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1.
Artigo em Inglês | MEDLINE | ID: mdl-32227984

RESUMO

Use of colloidal suspensions to generate structural colors has the potential to reduce the use of toxic metals or organic pigments in inkjet printing, coatings, cosmetics, and other applications, and is a promising avenue to create large-scale nanostructures that produce long-lasting colors. However, expanded use of structural colors requires a reduction in coffee-ring effects during printing, which currently requires intricately patterned substrates or high particle concentrations, and diversification of colors to compete with conventional printing inks. Here, we treat substrate surfaces with cold plasma to facilitate spontaneous assembly of particles into colloidal nanostructures, reducing the need for highly concentrated particle suspensions. Moreover, by employing binary mixtures, we can tune the lightness of the hue produced or change the hue itself, allowing us to cover wider regions of color space. We demonstrate the use of this cold-plasma approach on a variety of substrates, favoring substrate diversity on which printing can be performed. This methodology enables creation of high-resolution, complex designs and opens a path for extending the limits of anticounterfeiting applications by using binary mixtures.

2.
J R Soc Interface ; 17(162): 20190792, 2020 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-31992163

RESUMO

Contaminants decrease adhesive strength by interfering with substrate contact. Spider webs adhering to moths present an ideal model to investigate how natural adhesives overcome contamination because moths' sacrificial layer of scales rubs off on sticky silk, facilitating escape. However, Cyrtarachninae spiders have evolved gluey capture threads that adhere well to moths. Cyrtarachne capture threads contain large glue droplets oversaturated with water, readily flowing but also prone to drying out. Here, we compare the spreading and adhesion of Cyrtarachne akirai glue on intact mothwings, denuded cuticle and glass to the glue of a common orb-weaving spider, Larinioides cornutus, to understand how C. akirai glue overcomes dirty surfaces. Videos show that C. akirai's glue spreading accelerates along the underlying moth cuticle after the glue seeps beneath the moth scales-not seen on denuded cuticle or hydrophilic glass. Larinioides cornutus glue droplets failed to penetrate the moth scales, their force of adhesion thus limited by the strength of attachment of scales to the cuticle. The large size and low viscosity of C. akirai glue droplets function together to use the three-dimensional topography of the moth's scales against itself via capillary forces. Infrared spectroscopy shows C. akirai glue droplets readily lose free-flowing water. We hypothesize that this loss of water leads to increased viscosity during spreading, increasing cohesive forces during pull-off. This glue's two-phase behaviour shows how natural selection can leverage a defensive specialization of prey against themselves and highlights a new design principle for synthetic adhesives for adhering to troublesome surfaces.

3.
Langmuir ; 36(4): 837-843, 2020 Feb 04.
Artigo em Inglês | MEDLINE | ID: mdl-31898908

RESUMO

Use of a plasma-polymerized (pp) layer under a polyurethane (PU) coating on aluminum dramatically improves the corrosion resistance. Compared to conventional polymer coatings, pp coatings are highly cross-linked, have better adhesion to substrates, and result in lower emission of volatile organic contents. Although past research has focused on the properties of comparatively thick pp films and on the use of pp films alone to protect metals, we consider here very thin pp coatings as a primer layer to improve corrosion resistance. Electrochemical impedance spectroscopy combined with salt spray lab tests show that the corrosion resistance of a PU coating on top of a pp coating from hexamethyldisiloxane (HMDSO) is much better than that of a PU coating directly on Al 3003. The relatively poor pull-off adhesion between PU and pp-HMDSO is readily addressed using a gradient coating by depositing a pp maleic anhydride layer over the pp-HMDSO coating or by modifying the surface composition of the pp-HMDSO coating with N2 plasma. X-ray photon spectroscopy analysis of the failure interface from pull-off tests makes clear that failure does not occur at the interface between the pp coating and the metal substrate. Field tests show the performance of the coating system with PU on a gradient coating on Al 3003 to be superior to that of a coating system of PU on chromate-treated Al 3003.

