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1.  Measuring air layer volumes retained by submerged floating-ferns Salvinia and biomimetic superhydrophobic surfaces 
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.
PMCID: PMC4077374  PMID: 24991518
air layer; biomimetic; drag reduction; functional surfaces; plastron; Salvinia effect; volume measurement
2.  Functional superhydrophobic surfaces made of Janus micropillars 
Soft Matter  2014;11(3):506-515.
Particle coated micropillar arrays having hydrophobic sidewalls and hydrophilic silica tops are fabricated, enabling the top sides to be selectively post-functionalized. The so termed Janus pillars remain in the Cassie state even after chemical modification of the top faces.
We demonstrate the fabrication of superhydrophobic surfaces consisting of micropillars with hydrophobic sidewalls and hydrophilic tops, referred to as Janus micropillars. Therefore we first coat a micropillar array with a mono- or bilayer of polymeric particles, and merge the particles together to shield the top faces while hydrophobizing the walls. After removing the polymer film, the top faces of the micropillar arrays can be selectively chemically functionalised with hydrophilic groups. The Janus arrays remain superhydrophobic even after functionalisation as verified by laser scanning confocal microscopy. The robustness of the superhydrophobic behaviour proves that the stability of the entrapped air cushion is determined by the forces acting at the rim of the micropillars. This insight should stimulate a new way of designing super liquid-repellent surfaces with tunable liquid adhesion. In particular, combining superhydrophobicity with the functionalisation of the top faces of the protrusions with hydrophilic groups may have exciting new applications, including high-density microarrays for high-throughput screening of bioactive molecules, cells, or enzymes or efficient water condensation. However, so far chemical attachment of hydrophilic molecules has been accompanied with complete wetting of the surface underneath. The fabrication of superhydrophobic surfaces where the top faces of the protrusions can be selectively chemically post-functionalised with hydrophilic molecules, while retaining their superhydrophobic properties, is both promising and challenging.
PMCID: PMC4358088  PMID: 25415839
3.  Persistence of Metastability after Expansion of a Supercompressed Fluid Monolayer 
Fluid monolayers of 1-palmitoyl-2-oleoyl-phosphatidylcholine collapse from an air/water interface to form a three-dimensional bulk phase at the equilibrium spreading pressure (πe) of ~47 mN/m. This phase transition limits access to higher surface pressures under equilibrium conditions or during slow continuous compressions. We have shown previously that these films avoid collapse and become metastable when compressed on a captive bubble to surface pressures above 60 mN/m and that the metastability persists during expansion at least to πe. Here, we first documented the extent of this persistent metastability. Rates of isobaric collapse during expansion of the metastable films were up to 3 orders of magnitude slower than those during the initial compression to high surface pressures. Recovery of the ability to collapse depended on the surface pressure to which the films were expanded and how long they were held there. Films reverted after brief exposure to 20 mN/m and after 1 h at 35 mN/m. At πe, films remained capable of reaching high surface pressures during slow compressions after 65 h, although an increase in compressibility above 55 mN/m suggested somewhat increased rates of collapse. We also determined if the films remained metastable when they acquired sufficient free area to allow reinsertion of collapsed material. Faster isobaric expansion in the presence of more collapsed material and with further deviation below πe supported the existence of reinsertion. The persistence of metastability to πe shows that films with sufficient free area to allow reinsertion remain resistant to collapse. Observations that suggest heterogeneous reinsertion, however, argue that free area may be distributed heterogeneously and leave open the possibility that metastability persists because significant regions retain a restricted free area.
PMCID: PMC3520514  PMID: 15984255
4.  Tribology of bio-inspired nanowrinkled films on ultrasoft substrates 
Biomimetic design of new materials uses nature as antetype, learning from billions of years of evolution. This work emphasizes the mechanical and tribological properties of skin, combining both hardness and wear resistance of its surface (the stratum corneum) with high elasticity of the bulk (epidermis, dermis, hypodermis). The key for combination of such opposite properties is wrinkling, being consequence of intrinsic stresses in the bulk (soft tissue): Tribological contact to counterparts below the stress threshold for tissue trauma occurs on the thick hard stratum corneum layer pads, while tensile loads smooth out wrinkles in between these pads. Similar mechanism offers high tribological resistance to hard films on soft, flexible polymers, which is shown for diamond-like carbon (DLC) and titanium nitride thin films on ultrasoft polyurethane and harder polycarbonate substrates. The choice of these two compared substrate materials will show that ultra-soft substrate materials are decisive for the distinct tribological material. Hierarchical wrinkled structures of films on these substrates are due to high intrinsic compressive stress, which evolves during high energetic film growth. Incremental relaxation of these stresses occurs by compound deformation of film and elastic substrate surface, appearing in hierarchical nano-wrinkles. Nano-wrinkled topographies enable high elastic deformability of thin hard films, while overstressing results in zigzag film fracture along larger hierarchical wrinkle structures. Tribologically, these fracture mechanisms are highly important for ploughing and sliding of sharp and flat counterparts on hard-coated ultra-soft substrates like polyurethane. Concentration of polyurethane deformation under the applied normal loads occurs below these zigzag cracks. Unloading closes these cracks again. Even cyclic testing do not lead to film delamination and retain low friction behavior, if the adhesion to the substrate is high and the initial friction coefficient of the film against the sliding counterpart low, e.g. found for DLC.
