Holger Florian Bohn

The Salvinia Paradox: Superhydrophobic Surfaces with Hydrophilic Pins for Air Retention Under Water

[*] Prof. W. Barthlott, S. Wiersch, Dr. H. F. Bohn Nees-Institut für Biodiversität der Pflanzen Rheinische Friedrich-Wilhelms-Universität Meckenheimer Allee 170, 53115 Bonn (Germany) E-mail: barthlott@uni-bonn.de Prof. Th. Schimmel, Dr. M. Barczewski, Dr. S. Walheim, A. Weis, A. Kaltenmaier Institute of Applied Physics and Center for Functional Nanostructures (CFN) University of Karlsruhe Karlsruhe Institute of Technology (KIT) 76131 Karlsruhe (Germany) Institute of Nanotechnology and Center for Functional Nanostructures (CFN) Forschungszentrum Karlsruhe Karlsruhe Institute of Technology (KIT) 76021 Karlsruhe (Germany) E-mail: thomas.schimmel@physik.uni-karlsruhe.de Prof. K. Koch Biologie und Nanobiotechnologie Hochschule Rhein-Waal Landwehr 4, 47533 Kleve (Germany)

Plant surfaces with cuticular folds are slippery for beetles

Plant surfaces covered with three-dimensional (3D) waxes are known to strongly reduce insect adhesion, leading to slippery surfaces. Besides 3D epicuticular waxes, cuticular folds are a common microstructure found on plant surfaces, which have not been quantitatively investigated with regard to their influence on insect adhesion. We performed traction experiments with Colorado potato beetles on five plant surfaces with cuticular folds of different magnitude. For comparison, we also tested (i) smooth plant surfaces and (ii) plant surfaces possessing 3D epicuticular waxes. Traction forces on surfaces with medium cuticular folds, of about 0.5 µm in both height and thickness and a spacing of 0.5–1.5 µm, were reduced by an average of 88 per cent in comparison to smooth plant surfaces. Traction forces were reduced by the same order of magnitude as on plant surfaces covered with 3D epicuticular waxes. For surface characterization, we performed static contact angle measurements, which proved a strong effect of cuticular folds also on surface wettability. Surfaces possessing cuticular folds of greater magnitude showed higher contact angles up to superhydrophobicity. We hypothesize that cuticular folds reduce insect adhesion mainly due to a critical roughness, reducing the real contact area between the surface and the insects adhesive devices.

Comparative and functional morphology of hierarchically structured anti-adhesive surfaces in carnivorous plants and kettle trap flowers

Plant surfaces that are slippery for insects have evolved independently several times in the plant kingdom, mainly in the groups of carnivorous plants and kettle trap flowers. The surface morphologies of 53 species from both groups were investigated by scanning electron microscopy. It was found that the surfaces possess highly diverse topographical structures. We present a classification of 12 types of anti-adhesive surfaces, in regard to the assembly and hierarchy of their structural elements. The observed structural elements are different combinations of epidermal cell curvatures with cuticular folds or 3D epicuticular wax crystals and idioblastic elements.

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

Summary 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.

Plant surfaces with cuticular folds and their replicas: Influence of microstructuring and surface chemistry on the attachment of a leaf beetle

Plant surfaces covered either with epicuticular wax crystals or cuticular folds have been shown to strongly reduce the ability of insects to attach to them. However, the relative impact of surface structuring vs. surface chemistry on insect attachment remains unclear. To understand the mechanisms reducing adhesion of insects on plant surfaces in more detail, we performed traction experiments (i) on plant surfaces covered with cuticular folds of different dimensions, and on their (ii) untreated and (iii) hydrophobized replicas. As a reference, measurements were performed on replicas of smooth plant surfaces and of glass. Traction forces were measured with a highly sensitive force transducer, using tethered male Colorado potato beetles (Leptinotarsa decemlineata) as a model insect species. Contact angle measurements with water and diiodomethane were also performed to examine the physicochemical properties of the test surfaces. We found that surface structuring has a strong influence on the magnitude of the attachment force. In contrast, under the chosen experimental conditions, surface chemistry had no significant influence. Our results indicate that attachment of the beetles is reduced solely by the dimensions of the folds, with cuticular folds of about 0.5 μm in both height and width being the most effective. Contrary to the attachment of beetles, the wettability of the surfaces was considerably influenced by both surface structuring and chemistry. These results contribute to a better understanding of plant-insect interactions and the function of microstructured surfaces, and may facilitate the development of biomimetic anti-adhesive surfaces.

Surface microstructures of daisy florets (Asteraceae) and characterization of their anisotropic wetting.

