Experimental study on mechanism of stable drag reduction with hydrogel interface

The hydrogel is a novel material with drag-reducing ability, which has great potential to be applied in marine applications. In this paper, a series of superhydrophilic hydrogels were prepared by photoinitiated radical polymerization. The Couette flow and particle image velocimeter (PIV) observation in the water tunnel were conducted to investigate the drag-reducing characteristics of the prepared hydrogels. The results show that the hydrogel thickness of 1500 µm is the critical value corresponding to the maximum drag reduction rate of about 68%. The drag reduction is induced by the boundary slip phenomenon on the hydrophilic surface, and the further mechanism can be explained in two aspects. On one hand, the generated vorticities in the grooves between the macroscopic convex structures of the hydrogel induce the fluid sliding state converted into the rolling state. This state transformation reduces the near-wall velocity gradient, and thus promotes the generation of boundary slip. On the other hand, fluorescence particle experiments reveal that there exists water exchange between the inside and outside of the hydrogel. This exchange would promote the relative velocity, and thus strengthens the boundary slip. The present work is expected to make a certain contribution to reveal the drag reduction mechanism of hydrogel.