PMCID: PMC3962205  PMID: 24688710
Biomimetic; skin deformation; magnetron sputtering; polycarbonate; thermoplastic polyurethane; titanium nitride; diamond-like carbon
5.  Superhydrophobic Thin Films Fabricated by Reactive Layer-by-Layer Assembly of Azlactone-Functionalized Polymers 
We report an approach to the fabrication of superhydrophobic thin films that is based on the ‘reactive’ layer-by-layer assembly of azlactone-containing polymer multilayers. We demonstrate that films fabricated from alternating layers of the azlactone functionalized polymer poly(2-vinyl-4,4-dimethylazlactone) (PVDMA) and poly(ethyleneimine) (PEI) exhibit micro- and nanoscale surface features that result in water contact angles in excess of 150º. Our results reveal that the formation of these surface features is (i) dependent upon film thickness (i.e., the number of layers of PEI and PVDMA deposited) and (ii) that it is influenced strongly by the presence (or absence) of cyclic azlactone-functionalized oligomers that can form upon storage of the 2-vinyl-4,4-dimethylazlactone (VDMA) used to synthesize PVDMA. For example, films fabricated using polymers synthesized in the presence of these oligomers exhibited rough, textured surfaces and superhydrophobic behavior (i.e., advancing contact angles in excess of 150º). In contrast, films fabricated from PVDMA polymerized in the absence of this oligomer (e.g., using freshly distilled monomer) were smooth and only moderately hydrophobic (i.e., advancing contact angles of ~75º). The addition of authentic, independently synthesized oligomer to samples of distilled VDMA at specified and controlled concentrations permitted reproducible fabrication of superhydrophobic thin films on the surfaces of a variety of different substrates. The surfaces of these films were demonstrated to be superhydrophobic immediately after fabrication, but they became hydrophilic after exposure to water for six days. Additional experiments demonstrated that it was possible to stabilize and prolong the superhydrophobic properties of these films (e.g., advancing contact angles in excess of 150° even after complete submersion in water for at least six weeks) by exploiting the reactivity of residual azlactones to functionalize the surfaces of the films using hydrophobic amines (e.g., aliphatic or semi-fluorinated aliphatic amines). Our results demonstrate a straightforward and substrate-independent approach to the design of superhydrophobic and reactive polymer-based coatings of potential use in a broad range of fundamental and applied contexts.
PMCID: PMC2998398  PMID: 21151704
6.  A Mechanistic Study of Wetting Superhydrophobic Porous 3D Meshes 
Advanced functional materials  2013;23(29):3628-3637.
Superhydrophobic, porous, 3D materials composed of poly( ε -caprolactone) (PCL) and the hydrophobic polymer dopant poly(glycerol monostearate-co- ε -caprolactone) (PGC-C18) are fabricated using the electrospinning technique. These 3D materials are distinct from 2D superhydrophobic surfaces, with maintenance of air at the surface as well as within the bulk of the material. These superhydrophobic materials float in water, and when held underwater and pressed, an air bubble is released and will rise to the surface. By changing the PGC-C18 doping concentration in the meshes and/or the fiber size from the micro- to nanoscale, the long-term stability of the entrapped air layer is controlled. The rate of water infiltration into the meshes, and the resulting displacement of the entrapped air, is quantitatively measured using X-ray computed tomography. The properties of the meshes are further probed using surfactants and solvents of different surface tensions. Finally, the application of hydraulic pressure is used to quantify the breakthrough pressure to wet the meshes. The tools for fabrication and analysis of these superhydrophobic materials as well as the ability to control the robustness of the entrapped air layer are highly desirable for a number of existing and emerging applications.
PMCID: PMC4191864  PMID: 25309305
7.  Animal or Plant: Which Is the Better Fog Water Collector? 
PLoS ONE  2012;7(4):e34603.
Occasional fog is a critical water source utilised by plants and animals in the Namib Desert. Fog basking beetles (Onymacris unguicularis, Tenebrionidae) and Namib dune bushman grass (Stipagrostris sabulicola, Poaceae) collect water directly from the fog. While the beetles position themselves optimally for fog water collection on dune ridges, the grass occurs predominantly at the dune base where less fog water is available. Differences in the fog-water collecting abilities in animals and plants have never been addressed. Here we place beetles and grass side-by-side in a fog chamber and measure the amount of water they collect over time. Based on the accumulated amount of water over a two hour period, grass is the better fog collector. However, in contrast to the episodic cascading water run-off from the grass, the beetles obtain water in a steady flow from their elytra. This steady trickle from the beetles' elytra to their mouth could ensure that even short periods of fog basking – while exposed to predators – will yield water. Up to now there is no indication of specialised surface properties on the grass leafs, but the steady run-off from the beetles could point to specific property adaptations of their elytra surface.
PMCID: PMC3318004  PMID: 22509331
8.  Air Cleaning Technologies 
Executive Summary
This health technology policy assessment will answer the following questions:
When should in-room air cleaners be used?
How effective are in-room air cleaners?
Are in-room air cleaners that use combined HEPA and UVGI air cleaning technology more effective than those that use HEPA filtration alone?
What is the Plasmacluster ion air purifier in the pandemic influenza preparation plan?
The experience of severe acute respiratory syndrome (SARS) locally, nationally, and internationally underscored the importance of administrative, environmental, and personal protective infection control measures in health care facilities. In the aftermath of the SARS crisis, there was a need for a clearer understanding of Ontario’s capacity to manage suspected or confirmed cases of airborne infectious diseases. In so doing, the Walker Commission thought that more attention should be paid to the potential use of new technologies such as in-room air cleaning units. It recommended that the Medical Advisory Secretariat of the Ontario Ministry of Health and Long-Term Care evaluate the appropriate use and effectiveness of such new technologies.
Accordingly, the Ontario Health Technology Advisory Committee asked the Medical Advisory Secretariat to review the literature on the effectiveness and utility of in-room air cleaners that use high-efficiency particle air (HEPA) filters and ultraviolet germicidal irradiation (UVGI) air cleaning technology.
Additionally, the Ontario Health Technology Advisory Committee prioritized a request from the ministry’s Emergency Management Unit to investigate the possible role of the Plasmacluster ion air purifier manufactured by Sharp Electronics Corporation, in the pandemic influenza preparation plan.
