The superhydrophilic and superoleophilic leaf surface of Ruellia devosiana (Acanthaceae): a biological model for spreading of water and oil on surfaces

Folk medicine, biological activity, and chemical profiles of Brazilian Acanthaceae (Lamiales) - A review

INTRODUCTION

The botanical family Acanthaceae (order Lamiales) potentially comprises 4900 species in 191 genera with extensive morphological, habit and habitat diversity. The family is widely distributed throughout the world but is especially rich in tropical and subtropical regions. Many of its species have great ornamental importance and are broadly used for medicinal purposes in several countries of Asia and Africa. Brazil is a main center of diversity of the family, where they are distributed across all its biomes, mainly in the herbaceous-shrub stratum. Medicinal investigations about Brazilian species are scarce, the exception being a single native species, Justicia pectoralis Jacq., that is widely used and studied chemically.

AIM OF THE REVIEW

This work compiled studies that indicated folk medicinal use, investigated biological activity, or evaluated the chemical composition of Brazilian species of Acanthaceae.

MATERIAL AND METHODS

Medicinal uses, investigations of biological activities and chemical data were collected and summarized through bibliographic surveys. Tables were compiled to standardize the information and the appropriate references were gathered for each species. Registration of chemical components used in the treatment of ailments and in preserving health were emphasized with the aim of stimulating future investigations.

RESULTS

The breadths of habitats and morphologies of the family are directly related to its chemical diversity, as confirmed here for Brazilian species. Although the investigated species represent less than 9% of the total richness of the family in Brazil, they encompass a great diversity of chemical substances. The data indicated folk medicinal uses for 26 species and biological tests for 23, while 30 species were investigated chemically. Ruellia and Justicia were the most researched genera with 12 and 11 species, representing approximately 14% and 7% of Brazilian species of each genus, respectively. Two species are native to other countries but become naturalized in Brazil. Studies of native species were carried out in different countries around the world, with many reports of medicinal uses and biological tests. Examples of uses include anticancer and antidepressant actions, as well as activities against respiratory problems and other diseases.

CONCLUSIONS

This work highlights the chemical and biological diversity of the studied Brazilian species of Acanthaceae, which emphasizes the need to expand studies with native Brazilian species.

Plant-Actuated Micro-Nanorobotics Platforms: Structural Designs, Functional Prospects, and Biomedical Applications

Plants are anatomically and physiologically different from humans and animals; however, there are several possibilities to utilize the unique structures and physiological systems of plants and adapt them to new emerging technologies through a strategic biomimetic approach. Moreover, plants provide safe and sustainable results that can potentially solve the problem of mass-producing practical materials with hazardous and toxic side effects, particularly in the biomedical field, which requires high biocompatibility. In this review, it is investigated how micro-nanostructures available in plants (e.g., nanoparticles, nanofibers and their composites, nanoporous materials, and natural micromotors) are adapted and utilized in the design of suitable materials for a micro-nanorobot platform. How plants' work on micro- and nanoscale systems (e.g., surface roughness, osmotically induced movements such as nastic and tropic, and energy conversion and harvesting) that are unique to plants, can provide functionality on the platform and become further prospective resources are examined. Furthermore, implementation across organisms and fields, which is promising for future practical applications of the plant-actuated micro-nanorobot platform, especially on biomedical applications, is discussed. Finally, the challenges following its implementation in the micro-nanorobot platform are also presented to provide advanced adaptation in the future.

An ecological perspective on water shedding from leaves

Water shedding from leaves is a complex process depending on multiple leaf traits interacting with rain, wind, and air humidity, and with the entire plant and surrounding vegetation. Here, we synthesize current knowledge of the physics of water shedding with implications for plant physiology and ecology. We argue that the drop retention angle is a more meaningful parameter to characterize the water-shedding capacity of leaves than the commonly measured static contact angle. The understanding of the mechanics of water shedding is largely derived from laboratory experiments on artificial rather than natural surfaces, often on individual aspects such as surface wettability or drop impacts. In contrast, field studies attempting to identify the adaptive value of leaf traits linked to water shedding are largely correlative in nature, with inconclusive results. We make a strong case for taking the hypothesis-driven experimental approach of biomechanical laboratory studies into a real-world field setting to gain a comprehensive understanding of leaf water shedding in a whole-plant ecological and evolutionary context.

