Unusual photorheological properties of TiO₂ nanoparticle suspensions under UV light irradiation

Precise control of the rheological properties of nanofluids in a non-contact way is of particular interest for its application in various nanotechnologies such as in microfluidic chips. In this study, for the first time, it was found that the viscosity of TiO₂ nanoparticle suspensions can be precisely controlled by UV light irradiation of certain time. After 20 min irradiation of UV light, the viscosity increment of the suspensions compared to the base fluid was observed to reduce by 63.2%. This photorheological effect is irreversible unless ultra-sonication treatment was applied. More interestingly, the viscosity of the suspensions first decreases and then reversely increases with the irradiation time, namely, there is a critical value for the irradiation time. However, the UV irradiation has negligible influence on the thermal conductivity of the suspensions. It turns out that the UV irradiation can cause both increased particle size and hydrophilcity of TiO₂ nanoparticles in the suspensions. Accordingly, molecular dynamics (MD) simulation revealed that the increased hydrophilcity of particle surface can result in the decrease in the viscosity of solid-liquid systems. It can be concluded that the observed unusual photorheological properties should be attributed to the combined effect of particle aggregation process and hydrophilcity increase. Our study should be of great value for nanoparticle suspensions in practical applications related to fluid flow and heat transfer.