Clinical Need
Airborne transmission of infectious diseases depends in part on the concentration of breathable infectious pathogens (germs) in room air. Infection control is achieved by a combination of administrative, engineering, and personal protection methods. Engineering methods that are usually carried out by the building’s heating, ventilation, and air conditioning (HVAC) system function to prevent the spread of airborne infectious pathogens by diluting (dilution ventilation) and removing (exhaust ventilation) contaminated air from a room, controlling the direction of airflow and the air flow patterns in a building. However, general wear and tear over time may compromise the HVAC system’s effectiveness to maintain adequate indoor air quality. Likewise, economic issues may curtail the completion of necessary renovations to increase its effectiveness. Therefore, when exposure to airborne infectious pathogens is a risk, the use of an in-room air cleaner to reduce the concentration of airborne pathogens and prevent the spread of airborne infectious diseases has been proposed as an alternative to renovating a HVAC system.
Airborne transmission is the spread of infectious pathogens over large distances through the air. Infectious pathogens, which may include fungi, bacteria, and viruses, vary in size and can be dispersed into the air in drops of moisture after coughing or sneezing. Small drops of moisture carrying infectious pathogens are called droplet nuclei. Droplet nuclei are about 1 to 5μm in diameter. This small size in part allows them to remain suspended in the air for several hours and be carried by air currents over considerable distances. Large drops of moisture carrying infectious pathogens are called droplets. Droplets being larger than droplet nuclei, travel shorter distances (about 1 metre) before rapidly falling out of the air to the ground. Because droplet nuclei remain airborne for longer periods than do droplets, they are more amenable to engineering infection control methods than are droplets.
Droplet nuclei are responsible for the airborne transmission of infectious diseases such as tuberculosis, chicken pox (varicella), measles (rubeola), and dessiminated herpes zoster, whereas close contact is required for the direct transmission of infectious diseases transmitted by droplets, such as influenza (the flu) and SARS.
The Technology
In-room air cleaners are supplied as portable or fixed devices. Fixed devices can be attached to either a wall or ceiling and are preferred over portable units because they have a greater degree of reliability (if installed properly) for achieving adequate room air mixing and airflow patterns, which are important for optimal effectiveness.
Through a method of air recirculation, an in-room air cleaner can be used to increase room ventilation rates and if used to exhaust air out of the room it can create a negative-pressure room for airborne infection isolation (AII) when the building’s HVAC system cannot do so. A negative-pressure room is one where clean air flows into the room but contaminated air does not flow out of it. Contaminated room air is pulled into the in-room air cleaner and cleaned by passing through a series of filters, which remove the airborne infectious pathogens. The cleaned air is either recirculated into the room or exhausted outside the building. By filtering contaminated room air and then recirculating the cleaned air into the room, an in-room air cleaner can improve the room’s ventilation. By exhausting the filtered air to the outside the unit can create a negative-pressure room. There are many types of in-room air cleaners. They vary widely in the airflow rates through the unit, the type of air cleaning technology used, and the technical design.
Crucial to maximizing the efficiency of any in-room air cleaner is its strategic placement and set-up within a room, which should be done in consultation with ventilation engineers, infection control experts, and/or industrial hygienists. A poorly positioned air cleaner may disrupt airflow patterns within the room and through the air cleaner, thereby compromising its air cleaning efficiency.
The effectiveness of an in-room air cleaner to remove airborne pathogens from room air depends on several factors, including the airflow rate through the unit’s filter and the airflow patterns in the room. Tested under a variety of conditions, in-room air cleaners, including portable or ceiling mounted units with either a HEPA or a non-HEPA filter, portable units with UVGI lights only, or ceiling mounted units with combined HEPA filtration and UVGI lights, have been estimated to be between 30% and 90%, 99% and 12% and 80% effective, respectively. However, and although their effectiveness is variable, the United States Centers for Disease Control and Prevention has acknowledged in-room air cleaners as alternative technology for increasing room ventilation when this cannot be achieved by the building’s HVAC system with preference given to fixed recirculating systems over portable ones.
Importantly, the use of an in-room air cleaner does not preclude either the need for health care workers and visitors to use personal protective equipment (N95 mask or equivalent) when entering AII rooms or health care facilities from meeting current regulatory requirements for airflow rates (ventilation rates) in buildings and airflow differentials for effective negative-pressure rooms.
The Plasmacluster ion technology, developed in 2000, is an air purification technology. Its manufacturer, Sharp Electronics Corporation, says that it can disable airborne microorganisms through the generation of both positive and negative ions. (1) The functional unit is the hydroxyl, which is a molecule comprised of one oxygen molecule and one hydrogen atom.
Plasmacluster ion air purifier uses a multilayer filter system composed of a prefilter, a carbon filter, an antibacterial filter, and a HEPA filter, combined with an ion generator to purify the air. The ion generator uses an alternating plasma discharge to split water molecules into positively and negatively charged ions. When these ions are emitted into the air, they are surrounded by water molecules and form cluster ions which are attracted to airborne particles. The cluster ion surrounds the airborne particle, and the positive and negative ions react to form hydroxyls. These hydroxyls steal the airborne particle’s hydrogen atom, which creates a hole in the particle’s outer protein membrane, thereby rendering it inactive.
Because influenza is primarily acquired by large droplets and direct and indirect contact with an infectious person, any in-room air cleaner will have little benefit in controlling and preventing its spread. Therefore, there is no role for the Plasmacluster ion air purifier or any other in-room air cleaner in the control of the spread of influenza. Accordingly, for purposes of this review, the Medical Advisory Secretariat presents no further analysis of the Plasmacluster.
Review Strategy
The objective of the systematic review was to determine the effectiveness of in-room air cleaners with built in UVGI lights and HEPA filtration compared with those using HEPA filtration only.
The Medical Advisory Secretariat searched the databases of MEDLINE, EMBASE, Cochrane Database of Systematic Reviews, INAHATA (International Network of Agencies for Health Technology Assessment), Biosis Previews, Bacteriology Abstracts, Web of Science, Dissertation Abstracts, and NIOSHTIC 2.
A meta-analysis was conducted if adequate data was available from 2 or more studies and where statistical and clinical heterogeneity among studies was not an issue. Otherwise, a qualitative review was completed. The GRADE system was used to summarize the quality of the body of evidence comprised of 1 or more studies.
