Development and performance of a gelatin-based bio-polysaccharide drag reduction coating

The secreting drag reduction mucus in fish epiderm inspires the manufacturing of five gelatin–polysaccharide drag reduction coatings. First, a mixed solution composed of the gelatin and bio-polysaccharides [guar gum, xanthan gum, locust bean gum, tragacanth gum, or acacia gum] was poured into rectangular grooved polymethyl methacrylate (PMMA) plates, and bionic coatings were obtained after curing. Then, the surface characteristics of the coatings were characterized, and the internal micro-/nanoscale three dimensional (3D) net structures provided releasing access for the polysaccharide molecules. Importantly, a parametric study focusing on the gelatin and polysaccharide proportion affected the drag reduction of the coatings in a turbulent channel flow. Based on a smooth PMMA plate without a coating as a reference, the five developed coatings exhibited considerable drag-reducing effects with the corresponding maximum drag reduction rates that all exceeded 20%. There are three drag reduction mechanisms (polymer drag reduction, slip phenomenon, and wall flexibility) and one drag increase mechanism (surface roughness). Increasing the gelatin proportion affects the release rate of the drag-reducing agents, surface flexibility, and surface slip properties. Meanwhile, increasing the polysaccharide proportion promotes the release of polysaccharides, but increases the surface roughness. Thus, the effects of gelatin and polysaccharide are complicated due to competition between these mechanisms. Future works should focus on clarifying the complex mechanisms to improve the drag reduction efficiency of the gelatin-based bio-polysaccharide coatings. These biomimetic drag-reducing coatings could be further applied to underwater equipment.