Reduction of fluid friction on the surface coated with TiO₂ photocatalyst under UV illumination

Fluid friction in a cylindrical glass pipe coated with TiO₂ was investigated before and after UV illumination. The crystalline phase of TiO₂ was anatase. Its coating thickness was about 90 nm. The frictional drag was evaluated by measuring the pressure loss in the pipe. Before UV illumination, the pressure loss changed gradually from a laminar flow to the turbulent one in the transition range, where the Reynolds number was around 3,300. However, the pressure loss after UV illumination maintained a laminar flow until the Reynolds number was 6,000. The reduction rate of the frictional drag increased concomitantly with increasing flow velocity. Particle image velocimetry analysis revealed that the fluid friction would be reduced by suppressing unstable flow around the wall of the glass pipe. These results imply that highly hydrophilic TiO₂ surface effectively reduced fluid friction in a large Reynolds number range effectively, and that suitable surface wettability for the reduction of the fluid friction depends on the flow velocity.