A mathematical model and mesh-free numerical method for contact-line motion in lubrication theory

ABSTRACT

We introduce a mathematical model with a mesh-free numerical method to describe contact-line motion in lubrication theory. We show how the model resolves the singularity at the contact line, and generates smooth profiles for an evolving, spreading droplet. The model describes well the physics of droplet spreading-including Tanner's Law for the evolution of the contact line. The model can be configured to describe complete wetting or partial wetting, and we explore both cases numerically. In the case of partial wetting, the model also admits analytical solutions for the droplet profile, which we present here.

ARTICLE HIGHLIGHTS

We formulate a mathematical model to regularize the contact-line singularity for droplet spreading.The model can be solved using a fast, accurate mesh-free numerical method.Numerical simulations confirm that the model describes the quantitative aspects of droplet spreading well.

Robust bulk micro-nano hierarchical copper structures possessing exceptional bactericidal efficacy

Conventional copper (Cu) metal surfaces are well recognized for their bactericidal properties. However, their slow bacteria-killing potency has historically excluded them as a rapid bactericidal material. We report the development of a robust bulk superhydrophilic micro-nano hierarchical Cu structure that possesses exceptional bactericidal efficacy. It resulted in a 4.41 log₁₀ reduction (>99.99%) of the deadly Staphylococcus aureus (S. aureus) bacteria within 2 min vs. a 1.49 log₁₀ reduction (96.75%) after 240 min on common Cu surfaces. The adhered cells exhibited extensive blebbing, loss of structural integrity and leakage of vital intracellular material, demonstrating the rapid efficacy of the micro-nano Cu structure in destructing bacteria membrane integrity. The mechanism was attributed to the synergistic degradation of the cell envelope through enhanced release and therefore uptake of the cytotoxic Cu ions and the adhesion-driven mechanical strain due to its rapid ultimate superhydrophilicity (contact angle drops to 0° in 0.18 s). The scalable fabrication of this micro-nano Cu structure was enabled by integrating bespoke precursor alloy design with microstructure preconditioning for dealloying and demonstrated on 2000 mm² Cu surfaces. This development paves the way to the practical exploitation of Cu as a low-cost antibiotic-free fast bactericidal material.

Domino-like water transport on Tillandsia through flexible trichome wings

Tillandsia usneoides in epiphytic bromeliads takes up water through absorptive trichomes on the shoot surface under extreme environmental conditions. Although previous studies revealed the way by which T. usneoides absorbs water and prevents water loss, its water transport remains unclear. We characterized structures of trichome wings of T. usneoides. Wing length-to-thickness ratio of 136 and trichome interval (d)-to-wing length (l) ratio (d/l) smaller than 1 caused the water film to flatten the wings sequentially, resulting in domino-like water transport. A hinge-like linkage between wing and outer ring cells and the wing size longer than the elastocapillary length (L_EC ) brought about this unique reconfiguration, which is the flattening and recovery of wings. Tillandsia usneoides transported water rapidly on the surface as the water film propagated on the exterior trichomes with flexible wings and the transport distance at the macroscopic scale grew as t^x with x = 0.68 ± 0.04, unlike the conventional scaling of t^0.5 . Empirical and theoretical investigations proved our assumption that external water transport with the domino-like effect predominated over internal vascular transport. Biomimetic trichome wings simulated the domino-like water transport, highlighting the important role of flexible wing arrays.

Innovative fouling-resistant materials for industrial heat exchangers: a review

Fouling of heat exchangers (HEs) has become a major concern across the industrial sector. Fouling is an omnipresent phenomenon but is particularly prevalent in the dairy, oil, and energy industries. Reduced energy performance that results from fouling represents significant operating loss in terms of both maintenance and impact on product quality and safety. In most industries, cleaning or replacing HEs are currently the only viable solutions for controlling fouling. This review examines the latest advances in the development of innovative materials and coatings for HEs that could mitigate the need for costly and frequent cleaning and potentially extend their operational life. To better understand the correlation between surface properties and fouling occurrence, we begin by providing an overview of the main mechanisms underlying fouling. We then present selected key strategies, which can differ considerably, for developing antifouling surfaces and conclude by discussing the current trends in the search for ideal materials for a range of applications. In our presentation of all these aspects, emphasis is given wherever possible to the potential transfer of these innovative surfaces from the laboratory to the three industries most concerned by HE fouling problems: food, petrochemicals, and energy production.