4.
Environ Microbiol ; 22(3): 952-963, 2020 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-31390129

RESUMO

Chlorophyll (Chl) f and d are the most recently discovered chlorophylls, enabling cyanobacteria to harvest near-infrared radiation (NIR) at 700-780 nm for oxygenic photosynthesis. Little is known about the occurrence of these pigments in terrestrial habitats. Here, we provide first details on spectral photon irradiance within the photic zones of four terrestrial cave systems in concert with a detailed investigation of photopigmentation, light reflectance and microbial community composition. We frequently found Chl f and d along the photic zones of caves characterized by low light enriched in NIR and inhabited by cyanobacteria producing NIR-absorbing pigments. Surprisingly, deeper parts of caves still contained NIR, an effect likely attributable to the reflectance of specific wavelengths by the surface materials of cave walls. We argue that the stratification of microbial communities across the photic zones of cave entrances resembles the light-driven species distributions in forests and aquatic environments.

5.
J Exp Zool A Ecol Integr Physiol ; 333(2): 118-125, 2020 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-31742935

RESUMO

The remarkable properties of the gecko adhesive system have been intensively studied. Although many gecko-inspired synthetic adhesives have been designed and fabricated, few manage to capture the multifunctionality of the natural system. Analogous to previously documented self-cleaning, recent work demonstrated that gecko toe pads dry when geckos take steps on dry substrates (i.e., self-drying). Whether digital hyperextension (DH), the distal to proximal peeling of gecko toe pads, is involved in the self-drying process, had not been determined. Here, the effect of DH on self-drying was isolated by preventing DH from occurring during normal walking locomotion of Gekko gecko after toe pads were wetted. Our initial analysis revealed low statistical power, so we increased our sample size to determine the robustness of our result. We found that neither DH nor the DH-substrate interaction had a significant effect on the maximum shear adhesive force after self-drying. These results suggest that DH is not necessary for self-drying to occur. Interestingly, however, we discovered that shear adhesion is higher on a surface tending hydrophobic compared to a hydrophilic surface, demonstrating that gecko adhesion is sensitive to substrate wettability during the subdigital pad drying process. Furthermore, we also observed frequent damage to the adhesive system during shear adhesion testing post-drying, indicating that water may compromise the structural integrity of the adhesive structures. Our results not only have behavioral and ecological implications for free-ranging geckos but also have the potential to influence the design and fabrication of gecko-inspired synthetic adhesives that can regain adhesion after fouling with water.

6.
Langmuir ; 36(1): 447-455, 2020 Jan 14.
Artigo em Inglês | MEDLINE | ID: mdl-31826618

RESUMO

The properties of nanoconfined water arise in direct response to the properties of the interfaces that confine it. A great deal of research has focused on understanding how and why the physical properties of confined water differ greatly from the bulk. In this work, we have used all-atom molecular dynamics (MD) simulations to provide a detailed description of the structural and dynamical properties of nanoconfined water between two monolayers consisting of an archetypal ionic surfactant, cetrimonium bromide (CTAB, [CH3(CH2)15N(CH3)3]+Br-). Small differences in the area per surfactant of the monolayers impart a clear effect on the intrinsic density, mobility, and ordering of the interfacial water layer confined by the monolayers. We find that as the area per surfactant within a monolayer decreases, the mobility of the interfacial water molecules decreases in response. As the monolayer packing density decreases, we find that each individual CTAB molecule has a greater effect on the ordering of water molecules in its first hydration shell. In a denser monolayer, we observe that the effect of individual CTAB molecules on the ordering of water molecules is hindered by increased competition between headgroups. Therefore, when two monolayers with different areas per surfactant are used to confine a nanoscale water layer, we observe the emergence of noncentrosymmetry.

