Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces
Authors: François J. Peaudecerf, Julien R. Landel, Raymond E. Goldstein, Paolo Luzzatto-Fegiz
Journal: Proceedings of the National Academy of Sciences of the United States of America
Publication Date: 23 May, 2017
Department of: Mathematics
Bio-inspired surfaces for energy efficient maritime transport, provided surfactant molecules are not in the way…
Researchers at The University of Manchester, the University of Cambridge and the University of California Santa Barbara have recently shown that surfactants can strongly impact the drag reduction potential of bio-inspired superhydrophobic surfaces (SHSs).
Over the last 20 years, scientists have tried to cover the external surface of ships, submarines or the internal surface of pipelines with SHSs which could decrease their drag, thus considerably reducing 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.
This study reveals that surfactants, molecules naturally present in our environment, are the most likely cause for the reduced performance of SHSs, due to a phenomenon known as Marangoni forces. 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.
- Superhydrophobic surfaces (SHSs) are strongly water-repellent solid surfaces made using a combination of micro-texture, such as micron-size ridges or pillars, and hydrophobic materials or coating.
- SHSs have the property of repelling water to an astonishing degree. Known in nature as the ‘lotus leaf effect’, water rolls on the lotus leaf or SHSs as near-perfect beads, instead of spreading over them.
- As air bubbles are trapped in the micro-texture of SHSs, the drag between a SHS and the liquid flowing alongside is significantly diminished. This is due to the lower friction between liquid and air compared to the friction between liquid and solid.
- Drag is the resisting force opposing the relative motion of an object into a fluid: for instance, when a ship moves in water, or when petrol is pumped through a pipeline.
- Surfactants are long organic molecules which can have a hydrophobic tail and a hydrophilic head. They prefer to stay at the interface between a liquid and a gas, thus modifying the chemical and physical properties of the interface. Surfactants are used in many products such as detergents or soap.
- Marangoni forces, named after Carlo Marangoni who first studied them in 1865, can arise when the concentration of surfactants is not uniform, pointing towards lower concentrations.