Experimental investigation of drag reduction induced by hydrogels crosslinked with Fe³⁺ in water-soluble and water-insoluble liquid oils

Hydrogels have considerable potential for use in marine transportation, but few studies have examined their ability to reduce drag in liquid oils. In this work, we first prepared a series of hydrogels crosslinked by ferric ions (Fe³⁺) through photoinitiated radical polymerization, and then a rheometer was used to measure their performance in terms of reducing drag in flow fields containing a water-soluble aqueous glycerol solution and a water-insoluble dimethyl silicone oil (DSO). The results showed that all the hydrogels considered in our experiments could reduce drag in both experimental liquid oils, where their crosslinking with Fe³⁺ led to a decrease in drag. The higher viscosity of the aqueous glycerol solution induced a greater 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. This work revealed the mechanism of drag reduction induced by hydrogels that could be applied to marine transportation.