7.
Appl Environ Microbiol ; 86(5)2020 Feb 18.
Artigo em Inglês | MEDLINE | ID: mdl-31836580

RESUMO

Melanin is a pigment produced by organisms throughout all domains of life. Due to its unique physicochemical properties, biocompatibility, and biostability, there has been an increasing interest in the use of melanin for broad applications. In the vast majority of studies, melanin has been either chemically synthesized or isolated from animals, which has restricted its use to small-scale applications. Using bacteria as biocatalysts is a promising and economical alternative for the large-scale production of biomaterials. In this study, we engineered the marine bacterium Vibrio natriegens, one of the fastest-growing organisms, to synthesize melanin by expressing a heterologous tyrosinase gene and demonstrated that melanin production was much faster than in previously reported heterologous systems. The melanin of V. natriegens was characterized as a polymer derived from dihydroxyindole-2-carboxylic acid (DHICA) and, similarly to synthetic melanin, exhibited several characteristic and useful features. Electron microscopy analysis demonstrated that melanin produced from V. natriegens formed nanoparticles that were assembled as "melanin ghost" structures, and the photoprotective properties of these particles were validated by their protection of cells from UV irradiation. Using a novel electrochemical reverse engineering method, we observed that melanization conferred redox activity to V. natriegens Moreover, melanized bacteria were able to quickly adsorb the organic compound trinitrotoluene (TNT). Overall, the genetic tractability, rapid division time, and ease of culture provide a set of attractive properties that compare favorably to current E. coli production strains and warrant the further development of this chassis as a microbial factory for natural product biosynthesis.IMPORTANCE Melanins are macromolecules that are ubiquitous in nature and impart a large variety of biological functions, including structure, coloration, radiation resistance, free radical scavenging, and thermoregulation. Currently, in the majority of investigations, melanins are either chemically synthesized or extracted from animals, which presents significant challenges for large-scale production. Bacteria have been used as biocatalysts to synthesize a variety of biomaterials due to their fast growth and amenability to genetic engineering using synthetic biology tools. In this study, we engineered the extremely fast-growing bacterium V. natriegens to synthesize melanin nanoparticles by expressing a heterologous tyrosinase gene with inducible promoters. Characterization of the melanin produced from V. natriegens-produced tyrosinase revealed that it exhibited physical and chemical properties similar to those of natural and chemically synthesized melanins, including nanoparticle structure, protection against UV damage, and adsorption of toxic compounds. We anticipate that producing and controlling melanin structures at the nanoscale in this bacterial system with synthetic biology tools will enable the design and rapid production of novel biomaterials for multiple applications.

8.
J R Soc Interface ; 16(160): 20190388, 2019 Nov 29.
Artigo em Inglês | MEDLINE | ID: mdl-31771420

RESUMO

Adhesive attachment systems consisting of multiple tapes or strands are commonly found in nature, for example in spider web anchorages or in mussel byssal threads, and their structure has been found to be ingeniously architected in order to optimize mechanical properties: in particular, to maximize dissipated energy before full detachment. These properties emerge from the complex interplay between mechanical and geometric parameters, including tape stiffness, adhesive energy, attached and detached lengths and peeling angles, which determine the occurrence of three main mechanisms: elastic deformation, interface delamination and tape fracture. In this paper, we introduce a formalism to evaluate the mechanical performance of multiple tape attachments in different parameter ranges, where an optimal (not maximal) adhesion energy emerges. We also introduce a numerical model to simulate the multiple peeling behaviour of complex structures, illustrating its predictions in the case of the staple-pin architecture. Finally, we present a proof-of-principle experiment to illustrate the predicted behaviour. We expect the presented formalism and the numerical model to provide important tools for the design of bioinspired adhesive systems with tuneable or optimized detachment properties.

9.
Proc Natl Acad Sci U S A ; 116(51): 25484-25490, 2019 Dec 17.
Artigo em Inglês | MEDLINE | ID: mdl-31772024

RESUMO

A mechanistic understanding of adhesion in soft materials is critical in the fields of transportation (tires, gaskets, and seals), biomaterials, microcontact printing, and soft robotics. Measurements have long demonstrated that the apparent work of adhesion coming into contact is consistently lower than the intrinsic work of adhesion for the materials, and that there is adhesion hysteresis during separation, commonly explained by viscoelastic dissipation. Still lacking is a quantitative experimentally validated link between adhesion and measured topography. Here, we used in situ measurements of contact size to investigate the adhesion behavior of soft elastic polydimethylsiloxane hemispheres (modulus ranging from 0.7 to 10 MPa) on 4 different polycrystalline diamond substrates with topography characterized across 8 orders of magnitude, including down to the angstrom scale. The results show that the reduction in apparent work of adhesion is equal to the energy required to achieve conformal contact. Further, the energy loss during contact and removal is equal to the product of the intrinsic work of adhesion and the true contact area. These findings provide a simple mechanism to quantitatively link the widely observed adhesion hysteresis to roughness rather than viscoelastic dissipation.

