Nanofluids are superior to pure fluids in heat transfer applications, such as direct solar collectors, due to enhanced thermophysical characteristics. Optimal dispersion is crucial, and direct ultrasonication is the most effective method in dispersing nanoparticles and preparing stable nanofluids; however, the optimal sonication time remains uncertain across different nanofluid types. In this study, alumina (Al₂O₃)–water nanofluid (0.1 wt%) and sodium dodecyl sulfate (SDS) surfactant were prepared using ultrasonication (up to 240 min). This study linked sonication duration to performance indicators—including zeta potential, particle size distribution, optical absorbance, pH, conductivity, and solar thermal conversion—to investigate dispersion and Photothermal Conversion Efficiency (PTE) under a solar simulator. The results indicate that sonication for up to 180 min enhances zeta potential, reduces agglomerate size …