Slip length and drag reduction of superhydrophobic surfaces in shear-thinning fluid flows

We hypothesise that superhydrophobic surfaces can achieve effective interfacial slip and drag reduction even under non-Newtonian, shear-thinning fluid flows. Unlike Newtonian fluids, where slip is primarily influenced by viscosity and surface tension, we anticipate that the shear-thinning nature of these fluids may enhance slip length and drag reduction.

The superhydrophobic surfaces used in this study, featuring a dual-scale random topography, were fabricated via a spray coating process, and low-concentration xanthan gum solutions (50-250 ppm) were used as model shear-thinning fluids of low elasticity. Drag reduction was experimentally measured using a rheometric cone-and-plate system, while slip length was calculated through a newly developed integral expression for power-law fluids.

Experimental results revealed a significant increase in slip length for xanthan gum solutions compared to distilled water, indicating that shear-thinning effects lead to substantial increases in slip length. Specifically, xanthan gum solutions with a power-law index of approximately 0.8 achieved over a 25% increase in slip length compared to equivalent Newtonian fluids. Furthermore, analytical and numerical analyses confirmed that shear-thinning fluids enhance the slip effect over superhydrophobic surfaces while shear-thickening fluids diminish it. The strong agreement between theoretical and experimental results underscores the potential of superhydrophobic surfaces to reduce drag in shear-thinning fluid flows.