surfactants are commonly added to improve NP stability by inducing steric or electrostatic repulsion to counteract Van der Waals forces. In this study, NFs were prepared by ultrasonically dispersing Al2O3 (0.02–0.5 wt.%) in distilled water, stabilized with Rhamnolipids (RHL, 200–600 ppm) and synthetic surfactants (SDS, SLS, and PVP, 2000–4500 ppm). Stability was assessed using UV-spectroscopy, dynamic light scattering, zeta potential, and viscosity measurements. Results indicated that stability improved with surfactant concentration, peaking at 3750 ppm (SDS), 4250 ppm (SLS), and 550 ppm (RHL). However, stability declined beyond 0.1 wt.% NP concentration, as indicated by a reduction in zeta potential values and UV–Vis absorbance intensity and an increase in particle size, suggesting increased particle aggregation. While RHL showed slightly lower stability than SDS and SLS, it is an eco-friendly alternative. PVP was ineffective and is not recommended for further research. This study is among the first to systematically evaluate the performance of a bio-surfactant, Rhamnolipid, in stabilizing Al2O3 nanofluids and compare it directly with synthetic surfactants.