Summary of Findings
There were no existing health technology assessments on air cleaning technology located during the literature review. The literature search yielded 59 citations of which none were retained. One study was retrieved from a reference list of a guidance document from the United States Centers for Disease Control and Prevention, which evaluated an in-room air cleaner with combined UVGI lights and HEPA filtration under 2 conditions: UVGI lights on and UVGI lights off. Experiments were performed using different ventilation rates and using an aerosolized pathogen comprised of Mycobaterium parafortuitum, a surrogate for the bacterium that causes tuberculosis. Effectiveness was measured as equivalent air changes per hour (eACH). This single study formed the body of evidence for our systematic review research question.
Experimental Results
The eACH rate for the HEPA-UVGI in-room air cleaner was statistically significantly greater when the UV lights were on compared with when the UV lights were off. (P < .05). However, subsequent experiments could not attribute this to the UVGI. Consequently, the results are inconclusive and an estimate of effect (benefit) is uncertain.
The study was reviewed by a scientific expert and rated moderate for quality. Further analysis determined that there was some uncertainty in the directness of the outcome measure (eACH); thus, the GRADE level for the quality of the evidence was low indicating that an estimate of effect is very uncertain.
There is uncertainty in the benefits of using in-room air cleaners with combined UVGI lights and HEPA filtration over systems that use HEPA filtration alone. However, there are no known risks to using systems with combined UVGI and HEPA technology compared with those with HEPA alone. There is an increase in the burden of cost including capital costs (cost of the device), operating costs (electricity usage), and maintenance costs (cleaning and replacement of UVGI lights) to using an in-room air cleaner with combined UVGI and HEPA technology compared with those with HEPA alone. Given the uncertainty of the estimate of benefits, an in-room air cleaner with HEPA technology only may be an equally reasonable alternative to using one with combined UVGI and HEPA technology
In-room air cleaners may be used to protect health care staff from air borne infectious pathogens such as tuberculosis, chicken pox, measles, and dessiminated herpes zoster. In addition, and although in-room air cleaners are not effective at protecting staff and preventing the spread of droplet-transmitted diseases such as influenza and SARS, they may be deployed in situations with a novel/emerging infectious agent whose epidemiology is not yet defined and where airborne transmission is suspected.
It is preferable that in-room air cleaners be used with a fixed and permanent room placement when ventilation requirements must be improved and the HVAC system cannot be used. However, for acute (temporary) situations where a novel/emerging infectious agent presents whose epidemiology is not yet defined and where airborne transmission is suspected it may be prudent to use the in room air cleaner as a portable device until mode of transmission is confirmed. To maximize effectiveness, consultation with an environmental engineer and infection control expert should be undertaken before using an in-room air cleaner and protocols for maintenance and monitoring of these devices should be in place.
If properly installed and maintained, in room air cleaners with HEPA or combined HEPA and UVGI air cleaning technology are effective in removing airborne pathogens. However, there is only weak evidence available at this time regarding the benefit of using an in-room air cleaner with combined HEPA and UVGI air cleaner technology instead of those with HEPA filter technology only.
PMCID: PMC3382390  PMID: 23074468
9.  Effects of Sustained Interstitial Fluid Pressurization Under Migrating Contact Area, and Boundary Lubrication by Synovial Fluid, on Cartilage Friction 
This experimental study tests two hypotheses which address outstanding questions in cartilage lubrication: Can the friction coefficient remain low under sustained physiological loading conditions? How effective is synovial fluid in the lubrication of articular cartilage? Based on theory, it is hypothesized that migrating contact areas can maintain elevated cartilage interstitial fluid pressurization, thus a low friction coefficient, indefinitely. It is also hypothesized that the beneficial effects of synovial fluid stem from boundary lubrication rather than fluid-film lubrication.
Five experiments were conducted on immature bovine femoro-tibial joints, to compare the frictional response under migrating versus stationary contact areas; the frictional response in synovial fluid versus saline; the role of sliding velocity and the role of congruence on the friction coefficient.
Migrating contact areas could maintain a low friction coefficient under sustained physiological conditions of loading for at least one hour. Synovial fluid reduced the friction coefficient by a factor of ~1.5 relative to saline. However, interstitial fluid pressurization was far more effective, reducing the friction coefficient by a factor of ~60 relative to equilibrium (zero-pressure) conditions. It was confirmed that synovial fluid acts as a boundary lubricant.
These results emphasize the importance of interstitial fluid pressurization on the frictional response of cartilage. They imply that the mechanical integrity of cartilage must be maintained to produce low friction in articular joints. The more limited effectiveness of synovial fluid implies that intra-articular injections of lubricants in degenerated joints may have only limited effectiveness on their tribological properties.
PMCID: PMC2622427  PMID: 18395475
10.  Superhydrophobic Surface Based on a Coral-Like Hierarchical Structure of ZnO 
PLoS ONE  2010;5(12):e14475.
Fabrication of superhydrophobic surfaces has attracted much interest in the past decade. The fabrication methods that have been studied are chemical vapour deposition, the sol-gel method, etching technique, electrochemical deposition, the layer-by-layer deposition, and so on. Simple and inexpensive methods for manufacturing environmentally stable superhydrophobic surfaces have also been proposed lately. However, work referring to the influence of special structures on the wettability, such as hierarchical ZnO nanostructures, is rare.
This study presents a simple and reproducible method to fabricate a superhydrophobic surface with micro-scale roughness based on zinc oxide (ZnO) hierarchical structure, which is grown by the hydrothermal method with an alkaline aqueous solution. Coral-like structures of ZnO were fabricated on a glass substrate with a micro-scale roughness, while the antennas of the coral formed the nano-scale roughness. The fresh ZnO films exhibited excellent superhydrophilicity (the apparent contact angle for water droplet was about 0°), while the ability to be wet could be changed to superhydrophobicity after spin-coating Teflon (the apparent contact angle greater than 168°). The procedure reported here can be applied to substrates consisting of other materials and having various shapes.
