Manchester mathematician publishes in PNAS

Julien Landel and co-workers have shown in a recent study published in PNAS that surfactant can strongly impact the drag reduction potential of superhydrophobic surfaces, also known for their lotus effect.

lotus
Water drops roll as beads over the lotus leaf

Superhydrophobic surfaces (SHSs) have the property of repelling water from their surface to an astonishing degree. Known in nature as the ‘lotus leaf effect’, water rolls on the lotus leaf as near-perfect beads, instead of spreading over the leaf. Owing to a combination of micro-texture and chemistry, water droplets mostly stand above air bubbles trapped in the micro-texture. This is known as the Cassie-Baxter or fakir state!

This observation has recently inspired scientists to cover the external surface of ships, submarines or the internal surface of pipelines with SHSs in order to decrease their drag. As drag or friction between air and water is significantly less than between water and solid, SHSs have the potential to reduce considerably the energy used in maritime transport or pipe flows. However, laboratory experiments conducted in the past ten years have shown inconsistent results, with many studies reporting significantly decreased performance compared with theoretical and numerical predictions.

Julien Landel and his co-workers François Peaudecerf, Ray Goldstein (University of Cambridge) and Paolo Luzzatto-Fegiz (University of California Santa-Barbara) have recently shown that surfactant, organic molecules naturally present in our environment, are the most likely cause for the decreased performance of SHSs. This was found after two years of intensive research, using careful experimentation with confocal microscopy, numerical simulations and mathematical modelling. As even minute concentrations of surfactants can severely reduce the SHS performance, the impact of this discovery is important for many applications and could guide future design of SHSs.

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