Superhydrophobic surfaces of the water bugNotonecta glauca: a model for friction reduction and air retention

Abstract

SummarySuperhydrophobic surfaces of plants and animals are of great interest for biomimetic applications. Whereas the self-cleaning properties of superhydrophobic surfaces have been extensively investigated, their ability to retain an air film while submerged under water has not, in the past, received much attention. Nevertheless, air retaining surfaces are of great economic and ecological interest because an air film can reduce friction of solid bodies sliding through the water. This opens perspectives for biomimetic applications such as low friction fluid transport or friction reduction on ship hulls. For such applications the durability of the air film is most important. While the air film on most superhydrophobic surfaces usually lasts no longer than a few days, a few semi-aquatic plants and insects are able to hold an air film over a longer time period. Currently, we found high air film persistence under hydrostatic conditions for the elytra of the backswimmer Notonecta glauca which we therefore have chosen for further investigations. In this study, we compare the micro- and nanostructure of selected body parts (sternites, upper side of elytra, underside of elytra) in reference to their air retaining properties. Our investigations demonstrate outstanding air film persistence of the upper side of the elytra of Notonecta glauca under hydrostatic and hydrodynamic conditions. This hierarchically structured surface was able to hold a complete air film under hydrostatic conditions for longer than 130 days while on other body parts with simple structures the air film showed gaps (underside of elytra) or even vanished completely after a few days (sternites). Moreover, the upper side of the elytra was able to keep an air film up to flow velocities of 5 m/s. Obviously the complex surface structure with tiny dense microtrichia and two types of larger specially shaped setae is relevant for this outstanding ability. Besides high air film persistence, the observation of a considerable fluid velocity directly at the air–water interface indicates the ability to reduce friction significantly. The combination of these two abilities makes these hierarchically structured surfaces extremely interesting as a biomimetic model for low friction fluid transport or drag reduction on ship hulls.

Superhydrophobic Properties of Nanotextured Polypropylene Foils Fabricated by Roll-to-Roll Extrusion Coating

“…7 In nature, the superhydrophobic effect aka the lotus effect is mostly known from plant leafs 8 , but is also encountered in fauna, e.g. the water strider Notonecta glauca 9,10 . Fabrication of superhydrophobic materials usually require initial structuring with a hierarchical micro and nanopattern to increase surface roughness, 11,12 and a subsequent coating with low surface energy chemistry 13,14 .…”

Fabrication of a superhydrophobic surface with underwater air-retaining properties by electrostatic flocking

“…Another advantage for the hybrid sample was that the double structure may be helpful for air retention. 14,26,28 According to the CA result, it can be seen that the hydrophobic treatment was suitable as most of the ocking samples except samples 2B and 6 reached a CA of 140 to 150…”

Elasticity of the hair cover in air-retaining Salvinia surfaces

“…However, double-structured lotuseffect surfaces maintain the thin air layer only temporarily, but there are several aquatic organisms like the back-swimmer Notonecta or the floating fern Salvinia, which maintain air layers under water for a longer time period [5]. The ability of superhydrophobic surfaces to retain an air film under water drew increased attention in the last years [6][7][8][9][10][11]. In contrast to surfaces with lotus effect, air-retaining surfaces are not selfcleaning and are characterized by a much more complex elaborate surface architecture.…”