10.
Biomacromolecules ; 20(12): 4593-4601, 2019 Dec 09.
Artigo em Inglês | MEDLINE | ID: mdl-31696706

RESUMO

Eumelanin is a ubiquitous natural pigment that has a broad absorption across ultraviolet (UV, 100-400 nm) and visible wavelengths (400-700 nm) and can protect against radiation. Synthetic eumelanin with properties similar to natural eumelanin has been made using dopamine or dihydroxyindole. Here, we use solid-state nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy to elucidate the chemical structure of synthetic eumelanins (made from dopamine and l-3,4-dihydroxyphenylalanine precursors) and investigate how their structures change after intensive UVA (315-400 nm) exposure. We first confirm that polydopamine has indole units. Upon UV exposure, the pyrrole ring in this indole unit remains intact, and a fraction of the six-membered benzyl ring is broken and the indole potentially converted to furo[3,4-b]pyrrole. This change in the chemical structure is accompanied by a release of carbon dioxide. In addition, the sepia (natural) eumelanin used for comparison is more stable than the synthetic eumelanin. Understanding the UVA degradation mechanism of eumelanin will help reveal the role of eumelanin in skin cancer and in the design of more efficient UV stabilizers.

11.
Sci Adv ; 5(9): eaax1254, 2019 09.
Artigo em Inglês | MEDLINE | ID: mdl-31555734

RESUMO

Surface segregation in binary colloidal mixtures offers a simple way to control both surface and bulk properties without affecting their bulk composition. Here, we combine experiments and coarse-grained molecular dynamics (CG-MD) simulations to delineate the effects of particle chemistry and size on surface segregation in photonic colloidal assemblies from binary mixtures of melanin and silica particles of size ratio (D large /D small) ranging from 1.0 to ~2.2. We find that melanin and/or smaller particles segregate at the surface of micrometer-sized colloidal assemblies (supraballs) prepared by an emulsion process. Conversely, no such surface segregation occurs in films prepared by evaporative assembly. CG-MD simulations explain the experimental observations by showing that particles with the larger contact angle (melanin) are enriched at the supraball surface regardless of the relative strength of particle-interface interactions, a result with implications for the broad understanding and design of colloidal particle assemblies.

12.
Front Zool ; 16: 19, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-31210775

RESUMO

Background: Mucus and mucus glands are important features of the amphibian cutis. In tree frogs, the mucus glands and their secretions are crucial components of the adhesive digital pads of these animals. Despite a variety of hypothesised functions of these components in tree frog attachment, the functional morphology of the digital mucus glands and the chemistry of the digital mucus are barely known. Here, we use an interdisciplinary comparative approach to analyse these components, and discuss their roles in tree frog attachment. Results: Using synchrotron micro-computer-tomography, we discovered in the arboreal frog Hyla cinerea that the ventral digital mucus glands differ in their morphology from regular anuran mucus glands and form a subdermal gland cluster. We show the presence of this gland cluster also in several other-not exclusively arboreal-anuran families. Using cryo-histochemistry as well as infrared and sum frequency generation spectroscopy on the mucus of two arboreal (H. cinerea and Osteopilus septentrionalis) and of two terrestrial, non-climbing frog species (Pyxicephalus adspersus and Ceratophrys cranwelli), we find neutral and acidic polysaccharides, and indications for proteinaceous and lipid-like mucus components. The mucus chemistry varies only little between dorsal and ventral digital mucus in H. cinerea, ventral digital and abdominal mucus in H. cinerea and O. septentrionalis, and between the ventral abdominal mucus of all four studied species. Conclusions: The presence of a digital mucus gland cluster in various anuran families, as well as the absence of differences in the mucus chemistry between arboreal and non-arboreal frog species indicate an adaptation towards generic functional requirements as well as to attachment-related requirements. Overall, this study contributes to the understanding of the role of glands and their secretions in tree frog attachment and in bioadhesion in general, as well as the evolution of anurans.

