One falling liquid droplet impacting a sessile droplet and then undergoing oscillation is a common phenomenon in both nature and industry. However, to the best of our knowledge, the head-on collision of two nanofluid drops has not been studied when both the nanoparticle and surfactant exist in the drop. In this work, the characteristics of oscillation after head-on collision of two TiO2-water nanofluid drops were investigated experimentally. The effects of impact velocity, drop size, and nanoparticle concentration have been considered to understand how they influence the drop spreading, recoiling, and rebounding. For the influence of the drop size, it shows that in 0% and 0.001% of nanoparticle concentrations, the relative spreading radius of small drop size is higher than that with large size, while it is reversed in 0.01% and 0.1% of nanoparticle mass fractions. Interestingly, it was found that there is an optimal nanoparticle mass fraction of 0.1% for nanofluid droplets where the oscillation of head-on collision can be damped much more rapidly than that for nanoparticle-free droplets. A hypothesis considering the comprehensive interaction of surfactant redistribution, surfactant gradient, and particle initial effect was proposed to explain the observed phenomenon. This work will be of significant reference to various practical applications especially when the drop collision oscillation characteristics are supposed to be under control.
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