The new process is convenient and environmentally friendly compared to conventional methods. Furthermore, the hierarchical structure generates the extraordinary solid/gas/liquid three-phase contact interface, which is the essential characteristic for a superhydrophobic surface.
PMCID: PMC3012683  PMID: 21209931
11.  Fog-basking behaviour and water collection efficiency in Namib Desert Darkling beetles 
Frontiers in Zoology  2010;7:23.
In the Namib Desert fog represents an alternative water source. This is utilised by Darkling beetles (Tenebrionidae) that employ different strategies for obtaining the fog water. Some dig trenches in the sand, while others use their own bodies as fog collectors assuming a characteristic fog-basking stance. Two beetle species from the genus Onymacris have been observed to fog-bask on the ridges of the sand dunes. These beetles all have smooth elytra surfaces, while another species with elytra covered in bumps is reported to have specialised adaptations facilitating water capture by fog-basking. To resolve if these other beetles also fog-bask, and if an elytra covered in bumps is a more efficient fog water collector than a smooth one, we examined four Namib Desert beetles; the smooth Onymacris unguicularis and O. laeviceps and the bumpy Stenocara gracilipes and Physasterna cribripes. Here we describe the beetles' fog-basking behaviour, the details of their elytra structures, and determine how efficient their dorsal surface areas are at harvesting water from fog.
The beetles differ greatly in size. The largest P. cribripes has a dorsal surface area that is 1.39, 1.56, and 2.52 times larger than O. unguicularis, O. laeviceps, and S. gracilipes, respectively. In accordance with earlier reports, we found that the second largest O. unguicularis is the only one of the four beetles that assumes the head standing fog-basking behaviour, and that fog is necessary to trigger this behaviour. No differences were seen in the absolute amounts of fog water collected on the dorsal surface areas of the different beetles. However, data corrected according to the sizes of the beetles revealed differences. The better fog water harvesters were S. gracilipes and O. unguicularis while the large P. cribripes was the poorest. Examination of the elytra microstructures showed clear structural differences, but the elytra of all beetles were found to be completely hydrophobic.
The differences in fog water harvesting efficiency by the dorsal surface areas of beetles with very different elytra surface structures were minor. We therefore conclude that the fog-basking behaviour itself is a more important factor than structural adaptations when O. unguicularis collect water from fog.
PMCID: PMC2918599  PMID: 20637085
12.  Approaches to nanostructure control and functionalizations of polymer@silica hybrid nanograss generated by biomimetic silica mineralization on a self-assembled polyamine layer 
We report the rational control of the nanostructure and surface morphology of a polyamine@silica nanoribbon-based hybrid nanograss film, which was generated by performing a biomimetic silica mineralization reaction on a nanostructured linear polyethyleneimine (LPEI) layer preorganized on the inner wall of a glass tube. We found that the film thickness, size and density of the nanoribbons and the aggregation/orientation of the nanoribbons in the film were facile to tune by simple adjustment of the biomimetic silicification conditions and LPEI self-assembly on the substrate. Our LPEI-mediated nanograss process allows the facile and programmable generation of a wide range of nanostructures and surface morphologies without the need for complex molecular design or tedious techniques. This ribbon-based nanograss has characteristics of a LPEI@silica hybrid structure, suggesting that LPEI, as a polymeric secondary amine, is available for subsequent chemical reaction. This feature was exploited to functionalize the nanograss film with three representative species, namely porphyrin, Au nanoparticles and titania. Of particular note, the novel silica@titania composite nanograss surface demonstrated the ability to convert its wetting behavior between the extreme states (superhydrophobic–superhydrophilic) by surface hydrophobic treatment and UV irradiation. The anatase titania component in the nanograss film acts as a highly efficient photocatalyst for the decomposition of the low-surface-energy organic components attached to the nanosurface. The ease with which the nanostructure can be controlled and facilely functionalized makes our nanograss potentially important for device-based application in microfluidic, microreactor and biomedical fields.
PMCID: PMC3257501  PMID: 22259759
biomimetic silica mineralization; linear polyethyleneimine; nanofiber; nanograss; thin film
13.  Friction characteristics of Cd-rich carbonate films on calcite surfaces: implications for compositional differentiation at the nanometer scale 
Lateral Force Microscopy (LFM) studies were carried out on cleaved calcite sections in contact with solutions supersaturated with respect to otavite (CdCO3) or calcite-otavite solid solutions (SS) as a means to examine the potential for future application of LFM as a nanometer-scale mineral surface composition mapping technique. Layer-by-layer growth of surface films took place either by step advancement or by a surface nucleation and step advancement mechanisms. Friction vs. applied load data acquired on the films and the calcite substrate were successfully fitted to the Johnson Kendall Roberts (JKR) model for single asperity contacts. Following this model, friction differences between film and substrate at low loads were dictated by differences in adhesion, whereas at higher load they reflect differences in contact shear strength. In most experiments at fixed load, the film showed higher friction than the calcite surface, but the friction-load dependence for the different surfaces revealed that at low loads (0–40 nN), a calcian otavite film has lower friction than calcite; a result that is contrary to earlier LFM reports of the same system. Multilayer films of calcian-otavite displayed increasing friction with film thickness, consistent with the expectation that the film surface composition will become increasingly Cd-rich with increasing thickness. Both load- and thickness-dependence trends support the hypothesis that the contact shear strength correlates with the hydration enthalpy of the surface ions, thereby imparting friction sensitivity in the LFM to mineral-water interface composition.
PMCID: PMC2719640  PMID: 19549312
14.  A perspective on underwater photosynthesis in submerged terrestrial wetland plants 
AoB Plants  2011;2011:plr030.
Submergence inhibits photosynthesis by terrestrial wetland plants, but less so in species that possess leaf gas films when submerged. Floodwaters are often supersaturated with dissolved CO2 enabling photosynthesis by submerged terrestrial plants, although rates remain well-below those in air. This important adaptation that enhances survival in submerged conditions is reviewed.