13.
Biomacromolecules ; 20(7): 2577-2586, 2019 Jul 08.
Artigo em Inglês | MEDLINE | ID: mdl-31244021

RESUMO

Clinically used bio-based tissue sealants bring in the risk of animal-borne infections, non-degradability, allergic reactions, tissue compression, tissue necrosis, and poor wet adhesion. Motivated by these unsatisfactory properties of existing tissue sealants, herein, we designed a library of solvent- and initiator-free hydrophobic mussel-inspired degradable tissue adhesives that can stick and seal the epidermis, pericardium, and Glisson's capsule under physiologically relevant wet conditions. By varying the molar ratio of the functional groups, we obtained polyester adhesive sealants with similar surface energy and varying viscosity. The careful examination of the wetting behavior of these polyester adhesive sealants on tissue surfaces showed that the polyester adhesive sealant with lower viscosity has higher intrinsic work of adhesion, which allowed them to adhere to strongly hydrated surfaces such as pericardium and Glisson's capsule. Because of the lower intrinsic work of adhesion, the polyester adhesive sealant with higher viscosity only adhered to the relatively hydrophobic surface (epidermis). The strong wet adhesion to tissue surfaces, cell-compatibility, hydrolytic degradability, and radical scavenging nature of these polyester adhesive sealants make them potential candidates for wound closure procedures.

14.
Integr Comp Biol ; 59(1): 203-213, 2019 07 01.
Artigo em Inglês | MEDLINE | ID: mdl-31065674

RESUMO

It has been nearly 20 years since Autumn and colleagues established the central role of van der Waals intermolecular forces in how geckos stick. Much has been discovered about the structure and function of fibrillar adhesives in geckos and other taxa, and substantial success has been achieved in translating natural models into bioinspired synthetic adhesives. Nevertheless, synthetics still cannot match the multidimensional performance observed in the natural gecko system that is simultaneously robust to dirt and water, resilient over thousands of cycles, and purportedly competent on surfaces that are rough at drastically different length scales. Apparent insensitivity of adhesion to variability in roughness is particularly interesting from both a theoretical and applied perspective. Progress on understanding the extent to which and the basis of how the gecko adhesive system is robust to variation in roughness is impeded by the complexity of quantifying roughness of natural surfaces and a dearth of data on free-ranging gecko substrate use. Here we review the main challenges in characterizing rough surfaces as they relate to collecting relevant estimates of variation in gecko adhesive performance across different substrates in their natural habitats. In response to these challenges, we propose a practical protocol (borrowing from thermal biophysical ecological methods) that will enable researchers to design detailed studies of structure-function relationships of the gecko fibrillar system. Employing such an approach will help provide specific hypotheses about how adhesive pad structure translates into a capacity for robust gecko adhesion across large variation in substrate roughness. Preliminary data we present on this approach suggest its promise in advancing the study of how geckos deal with roughness variation. We argue and outline how such data can help advance development of design parameters to improve bioinspired adhesives based on the gecko fibrillar system.


Assuntos
Lagartos/fisiologia , Locomoção/fisiologia , Adesividade , Animais , Propriedades de Superfície
15.
Langmuir ; 35(21): 7035-7042, 2019 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-31035758

