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

Abstract

SummarySome 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.

Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications

“…Persistent air layers under water (Salvinia effect) are discussed in [14,15,[53][54][55][56][57][58][59][60][61][62].…”

“…Thus, we find many examples of biological surface compartmentation; the dimensions vary from the microscopic in Eschscholtzia or Aeginetia seeds (figures 18a and 24) to the much larger units the entire leaf of Salvinia cucullata ( figure 18b,c). The margins of the compartments show an elaborated architecture [53], a particular 'edge effect' preventing the escape of air [60]. These biological construction details provide important information for biomimetic technical surfaces ( §9).…”

Unraveling the Salvinia Paradox: Design Principles for Submerged Superhydrophobicity

“…[ 1 ] Moving toward submerged applications requires a new inspiration: a promising candidate is the Salvinia molesta (Figure 1 a), because of its superior gas trapping capabilities. [ 6,[18][19][20] The gas entrapped within surface asperities can be either air or the vapor phase coexisting with the liquid: albeit the partial pressure of the other gases stabilizes the Cassie state, their presence is not a requirement for (meta)stable superhydrophobicity [ 21 ] (see the Supporting Information for additional details on the role of dissolved gases). The entrapped gas may be lost through different mechanisms, analyzed in detail below, determining the failure of superhydrophobicity:…”

Elasticity of the hair cover in air-retaining Salvinia surfaces

“…2 Surface structures of the leaves of four different Salvinia species: a S. molesta, b S. minima, c S. cucullata, d S. oblongifolia. Images are all taken with the digital microscope and shown in the same scale [17] …”