Extended Gibbs Free Energy and Laplace Pressure of Ordered Hexagonal Close-Packed Spherical Particles: A Wettability Study

The wetting property of spherical particles in a hexagonal close-packed (HCP) ordering from extended Gibbs free energy (GFE) and Laplace pressure view points is studied. A formalism is proposed to predict the contact angle (θ) of a droplet on the HCP films and penetration angle (α) of the liquid on the spherical particles. Then, the extended Laplace pressure for the layered HCP ordering is calculated and a correlation between the wetting angle, sign of pressure, and pressure gradient is achieved. Our results show that the sign and the slope of pressure are important criteria for determining the wettability state and it is found that the contact angle is independent of the particle radius, as supported by various experimental reports. The pressure gradient for the HCP films with Young contact angle higher than (lower than) a critical contact angle, 135° (45°), is positive (negative), indicating the superhydrophobicity (superhydrophilicity) state of the surface. To validate the proposed formulation, theoretical calculations are compared with the reported experimental measurements, showing a good agreement.

Nature-Inspired Structures Applied in Heat Transfer Enhancement and Drag Reduction

Heat exchangers are general equipment for energy exchange in the industrial field. Enhancing the heat transfer of a heat exchanger with low pump energy consumption is beneficial to the maximum utilization of energy. The optimization design for enhanced heat transfer structure is an effective method to improve the heat transfer coefficient. Present research shows that the biomimetic structures applied in different equipment could enhance heat transfer and reduce flow resistance significantly. Firstly, six biomimetic structures including the fractal-tree-like structure, conical column structure, hybrid wetting structure, scale structure, concave-convex structure and superhydrophobic micro-nano structure were summarized in this paper. The biomimetic structure characteristics and heat transfer enhancement and drag reduction mechanisms were analyzed. Secondly, four processing methods including photolithography, nanoimprinting, femtosecond laser processing and 3D printing were introduced as the reference of biomimetic structure machining. Finally, according to the systemic summary of the research review, the prospect of biomimetic heat transfer structure optimization was proposed.

Microscale mechanism of microstructure, micromorphology and Janus wettability of the banana leaf surface

As a result of natural selection, the adaxial and abaxial sides of banana leaves show different wetting states and anisotropy. Janus wettability between the adaxial and abaxial sides of the banana leaf surface is revealed for the first time in this work. This has relevance for the preparation of bionic materials and an important role in the efficient and high-quality production management of pesticide spraying in banana orchards. The main purpose of this research is to analyze and study the microscale mechanism and coupling relationship between the Janus wettability of banana leaf surface and the microstructure and micromorphology. We adopt advanced modern instrument analysis technology, such as contact angle (CA) measurements, field emission scanning electron microscopy (FESEM), X-ray spectrometric analysis (EDS), and Fourier transform infrared spectroscopy (FTIR), and performed tests on the adaxial and abaxial sides of banana leaves to investigate the cause of Janus wettability. The results show that banana leaves exhibit different degrees of anisotropy, mainly due to the surface micromorphology. Banana leaves exhibit a hydrophilic Wenzel state on the adaxial side and a weakly hydrophobic Cassie-Baxter state on the abaxial side. We focused on studying the coupling effect and found that the main coupling element impacting the Janus wettability of the banana leaf surface is the nanopillars microstructure, and the secondary coupling element is the content of hydrophilic functional groups on the surface. This work may lead to the design and fabrication of Janus wetting surfaces by mimicking the nanopillar structure on banana leaf surfaces and help explore the potential application of efficient and high-quality pesticide spraying in banana orchards.

Disentangling the role of surface topography and intrinsic wettability in the prey capture mechanism of Nepenthes pitcher plants

Nepenthes pitcher plants capture prey with leaves specialised as pitfall traps. Insects are trapped when they 'aquaplane' on the pitcher rim (peristome), a surface structured with macroscopic and microscopic radial ridges. What is the functional significance of this hierarchical surface topography? Here, we use insect pad friction measurements, photolithography, wetting experiments and physical modelling to demonstrate that the ridges enhance the trap's efficacy by satisfying two functional demands on prey capture: Macroscopic ridges restrict lateral but enhance radial spreading of water, thereby creating continuous slippery tracks which facilitate prey capture when little water is present. Microscopic ridges, in turn, ensure that the water film between insect pad and peristome remains stable, causing insects to aquaplane. In combination, the hierarchical ridge structure hence renders the peristome wettable, and water films continuous, so avoiding the need for a strongly hydrophilic surface chemistry, which would compromise resistance to desiccation and attract detrimental contamination.

