Investigation on superhydrophilic surface with porous structure: Drag reduction or drag increasing

In this paper, different apertures of superhydrophilic porous TiO2 films are fabricated by sol-gel method with PEG2000 additive. The drag reduction effect of the films is investigated through experiment of falling-ball method and FEM simulation. The larger PEG2000 content, the bigger size and the more regular and homogeneous of the pores. The joint action of anatase crystal, hydroxyl groups and water molecules absorbed on the surface and porous structure causes the superhydrophilicity. The resistance test shows that when the addition amount of PEG2000 changes from 0.25 g to 2 g, the drag reduction efficiency increases from -17.9% to 8.6% and -16.8% to 9.4% with 12 mm and 25 mm diameter balls. The drag reduction mechanism can be obtained from Finite Element Method simulation. The result shows that vortexes are formed in the pore which can reduce the frictional drag by lowering the velocity gradient and giving a frictional driving force. Meanwhile, an "extra press drag" is formed between the front and back wall. If the degree of friction drag reduction is greater than the degree of press drag increase, the film will show the effect of drag reduction overall. (C) 2017 Published by Elsevier B.V.