The surface microstructures on ray florets of 62 species were characterized and compared with modern phylogenetic data of species affiliation in Asteraceae to determine sculptural patterns and their occurrence in the tribes of Asteraceae. Their wettability was studied to identify structural-induced droplet adhesion, which can be used for the development of artificial surfaces for water harvesting and passive surface water transport. The wettability was characterized by contact angle (CA) and tilt angle measurements, performed on fresh ray florets and their epoxy resin replica. The CAs on ray florets varied between 104° and 156°, but water droplets did not roll off when surface was tilted at 90°. Elongated cell structures and cuticle folding orientated in the same direction as the cell elongation caused capillary forces, leading to anisotropic wetting, with extension of water droplets along the length axis of epidermis cells. The strongest elongation of the droplets was also supported by a parallel, cell-overlapping cuticle striation. In artificial surfaces made of epoxy replica of ray florets, this effect was enhanced. The distribution of the identified four structural types exhibits a strong phylogenetic signal and allows the inference of an evolutionary trend in the modification of floret epidermal cells.

Impact of cell shape in hierarchically structured plant surfaces on the attachment of male Colorado potato beetles (Leptinotarsa decemlineata)

Summary Plant surfaces showing hierarchical structuring are frequently found in plant organs such as leaves, petals, fruits and stems. In our study we focus on the level of cell shape and on the level of superimposed microstructuring, leading to hierarchical surfaces if both levels are present. While it has been shown that epicuticular wax crystals and cuticular folds strongly reduce insect attachment, and that smooth papillate epidermal cells in petals improve the grip of pollinators, the impact of hierarchical surface structuring of plant surfaces possessing convex or papillate cells on insect attachment remains unclear. We performed traction experiments with male Colorado potato beetles on nine different plant surfaces with different structures. The selected plant surfaces showed epidermal cells with either tabular, convex or papillate cell shape, covered either with flat films of wax, epicuticular wax crystals or with cuticular folds. On surfaces possessing either superimposed wax crystals or cuticular folds we found traction forces to be almost one order of magnitude lower than on surfaces covered only with flat films of wax. Independent of superimposed microstructures we found that convex and papillate epidermal cell shapes slightly enhance the attachment ability of the beetles. Thus, in plant surfaces, cell shape and superimposed microstructuring yield contrary effects on the attachment of the Colorado potato beetle, with convex or papillate cells enhancing attachment and both wax crystals or cuticular folds reducing attachment. However, the overall magnitude of traction force mainly depends on the presence or absence of superimposed microstructuring.

A Passionate Free Climber: Structural Development and Functional Morphology of the Adhesive Tendrils in Passiflora discophora

Premise of research. Passiflora discophora is exceptional among passion flowers for its climbing strategy, using branched tendrils with terminal adhesive pads instead of coiling tendrils as typical within this family. This article investigates the structural development and morphology of these adhesive pads and aims to understand the underlying structure-function relationship of the attachment process. Based on our results, we discuss possible mechanical consequences of the tendril structure and compare our findings with similar attachment systems in unrelated species in order to identify general strategies of this mode of attachment. Methodology. We investigated the temporal development of the attachment process, including detailed studies of the morphology and anatomy of the adhesive pads, using LM with different staining procedures and SEM. Pivotal results. Young tendrils establish initial contact with a supporting substrate by interlocking with their hook-shaped tips. Touch stimuli induce the tips to develop into adhesive pads by callus-like growth of papillate epidermal cells. Fully grown pads are hemispherical on flat substrates or completely fill out larger cavities of the substrate. By apical cell division, the pad tissue perfectly mimics the microtopography of the substrate and also grows into minute gaps and fissures, establishing firm anchorage by optimal form closure. Additionally, an extracellular substance is visible at the interface between pad and substrate surface, which might act as adhesive. Conclusions. The opportunistic growth and cellular structure make the tendrils and adhesive pads of P. discophora a highly adaptive attachment system. Comparison with other not closely related taxa reveals general principles of this climbing mode, namely, (1) branched tendrils with multiple adhesive pads, (2) papillate cells establishing optimal form closure with the substrate and in some species additionally secreting adhesive substances, (3) free coiling of axes, and (4) persisting anchorage after senescence.

Straightforward and precise approach to replicate complex hierarchical structures from plant surfaces onto soft matter polymer

The surfaces of plant leaves are rarely smooth and often possess a species-specific micro- and/or nano-structuring. These structures usually influence the surface functionality of the leaves such as wettability, optical properties, friction and adhesion in insect–plant interactions. This work presents a simple, convenient, inexpensive and precise two-step micro-replication technique to transfer surface microstructures of plant leaves onto highly transparent soft polymer material. Leaves of three different plants with variable size (0.5–100 µm), shape and complexity (hierarchical levels) of their surface microstructures were selected as model bio-templates. A thermoset epoxy resin was used at ambient conditions to produce negative moulds directly from fresh plant leaves. An alkaline chemical treatment was established to remove the entirety of the leaf material from the cured negative epoxy mould when necessary, i.e. for highly complex hierarchical structures. Obtained moulds were filled up afterwards with low viscosity silicone elastomer (PDMS) to obtain positive surface replicas. Comparative scanning electron microscopy investigations (original plant leaves and replicated polymeric surfaces) reveal the high precision and versatility of this replication technique. This technique has promising future application for the development of bioinspired functional surfaces. Additionally, the fabricated polymer replicas provide a model to systematically investigate the structural key points of surface functionalities.