Substantial Improvement of Oil Aerosol Filtration Performance Using In-Plane Asymmetric Wettability

Oil aerosol usually causes air pollution, health issues, and corrosion to equipment. The removal of aerosol oil particles from the air is a crucial process in industrial production and daily life. Although fibrous filters have been a widely used material for the separation of oil aerosol from the air, it is still a challenge to separate submicrometer aerosol oil particles with both high filtration efficiency and low resistance. Herein, we report a novel approach to markedly reduce the pressure drop of a fibrous filter and simultaneously increase its aerosol filtration efficiency, only by surface treatment to make the filter have in-plane alternating superoleophilic and superoleophobic patterns. We used a spraying method to prepare superoleophobic and superoleophilic patterns on the filter. The best filtration results were achieved when two layers of the patterned filters that have superoleophobic and superoleophilic strips (both width, 5 mm) were stacked in a way that the opposite wetting surfaces contacted each other between the layers. The filter showed a much-reduced filtration resistance and the pressure drop (4.16 kPa) at the pseudo-steady state being at least 45% lower when compared to the two-layer controls with a homogeneous surface wettability (i.e., untreated surface, superoleophobicity, and superoleophilicity). It also showed higher filtration efficiency (98.37% for small oil mists and 99.99% for large oil mists) and over two times higher quality factor (0.99 kPa^-1 for small oil mists and 2.27 kPa^-1 for large oil mists). The asymmetric wettability leads to the formation of unobstructed channels for the air stream to penetrate through the filter matrix, leading to a low resistance with improved oil capture efficiency. The pattern strip width showed an effect on filtration performance. This unexpected finding may provide a novel approach to designing high-performance, low energy consumption, and long-life coalescence filters.

A study on the wetting properties of broccoli leaf surfaces and their time dependent self-healing after mechanical damage

Plants are protected from the elements by a complex hierarchical epicuticular wax layer which has inspired the creation of super-hydrophobic and self-cleaning surfaces. Although many studies have been conducted on different plant wax systems to determine the mechanisms of water repulsion hardly any have studied the recovery of the epicuticular wax layer. In the current study the wetting properties and crystallographic nature of the wax surface of Brassica oleracea var. italica (broccoli) has been studied, as well as the time-dependent recovery of the surface after mechanical damage. It was found that the surface of the broccoli leaves is not only super-repulsive and self-cleaning in regards to water but also in regards to glycerol and formamide, both of which have considerably lower surface tension values. Furthermore, it was shown that the surface properties do indeed recover after damage and that this recovery is multi-stepped and strongly dependent on the recovery of the roughness of the surface.

Dynamic Spreading of Droplets on Lyophilic Micropillar-Arrayed Surfaces

The wetting kinetics of droplets on lyophilic pillar-arrayed substrates is the driving mechanism of several natural phenomena (e.g., insect capturing by Nepenthes) and many industrial technologies (e.g., gas-liquid separation). For a lyophilic pillar-arrayed surface, a fringe film is formed ahead of the contact line, resulting in distinct wetting kinetics, which needs further investigation. In this study, Si(100) substrates with square micropillars were used to investigate the early spreading of droplets on lyophilic pillar-arrayed surfaces through the droplet-spreading method. A fringe film was observed ahead of the contact line for micropillar-arrayed surfaces. The spreading radius was enhanced by micropillars and mainly caused by liquid penetration into the pillar forest, resulting in alteration of the dissipation mechanism. The early spreading of droplets on lyophilic micropillar-arrayed surface was affected only by the solid fraction and independent of the pillar height. A semitheoretical model without adjustable parameters was established on the basis of the global energetic equation, considering the local dissipation, viscous dissipation, and the dissipation in the precursor film. The prediction of the model agrees with the experimental results. Our semitheoretical model may aid in predicting the wetting kinetics on lyophilic pillar-arrayed substrates and assist the design of pillar-arrayed surfaces in practical applications.