Background and aims
Wetland plants inhabit flood-prone areas and therefore can experience episodes of complete submergence. Submergence impedes exchange of O2 and CO2 between leaves and the environment, and light availability is also reduced. The present review examines limitations to underwater net photosynthesis (PN) by terrestrial (i.e. usually emergent) wetland plants, as compared with submerged aquatic plants, with focus on leaf traits for enhanced CO2 acquisition.
Floodwaters are variable in dissolved O2, CO2, light and temperature, and these parameters influence underwater PN and the growth and survival of submerged plants. Aquatic species possess morphological and anatomical leaf traits that reduce diffusion limitations to CO2 uptake and thus aid PN under water. Many aquatic plants also have carbon-concentrating mechanisms to increase CO2 at Rubisco. Terrestrial wetland plants generally lack the numerous beneficial leaf traits possessed by aquatic plants, so submergence markedly reduces PN. Some terrestrial species, however, produce new leaves with a thinner cuticle and higher specific leaf area, whereas others have leaves with hydrophobic surfaces so that gas films are retained when submerged; both improve CO2 entry.
Submergence inhibits PN by terrestrial wetland plants, but less so in species that produce new leaves under water or in those with leaf gas films. Leaves with a thinner cuticle, or those with gas films, have improved gas diffusion with floodwaters, so that underwater PN is enhanced. Underwater PN provides sugars and O2 to submerged plants. Floodwaters often contain dissolved CO2 above levels in equilibrium with air, enabling at least some PN by terrestrial species when submerged, although rates remain well below those in air.
PMCID: PMC3249690  PMID: 22476500
This study presents an analysis of the contact of a rippled rigid impermeable indenter against a cartilage layer, which represents a first simulation of the contact of rough cartilage surfaces with lubricant entrapment. Cartilage was modeled with the biphasic theory for hydrated soft tissues, to account for fluid flow into or out of the lubricant pool. The findings of this study demonstrate that under contact creep, the trapped lubricant pool gets depleted within a time period on the order of seconds or minutes as a result of lubricant flow into the articular cartilage. Prior to depletion, hydrostatic fluid load across the contact interface may be enhanced by the presence of the trapped lubricant pool, depending on the initial geometry of the lubricant pool. According to friction models based on the biphasic nature of the tissue, this enhancement in fluid load support produces a smaller minimum friction coefficient than would otherwise be predicted without a lubricant pool. The results of this study support the hypothesis that trapped lubricant decreases the initial friction coefficient following load application, independently of squeeze-film lubrication effects.
PMCID: PMC2842086  PMID: 14618917
16.  Meibomian Lipid Films and the Impact of Temperature 
There is evidence that, in cold conditions, the temperature of human eyelids and of the ocular surface drops well below normal physiological levels. This may have a detrimental impact on the stability and functionality of the human tear film and the tear film lipid layer, which was indeed demonstrated in in vitro experiments with meibomian lipid films.
There is evidence that, in cold conditions, the temperature of human eyelids and of the ocular surface drops well below normal physiological levels. This may have a detrimental impact on the stability and functionality of the human tear film and the tear film lipid layer. The goal of this project was to quantitatively examine the possible impact of temperature on the latter.
Meibum samples were collected by using a soft-squeezing technique and were studied in a Langmuir trough. The obtained surface pressure and area isotherms were analyzed to determine the biophysical parameters of thin meibomian lipid film (MLF): the lift-off area, collapse pressure, two-dimensional elasticity, and hysteresis and their dependence on temperature.
MLF was found to be highly susceptible to changes in temperature. At temperatures below the physiological level, the MLF became stiff and shrank considerably. The shrinkage left a large portion of the air–water interface uncovered with lipid molecules. The effect was shown to be reversible. On reheating, the lipids melted and respread to restore the original film. There was a fundamental difference observed between three-dimensional melting of dry meibum in bulk and the two-dimensional melting in MLF at the air–water interface. Bulk meibum melted in a narrower temperature range and showed a much higher cooperativity of melting.
Temperature critically influences MLF. Low temperature leads to stiffening of the film, which loses its ability to form continuous layers at the air–water interface. These effects were shown be of a cooperative nature, manifesting in relatively narrow concentration and temperature ranges.
PMCID: PMC3061497  PMID: 20610839
17.  Asymmetric Free-Standing Film with Multifunctional Anti-Bacterial and Self-Cleaning Properties 
ACS applied materials & interfaces  2012;4(9):4476-4483.
A superhydrophobic/hydrophilic asymmetric free-standing film has been created using layer-by-layer assembly technique. Poly(ethylene-imine)-Ag+ complex (PEI-Ag+) at pH 9.0 was assembled with poly(acrylic acid) (PAA) at pH 3.2 on a Teflon substrate to yield a micro-nanostructured surface that can be turned to be superhydrophobic after being coated with a low surface energy compound. Silver nanoparticle loaded free-standing film with one surface being superhydrophobic while the other surface is hydrophilic was then obtained after detachment from the substrate. The superhydrophobicity enabled the upper surface with anti adhesion and self-cleaning properties and the hydrophilic bottom surface can release silver ions as antibiotic agent. The broad-spectrum antimicrobial capability of silver ions released from the bottom surface coupled with superhydrophobic barrier protection of the upper surface may make the free-standing film a new therapy for open wound.
PMCID: PMC4111538  PMID: 22947922
Asymmetric; superhydrophobic; free-standing films; anti adhesion; drug release; layer-by-layer assembly
18.  Noncontact friction via capillary shear interaction at nanoscale 
Nature Communications  2015;6:7359.
Friction in an ambient condition involves highly nonlinear interactions of capillary force, induced by the capillary-condensed water nanobridges between contact or noncontact asperities of two sliding surfaces. Since the real contact area of sliding solids is much smaller than the apparent contact area, the nanobridges formed on the distant asperities can contribute significantly to the overall friction. Therefore, it is essential to understand how the water nanobridges mediate the ‘noncontact' friction, which helps narrow the gap between our knowledge of friction on the microscopic and macroscopic scales. Here we show, by using noncontact dynamic force spectroscopy, the single capillary bridge generates noncontact friction via its shear interaction. The pinning–depinning dynamics of the nanobridge's contact line produces nonviscous damping, which occurs even without normal load and dominates the capillary-induced hydrodynamic damping. The novel nanofriction mechanism may provide a deeper microscopic view of macroscopic friction in air where numerous asperities exist.