RESUMO

Bacterial adhesion is described as a multistep process of interactions between microbes and the substrate, beginning with reversible contact, followed by irreversible adhesion. We explore the influence of substrate zeta potential on adhesion of Mycobacterium smegmatis, a nonpathogenic bacterial model for tuberculosis-causing Mycobacterium tuberculosis and a common foulant of reverse osmosis filtration systems. Substrates having a range of zeta potentials were prepared by coating silica with the polycation, poly(diallyldimethyl ammonium chloride) (pDADMAC), by adjusting the pH of alumina, a pH-responsive material, and by coating silica with a hydrophobic self-assembled monolayer coating of octadecyltrichlorosilane. Our observations using these surfaces demonstrated that adhesion of M. smegmatis increased significantly by more than 200% on the silica-pDADMAC system and more than 300% on alumina substrates, as zeta potential became less negative, and that the variation of pH did not affect adhesion on alumina surfaces. Live and heat-killed bacteria were studied to investigate the contribution of biological response to adhesion with respect to zeta potential. While approximately 60% fewer heat-killed M. smegmatis adhered to pDADMAC-coated silica substrates, the trend of significantly increasing adhesion with less negative zeta potential was still observed. These results show the influence of zeta potential on adhesion of M. smegmatis, which is a separate process from that of the biological response. Across the range of substrate surface chemistries, hydrophobicities, and zeta potentials tested, adhesion of M. smegmatis can primarily be controlled by zeta potential. The bacterial zeta potential was not changed by the various experimental conditions and was -28.3 ± 2.4 mV.

16.
ACS Appl Mater Interfaces ; 11(23): 21159-21165, 2019 Jun 12.
Artigo em Inglês | MEDLINE | ID: mdl-31094502

RESUMO

Nanostructured materials producing structural colors have great potential in replacing toxic metals or organic pigments. Electrophoretic deposition (EPD) is a promising method for producing these materials on a large scale, but it requires improvements in brightness, saturation, and mechanical stability. Herein, we use EPD assembly to codeposit silica (SiO2) particles with precursors of synthetic melanin, polydopamine (PDA), to produce mechanically robust, wide-angle structurally colored coatings. We use spectrophotometry to show that PDA precursors enhance the saturation of structural colors and nanoscratch testing to demonstrate that they stabilize particles within the EPD coatings. Stabilization by PDA precursors allows us to coat flexible substrates that can sustain bending stresses, opening an avenue for electroprinting on flexible materials.

17.
Phys Rev Lett ; 122(12): 128004, 2019 Mar 29.
Artigo em Inglês | MEDLINE | ID: mdl-30978075

RESUMO

The friction between a liquid swollen soft elastomer and a solid surface depends on the state of a confined liquid. To measure the physical state of the confined liquid, an interface-sensitive sum frequency generation spectroscopy technique was used to probe the contact region. We find that during sliding (friction) and pull-off (adhesion) experiments of pentadecane-swollen poly(dimethyl siloxane) lenses submerged in linear alkane (pentadecane) on a sapphire substrate, crystallinelike ordering of the liquid occurs only at the contact line, where we anticipate the highest shear. This crystallinelike structure of pentadecane molecules is transient and shows Arrhenius temperature dependence with unusually long relaxation times (hundreds of seconds) and an activation energy (50 kJ/mole), which is twice that of the bulk pentadecane liquid, at temperatures that are 14-70 °C higher than the bulk melting temperature (T_{m}=9 °C). This unusual long-lived crystallinelike ordering may explain why these systems show higher friction coefficients (boundary lubrication) compared to values predicted using bulk viscosity of pentadecane (hydrodynamic lubrication).