Superlyophilic Interfaces and Their Applications

Superlyophilic interfaces denote interfaces displaying strong affinity to diverse liquids, including superhydrophilic, superoleophilic, and superamphiphilic interfaces. When coming in contact with these interfaces, water or oil droplets tend to spread completely with contact angles close to 0°, presenting versatile applications including self-cleaning, antifogging, controllable liquid transport, liquid separation, and so forth. Inspired by nature, scientists have developed various kinds of artificial superlyophilic (SLPL) interfaces in the past decades. In terms of dimensional characteristics, the artificial SLPL interfaces can be divided into four categories: i) 0D particles, whose dispersibility or catalytic performance can be notably enhanced by superlyophilicity; ii) 1D micro-/nanofibers or nanotubes/channels, which can efficiently transfer liquids with SLPL interfaces; iii) 2D flat SLPL interfaces, on which different functional molecules can be deposited uniformly, forming ultrathin and smooth films; and iv) 3D structures, which can be obtained by either constructing 0D, 1D, or 2D SLPL materials separately or directly fabricating random SLPL frameworks, and can always be used as functional coatings or bulk materials. Here, natural and artificial SLPL interfaces are briefly introduced, followed by a short discussion of the limit between lyophilicity and lyophobicity, and then a snapshot of methods to generate SLPL interfaces is given. Specific focus is placed on recent achievements of constructing SLPL interfaces from zero to three dimensions. Following that, broad applications of SLPL interfaces in commercial areas will be introduced. Finally, a short summary and outlook for future challenges in this field is presented.

Plant Surfaces: Structures and Functions for Biomimetic Innovations

An overview of plant surface structures and their evolution is presented. It combines surface chemistry and architecture with their functions and refers to possible biomimetic applications. Within some 3.5 billion years biological species evolved highly complex multifunctional surfaces for interacting with their environments: some 10 million living prototypes (i.e., estimated number of existing plants and animals) for engineers. The complexity of the hierarchical structures and their functionality in biological organisms surpasses all abiotic natural surfaces: even superhydrophobicity is restricted in nature to living organisms and was probably a key evolutionary step with the invasion of terrestrial habitats some 350-450 million years ago in plants and insects. Special attention should be paid to the fact that global environmental change implies a dramatic loss of species and with it the biological role models. Plants, the dominating group of organisms on our planet, are sessile organisms with large multifunctional surfaces and thus exhibit particular intriguing features. Superhydrophilicity and superhydrophobicity are focal points in this work. We estimate that superhydrophobic plant leaves (e.g., grasses) comprise in total an area of around 250 million km2, which is about 50% of the total surface of our planet. A survey of structures and functions based on own examinations of almost 20,000 species is provided, for further references we refer to Barthlott et al. (Philos. Trans. R. Soc. A 374: 20160191, 1). A basic difference exists between aquatic non-vascular and land-living vascular plants; the latter exhibit a particular intriguing surface chemistry and architecture. The diversity of features is described in detail according to their hierarchical structural order. The first underlying and essential feature is the polymer cuticle superimposed by epicuticular wax and the curvature of single cells up to complex multicellular structures. A descriptive terminology for this diversity is provided. Simplified, the functions of plant surface characteristics may be grouped into six categories: (1) mechanical properties, (2) influence on reflection and absorption of spectral radiation, (3) reduction of water loss or increase of water uptake, moisture harvesting, (4) adhesion and non-adhesion (lotus effect, insect trapping), (5) drag and turbulence increase, or (6) air retention under water for drag reduction or gas exchange (Salvinia effect). This list is far from complete. A short overview of the history of bionics and the impressive spectrum of existing and anticipated biomimetic applications are provided. The major challenge for engineers and materials scientists, the durability of the fragile nanocoatings, is also discussed.

Influence of surface structure and chemistry on water droplet splashing

Water droplet splashing and aerosolization play a role in human hygiene and health systems as well as in crop culturing. Prevention or reduction of splashing can prevent transmission of diseases between animals and plants and keep technical systems such as pipe or bottling systems free of contamination. This study demonstrates to what extent the surface chemistry and structures influence the water droplet splashing behaviour. Smooth surfaces and structured replicas of Calathea zebrina (Sims) Lindl. leaves were produced. Modification of their wettability was done by coating with hydrophobizing and hydrophilizing agents. Their wetting was characterized by contact angle measurement and splashing behaviour was observed with a high-speed video camera. Hydrophobic and superhydrophilic surfaces generally showed fewer tendencies to splash than hydrophobic ones. Structuring amplified the underlying behaviour of the surface chemistries, increasing hydrophobic surfaces' tendency to splash and decreasing splash on hydrophilic surfaces by quickly transporting water off the impact point by capillary forces. The non-porous surface structures found in C. zebrina could easily be applied to technical products such as plastic foils or mats and coated with hydrophilizing agents to suppress splash in areas of increased hygiene requirements or wherever pooling of liquids is not desirable.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.