The contribution from water bridges at nanoscale between rough surfaces is important for macroscopic friction under ambient conditions. Here, Lee et al. show that water nanobridge produce noncontact friction originated from the pinning–depinning dynamics of the contact line at the interface.
PMCID: PMC4490357  PMID: 26066909
19.  Superhydrophobic Photosensitizers. Mechanistic Studies of 1O2 Generation in the Plastron and Solid/Liquid Droplet Interface 
Journal of the American Chemical Society  2013;135(50):18990-18998.
We describe here a physical-organic study of the first triphasic superhydrophobic sensitizer for photooxidations in water droplets. Control of synthetic parameters enables the mechanistic study of “borderline” two- and three-phase superhydrophobic sensitizer surfaces where 1O2 is generated in compartments that are wetted, partially wetted, or remain dry in the plastron (i.e., air layer beneath the droplet). The superhydrophobic surface is synthesized by partially embedding silicon phthalocyanine (Pc) sensitizing particles to specific locations on polydimethylsiloxane (PDMS) posts printed in a square array (1 mm tall posts on 0.5 mm pitch). In the presence of red light and oxygen, singlet oxygen is formed on the superhydrophobic surface and reacts with 9,10-anthracene dipropionate dianion (1) within a freestanding water droplet to produce an endoperoxide in 54–72% yields. Control of the 1O2 chemistry was achieved by the synthesis of superhydrophobic surfaces enriched with Pc particles either at the PDMS end-tips or at PDMS post bases. Much of the 1O2 that reacts with anthracene 1 in the droplets was generated by the sensitizer “wetted” at the Pc particle/water droplet interface and gave the highest endoperoxide yields. About 20% of the 1O2 can be introduced into the droplet from the plastron. The results indicate that the superhydrophobic sensitizer surface offers a unique system to study 1O2 transfer routes where a balance of gas and liquid contributions of 1O2 is tunable within the same superhydrophobic surface.
PMCID: PMC3930608  PMID: 24295210
20.  Assembly and Structure of α-Helical Peptide Films on Hydrophobic Fluorocarbon Surfaces 
Biointerphases  2010;5(1):9-16.
The structure, orientation and formation of amphiphilic α-helix model peptide films on fluorocarbon surfaces has been monitored with sum frequency generation (SFG) vibrational spectroscopy, near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The α-helix peptide is a 14-mer of hydrophilic lysine and hydrophobic leucine residues with a hydrophobic periodicity of 3.5. This periodicity yields a rigid amphiphilic peptide with leucine and lysine side chains located on opposite sides. XPS composition analysis confirms the formation of a peptide film that covers about 75% of the surface. NEXAFS data are consistent with chemically intact adsorption of the peptides. A weak linear dichroism of the amide π* is likely due to the broad distribution of amide bond orientations inherent to the α-helical secondary structure. SFG spectra exhibit strong peaks near 2865 cm−1 and 2935 cm−1 related to aligned leucine side chains interacting with the hydrophobic surface. Water modes near 3200 cm−1 and 3400 cm−1 indicate ordering of water molecules in the adsorbed--peptide fluorocarbon surface interfacial region. Amide I peaks observed near 1655 cm−1 confirm that the secondary structure is preserved in the adsorbed peptide. A kinetic study of the film formation process using XPS and SFG showed rapid adsorption of the peptides followed by a longer assembly process. Peptide SFG spectra taken at the air–buffer interface showed features related to well ordered peptide films. Moving samples through the buffer surface led to the transfer of ordered peptide films onto the substrates.
PMCID: PMC3912757  PMID: 20408730
21.  Compound Microstructures and Wax Layer of Beetle Elytral Surfaces and Their Influence on Wetting Properties 
PLoS ONE  2012;7(10):e46710.
A beetles’ first line of defense against environmental hazards is their mesothoracic elytra – rigid, protective forewings. In order to study the interaction of these wings with water, the surface microstructures of various beetles’ elytra were observed by Environment Scanning Electron Microscopy (ESEM) and Atomic Force Microscopy (AFM). Chemistry components were ascertained using X-ray photoelectron spectroscopy (XPS). All the beetles of various habitats (including desert, plant, dung, land and water) exhibited compound microstructures on their elytra. The wetting properties of these elytra were identified using an optical contact angle meter. In general the native elytra exhibited hydrophilic or weak hydrophobic properties with contact angles (CAs) ranging from 47.5° to 109.1°. After treatment with chloroform, the CAs all increased on the rougher elytral surfaces. The presence of wax is not the only determinant of hydrophobic properties, but rather a combination with microscopic structures found on the surfaces. Irregularities and the presence or absence of tiny cracks, hairs (or setae), pores and protrusions are important factors which influence the wetting properties. Rougher elytral surfaces tended to present a stronger hydrophobicity. Effects on hydrophobicity, such as surface microstructures, chemistry, environment and aging (referring to the time after emergence), are also included and discussed. Our results also provide insights into the motion of water droplets when in contact with beetle elytra.
PMCID: PMC3464267  PMID: 23056414
22.  Role of Structure and Glycosylation of Adsorbed Protein Films in Biolubrication 
PLoS ONE  2012;7(8):e42600.