18.
Integr Comp Biol ; 59(1): 61-69, 2019 07 01.
Artigo em Inglês | MEDLINE | ID: mdl-30912804

RESUMO

The remarkable ability of geckos to adhere to a wide-variety of surfaces has served as an inspiration for hundreds of studies spanning the disciplines of biomechanics, functional morphology, ecology, evolution, materials science, chemistry, and physics. The multifunctional properties (e.g., self-cleaning, controlled releasability, reversibility) and adhesive performance of the gekkotan adhesive system have motivated researchers to design and fabricate gecko-inspired synthetic adhesives of various materials and properties. However, many challenges remain in our attempts to replicate the properties and performance of this complex, hierarchical fibrillar adhesive system, stemming from fundamental, but unanswered, questions about how fibrillar adhesion operates. Such questions involve the role of fibril morphology in adhesive performance and how the gekkotan adhesive apparatus is utilized in nature. Similar fibrillar adhesive systems have, however, evolved independently in two other lineages of lizards (anoles and skinks) and potentially provide alternate avenues for addressing these fundamental questions. Anoles are the most promising group because they have been the subject of intensive ecological and evolutionary study for several decades, are highly speciose, and indeed are advocated as squamate model organisms. Surprisingly, however, comparatively little is known about the morphology, performance, and properties of their convergently-evolved adhesive arrays. Although many researchers consider the performance of the adhesive system of Anolis lizards to be less accomplished than its gekkotan counterpart, we argue here that Anolis lizards are prime candidates for exploring the fundamentals of fibrillar adhesion. Studying the less complex morphology of the anoline adhesive system has the potential to enhance our understanding of fibril morphology and its relationship to the multifunctional performance of fibrillar adhesive systems. Furthermore, the abundance of existing data on the ecology and evolution of anoles provides an excellent framework for testing hypotheses about the influence of habitat microstructure on the performance, behavior, and evolution of lizards with subdigital adhesive pads.


Assuntos
Adesivos/química , Extremidades/fisiologia , Lagartos/fisiologia , Adesividade , Animais , Propriedades de Superfície
19.
Phys Chem Chem Phys ; 21(5): 2513-2518, 2019 Jan 30.
Artigo em Inglês | MEDLINE | ID: mdl-30656292

RESUMO

Quantification of interfacial composition and interfacial energy is essential for understanding prevalent phenomena such as purification and adhesion. However, for high-energy planar solid surfaces, traditional approaches for determining both parameters are inadequate. We take advantage of interface-sensitive spectroscopy to calculate the interfacial composition for acetone-chloroform, tetrahydrofuran-benzene, and N,N-dimethylformamide (DMF)-benzene mixtures. We calculate the differences in interfacial energy for the two components of each mixture from the adsorption isotherms and compare with that obtained from acid-base and dispersive interactions. The interfacial energy calculated using interfacial segregation agrees with the interfacial energy calculated by acid-base and dispersive interactions. The comparison illustrates how molecular interactions control macroscopic interfacial segregation. In all three mixtures, acid-base interactions dominate interfacial segregation. Comparing the two approaches for DMF-benzene mixtures leads to evidence of DMF dimerization in benzene. Using the present approach, the interfacial composition and interfacial energy can now be understood for interfacial behaviors including wetting and self-assembly.

20.
R Soc Open Sci ; 5(11): 181296, 2018 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-30564415

RESUMO

Orb webs produced by araneoid spiders depend upon aggregate glue-coated capture threads to retain their prey. Moths are challenging prey for most spiders because their scales detach and contaminate the glue droplets, significantly decreasing adhesion. Cyrtarachne are moth-specialist orb-weaving spiders whose capture threads adhere well to moths. We compare the adhesive properties and chemistry of Cyrtarachne aggregate glue to other orb-weaving spiders to test hypotheses about their structure, chemistry and performance that could explain the strength of Cyrtarachne glue. We show that the unusually large glue droplets on Cyrtarachne capture threads make them approximately 8 times more adhesive on glass substrate than capture threads from typical orb-weaving species, but Cyrtarachne adhesion is similar to that of other species after normalization by glue volume. Glue viscosity reversibly changes over 1000-fold in response to atmospheric humidity, and the adhesive strength of many species of orb spiders is maximized at a viscosity of approximately 105-106 cst where the contributions of spreading and bulk cohesion are optimized. By contrast, viscosity of Cyrtarachne aggregate glue droplets is approximately 1000 times lower at maximum adhesive humidity, likely facilitating rapid spreading across moth scales. Water uptake by glue droplets is controlled, in part, by hygroscopic low molecular weight compounds. NMR showed evidence that Cyrtarachne glue contains a variety of unknown low molecular weight compounds. These compounds may help explain how Cyrtarachne produces such exceptionally large and low viscosity glue droplets, and also why these glue droplets rapidly lose water volume after brief ageing or exposure to even slightly dry (e.g. < 80% RH) conditions, permanently reducing their adhesion. We hypothesize that the combination of large glue droplet size and low viscosity helps Cyrtarachne glue to penetrate the gaps between moth scales.

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