Fog collecting biomimetic surfaces: Influence of microstructure and wettability

We analyzed the fog collection efficiency of three different sets of samples: replica (with and without microstructures), copper wire (smooth and microgrooved) and polyolefin mesh (hydrophilic, superhydrophilic and hydrophobic). The collection efficiency of the samples was compared in each set separately to investigate the influence of microstructures and/or the wettability of the surfaces on fog collection. Based on the controlled experimental conditions chosen here large differences in the efficiency were found. We found that microstructured plant replica samples collected 2-3 times higher amounts of water than that of unstructured (smooth) samples. Copper wire samples showed similar results. Moreover, microgrooved wires had a faster dripping of water droplets than that of smooth wires. The superhydrophilic mesh tested here was proved more efficient than any other mesh samples with different wettability. The amount of collected fog by superhydrophilic mesh was about 5 times higher than that of hydrophilic (untreated) mesh and was about 2 times higher than that of hydrophobic mesh.

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.

Stomatal penetration by aqueous solutions--an update involving leaf surface particles

The recent visualization of stomatal nanoparticle uptake ended a 40-yr-old paradigm. Assuming clean, hydrophobic leaf surfaces, the paradigm considered stomatal liquid water transport to be impossible as a result of water surface tension. However, real leaves are not clean, and deposited aerosols may change hydrophobicity and water surface tension. Droplets containing NaCl, NaClO(3), (NH(4))(2) SO(4), glyphosate, an organosilicone surfactant or various combinations thereof were evaporated on stomatous abaxial and astomatous adaxial surfaces of apple (Malus domestica) leaves. The effects on photosynthesis, necrosis and biomass were determined. Observed using an environmental scanning electron microscope, NaCl and NaClO(3) crystals on hydrophobic tomato (Solanum lycopersicum) cuticles underwent several humidity cycles, causing repeated deliquescence and efflorescence of the salts. All physiological parameters were more strongly affected by abaxial than adaxial treatments. Spatial expansion and dendritic crystallization of the salts occurred and cuticular hydrophobicity was decreased more rapidly by NaClO(3) than NaCl. The results confirmed the stomatal uptake of aqueous solutions. Humidity fluctuations promote the spatial expansion of salts into the stomata. The ion-specific effects point to the Hofmeister series: chaotropic ions reduce surface tension, probably contributing to the defoliant action of NaClO(3), whereas the salt spray tolerance of coastal plants is probably linked to the kosmotropic nature of chloride ions.

Superhydrophilic and superhydrophobic nanostructured surfaces via plasma treatment

Polyethylene terephthalate (PET) films have been structured with isolated nanofibrils and fibril bundles using oxidative plasma treatments with increasing etching ratios. The transition from fibrils to bundles was smooth and it was associated with a significant reduction in the overall top area fraction and with the development of a second organisation level at a larger length scale. This increased complexity was reflected in the surface properties. The surfaces with two-level substructures showed superhydrophilic and superhydrophobic properties depending on the surface chemistry. These properties were preserved during prolonged storage and resisted moderate mechanical stress. By combining different contact angle and drop impact measurements, the optimum surface design and plasma processing parameters for maximizing stability of the superhydrophobic or superhydrophilic properties of the PET films were identified.

The insect-trapping rim of Nepenthes pitchers: surface structure and function

Carnivorous pitcher plants of the genus Nepenthes capture prey with a pitfall trap that relies on a micro-structured, slippery surface. The upper pitcher rim (peristome) is fully wettable and causes insects to slip by aquaplaning on a thin water film. The high wettability of the peristome is probably achieved by a combination of hydrophilic surface chemistry, surface roughness and the presence of hygroscopic nectar. Insect foot attachment could be prevented by the delayed drainage of the thin water film between the adhesive pad and the surface. Drainage should be faster for insects with a hairy adhesive system; however, they slip equally on the wet peristome. Therefore the stability of the water film against dewetting appears to be the key factor for aquaplaning. New experimental techniques may help to clarify the detailed function of the pitcher plant peristome and to explore its potential for biomimetic applications.