Experimental investigation of drag reduction induced by hydrogels crosslinked with Fe ³⁺ in water-soluble and -insoluble liquid oils

Hydrogels have considerable potential for use in marine transportation, but few studies have examined its attribute of drag reduction in liquid oils. In this paper, we first prepared a series of hydrogels crosslinked by ferric ion (Fe³⁺) through photoinitiated radical polymerization, and then a rheometer was used to measure their performance in terms of reducing drag in flow fields containing the water-soluble aqueous glycerol solution and the water-insoluble dimethyl silicone oil (DSO). The results showed all the hydrogels considered in our experiments could reduce drag in both experimental liquid oils, where their crosslinking with Fe³⁺ led to the decline of drag reduction. The higher viscosity of the aqueous glycerol solution induced a higher reduction in drag until it reached a value of 18 mPa·s. The hydrogels exhibited different mechanisms of drag reduction in different liquids. Drag reduction was induced in the aqueous glycerol solution by the boundary slip on the lubricating layer formed by the dissolution of water and glycerol, while that in DSO was induced by the boundary slip on the water layer of the surface of the hydrogel due to the insolubility of DSO in water. The work here revealed the mechanism of drag reduction induced by hydrogels that could be applied to marine transportation.