Water forms the basis of lubrication in the human body, but is unable to provide sufficient lubrication without additives. The importance of biolubrication becomes evident upon aging and disease, particularly under conditions that affect secretion or composition of body fluids. Insufficient biolubrication, may impede proper speech, mastication and swallowing, underlie excessive friction and wear of articulating cartilage surfaces in hips and knees, cause vaginal dryness, and result in dry, irritated eyes. Currently, our understanding of biolubrication is insufficient to design effective therapeutics to restore biolubrication. Aim of this study was to establish the role of structure and glycosylation of adsorbed protein films in biolubrication, taking the oral cavity as a model and making use of its dynamics with daily perturbations due to different glandular secretions, speech, drinking and eating, and tooth brushing. Using different surface analytical techniques (a quartz crystal microbalance with dissipation monitoring, colloidal probe atomic force microscopy, contact angle measurements and X-ray photo-electron spectroscopy), we demonstrated that adsorbed salivary conditioning films in vitro are more lubricious when their hydrophilicity and degree of glycosylation increase, meanwhile decreasing their structural softness. High-molecular-weight, glycosylated proteins adsorbing in loops and trains, are described as necessary scaffolds impeding removal of water during loading of articulating surfaces. Comparing in vitro and in vivo water contact angles measured intra-orally, these findings were extrapolated to the in vivo situation. Accordingly, lubricating properties of teeth, as perceived in 20 volunteers comprising of equal numbers of male and female subjects, could be related with structural softness and glycosylation of adsorbed protein films on tooth surfaces. Summarizing, biolubrication is due to a combination of structure and glycosylation of adsorbed protein films, providing an important clue to design effective therapeutics to restore biolubrication in patients with insufficient biolubrication.
PMCID: PMC3419733  PMID: 22916138
23.  Tribological properties of undoped and boron-doped nanocrystalline diamond films 
Thin solid films  2009;517(2):800-804.
Undoped and boron-doped nanocrystalline (NCD) diamond films were deposited on mirror polished Ti–6Al–4V substrates in a Microwave Plasma Assisted Chemical Vapor Deposition system. Sliding wear tests were conducted in ambient air with a nanotribometer. A systematic study of the tribological properties for both undoped and boron-doped NCD films were carried out. It was found for diamond/diamond sliding, coefficient of friction decreases with increasing normal loads. It was also found that the wear rate of boron-doped NCD films is about 10 times higher than that of undoped films. A wear rate of ~5.2×10−9 mm3/Nm was found for undoped NCD films. This value is comparable to the best known value of that of polished polycrystalline diamond films. Although no surface deformation, film delamination or micro-cracking were observed for undoped films, boron-doped NCD film undergoes a critical failure at a normal stress of 2.2 GPa, above which surface deformation is evident. Combined with high hardness and modulus, tunable conductivity and improved open air thermal stability, boron-doped nanocrystalline diamond film has tremendous potentials for applications such as Atomic Force Microscope probes, Micro-Electro-Mechanical System devices and biomedical sensors.
PMCID: PMC2776763  PMID: 19946362
Nanocrystalline diamond; Tribology; Microwave Plasma Assisted Chemical Vapor; Deposition; Boron doping
24.  Flexible conformable hydrophobized surfaces for turbulent flow drag reduction 
Scientific Reports  2015;5:10267.
In recent years extensive work has been focused onto using superhydrophobic surfaces for drag reduction applications. Superhydrophobic surfaces retain a gas layer, called a plastron, when submerged underwater in the Cassie-Baxter state with water in contact with the tops of surface roughness features. In this state the plastron allows slip to occur across the surface which results in a drag reduction. In this work we report flexible and relatively large area superhydrophobic surfaces produced using two different methods: Large roughness features were created by electrodeposition on copper meshes; Small roughness features were created by embedding carbon nanoparticles (soot) into Polydimethylsiloxane (PDMS). Both samples were made into cylinders with a diameter under 12 mm. To characterize the samples, scanning electron microscope (SEM) images and confocal microscope images were taken. The confocal microscope images were taken with each sample submerged in water to show the extent of the plastron. The hydrophobized electrodeposited copper mesh cylinders showed drag reductions of up to 32% when comparing the superhydrophobic state with a wetted out state. The soot covered cylinders achieved a 30% drag reduction when comparing the superhydrophobic state to a plain cylinder. These results were obtained for turbulent flows with Reynolds numbers 10,000 to 32,500.
PMCID: PMC4432562  PMID: 25975704
25.  Electrical characterization of single molecule and Langmuir–Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative 
Monolayer Langmuir–Blodgett (LB) films of 1,4-bis(pyridin-4-ylethynyl)benzene (1) together with the “STM touch-to-contact” method have been used to study the nature of metal–monolayer–metal junctions in which the pyridyl group provides the contact at both molecule–surface interfaces. Surface pressure vs area per molecule isotherms and Brewster angle microscopy images indicate that 1 forms true monolayers at the air–water interface. LB films of 1 were fabricated by deposition of the Langmuir films onto solid supports resulting in monolayers with surface coverage of 0.98 × 10−9 mol·cm−2. The morphology of the LB films that incorporate compound 1 was studied using atomic force microscopy (AFM). AFM images indicate the formation of homogeneous, monomolecular films at a surface pressure of transference of 16 mN·m−1. The UV–vis spectra of the Langmuir and LB films reveal that 1 forms two dimensional J-aggregates. Scanning tunneling microscopy (STM), in particular the “STM touch-to-contact” method, was used to determine the electrical properties of LB films of 1. From these STM studies symmetrical I–V curves were obtained. A junction conductance of 5.17 × 10−5 G 0 results from the analysis of the pseudolinear (ohmic) region of the I–V curves. This value is higher than that of the conductance values of LB films of phenylene-ethynylene derivatives contacted by amines, thiols, carboxylate, trimethylsilylethynyl or acetylide groups. In addition, the single molecule I–V curve of 1 determined using the I(s) method is in good agreement with the I–V curve obtained for the LB film, and both curves fit well with the Simmons model. Together, these results not only indicate that the mechanism of transport through these metal–molecule–metal junctions is non-resonant tunneling, but that lateral interactions between molecules within the LB film do not strongly influence the molecule conductance. The results presented here complement earlier studies of single molecule conductance of 1 using STM-BJ methods, and support the growing evidence that the pyridyl group is an efficient and effective anchoring group in sandwiched metal–monolayer–metal junctions prepared under a number of different conditions.
PMCID: PMC4464395  PMID: 26171291
Langmuir–Blodgett films; molecular electronics; STM touch-to-contact method

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