Measuring air layer volumes retained by submerged floating-ferns Salvinia and biomimetic superhydrophobic surfaces.

Matthias J Mayser,Meike Reker,Holger F Bohn,Wilhelm Barthlott

doi:10.3762/bjnano.5.93

Abstract

Some plants and animals feature superhydrophobic surfaces capable of retaining a layer of air when submerged under water. Long-term air retaining surfaces (Salvinia-effect) are of high interest for biomimetic applications like drag reduction in ship coatings of up to 30%. Here we present a novel method for measuring air volumes and air loss under water. We recorded the buoyancy force of the air layer on leaf surfaces of four different Salvinia species and on one biomimetic surface using a highly sensitive custom made strain gauge force transducer setup. The volume of air held by a surface was quantified by comparing the buoyancy force of the specimen with and then without an air layer. Air volumes retained by the Salvinia-surfaces ranged between 0.15 and 1 L/m2 depending on differences in surface architecture. We verified the precision of the method by comparing the measured air volumes with theoretical volume calculations and could find a good agreement between both values. In this context we present techniques to calculate air volumes on surfaces with complex microstructures. The introduced method also allows to measure decrease or increase of air layers with high accuracy in real-time to understand dynamic processes.

Highlights

Since the description of hierarchically structured, superhydrophobic, self-cleaning plant surfaces (Lotus-effect) [1,2] there has been an increasing interest in superhydrophobic surfaces [3,4,5]
The slightly lower values might be explained by the shape of the air water interface, which is not smooth but sagging in between the pillars, so that the real air volume should be slightly smaller than the theoretical value [37]
We have presented a reliable method for precisely measuring volumes of air layers on submerged superhydrophobic surfaces

Summary

Introduction

Since the description of hierarchically structured, superhydrophobic, self-cleaning plant surfaces (Lotus-effect) [1,2] there has been an increasing interest in superhydrophobic surfaces [3,4,5]. E.g., the leaves of Lotus (Nelumbo nucifera) provide very high contact angles and low hysteresis [1], the air layers that are held between the surface structures persist only for short periods of time [22]. For the validation of the method we use the microstructured wafer replicas as a defined control surface and compare the measured values with the calculated theoretical air volumes.

Results

Conclusion