We have measured the melting and freezing behavior of linear alkanes confined between cross-linked poly(dimethylsiloxane) (PDMS) elastomers and solid sapphire substrates. Small molecules are often used as lubricants to reduce friction or as plasticizers, but very little is directly known about the migration or changes in physical properties of these small molecules at interfaces, particularly the changes in transition temperatures upon confinement. Our previous studies highlighted striking differences between the crystal structure of confined and unconfined pentadecane crystals in contact with sapphire substrates. Here, we have used surface-sensitive infrared–visible sum-frequency-generation spectroscopy (SFG) to study the melting temperatures (Tm) of alkanes in nanometer thick interfacial regions between swollen PDMS elastomers in contact with sapphire substrate. We find that confined alkanes show depression in Tm compared to the melting temperature of unconfined bulk alkanes. The depression in Tm is a function of chain length, and these differences were smallest for shorter alkanes and largest for 19 unit long alkanes. In comparison, the DSC results for swollen PDMS elastomer show a broad distribution of melting points corresponding to different sizes of crystals formed within the network. The Tm for confined alkanes has been modeled using the combination of Flory–Rehner and Gibbs–Thomson models, and the depression in Tm is related to the thickness of the confined alkanes. These findings have important implications in understanding friction and adhesion of soft elastomeric materials and also the effects of confinement between two solid materials.

The water/graphene interface has received considerable attention in the past decade due to its relevance in various potential applications including energy storage, sensing, desalination, and catalysis. Most of our knowledge about the interfacial water structure next to graphene stems from simulations, which use experimentally measured water contact angles (WCAs) on graphene (or graphite) to estimate the water–graphene interaction strength. However, the existence of a wide spectrum of reported WCAs on supported graphene and graphitic surfaces makes it difficult to interpret the water–graphene interactions. Here, we have used surface-sensitive infrared-visible sum frequency generation (SFG) spectroscopy to probe the interfacial water structure next to graphene supported on a sapphire substrate. In addition, the ice nucleation properties of graphene have been explored by performing in situ freezing experiments as graphitic surfaces are considered good ice nucleators. For graphene supported on sapphire, we observed a strong SFG peak associated with highly coordinated, ordered water next to graphene. Similar ordering was not detected next to bare sapphire, implying that the observed ordering of water molecules in the former case is a consequence of the presence of graphene. Our analysis indicates that graphene behaves like a hydrophobic (or negatively charged) surface, leading to enhanced ordering of water molecules. Although liquid water orders next to graphene, the ice formed is proton disordered. This research sheds light on water–graphene interactions relevant in optimizing the performance of graphene in various applications.Keywords: freezing; graphene; ice; sum frequency generation; water;

Structures of amphiphilic molecules at the liquid/solid and solid/solid interfaces are important in understanding lubrication, colloid stabilization, chromatography, and nucleation. Here, we have used interface-sensitive sum frequency generation (SFG) spectroscopy to characterize the interfacial structures of long-chain alcohols above and below the bulk melting temperature (Tm). The melting temperature of the ordered hexadecanol monolayer was measured to be around 30 °C above the bulk Tm, consistent with the transition temperature reported using X-ray reflectivity [ Phys. Rev. Lett. 2011, 106, 137801]. The disruption of hydrogen bonds between the sapphire and the alcohol hydroxyl groups was directly measured as a function of temperature. The strength of this hydrogen-bonding interaction, which explained the monolayer thermal stability above Tm, was calculated using the Badger–Bauer equation. Below Tm, the ordered self-assembled monolayer influenced the structure of the interfacial crystalline layer, and the transition from the ordered monolayer to the bulk crystalline phases (α rotator phase, β crystalline phase, and γ crystalline phase) resulted in packing frustrations at the interface.

AbstractRecognizing the potential for synthetic adhesives that can function in wet environments, elements of mussel foot proteins such as L-3,4-dihydroxyphenylalanine (DOPA) and phosphoserine have been incorporated into synthetic adhesives. Such adhesives have corroborated the advantage of surface active groups like DOPA, but have not yet demonstrated superior performance in wet or underwater environments, without using organic solvents. What has been conspicuously absent from such designs is the effect of hydrophobic components in the performance of underwater adhesives. Herein it is shown that incorporation of hydrophobic groups in low modulus polyester adhesives provides very high lap-shear strength and resistance to water penetration. In addition to the excellent performance in wet conditions, the designed adhesive can be applied underwater without any solvent, is biodegradable, and is designed from soybean oil, which is a readily available and renewable resource.

We demonstrate the surfactant-free production of polymer nanoparticles using a continuous membrane-based tangential flow cell. Co-current streams of water and polymethylmethacrylate (PMMA)/acetone/water solution were separated by a porous regenerated cellulose (RC) membrane. The water concentration in the PMMA solution was adjusted so that as additional water diffused through the RC membrane, the PMMA solution composition crossed the two phase boundary to precipitate PMMA nanoparticles. The size of these nanoparticles varied with the concentration of the PMMA feed and the amount of water diffusing across the membrane. The size distribution of PMMA particles produced in a continuous flow membrane cell was much narrower than those produced by drop-wise water addition or batch dialysis precipitation of PMMA particles. A continuous production of polymer nanoparticles of high purity and narrow polydispersity are important requirements for biomedical applications such as delivering therapeutics.Download high-res image (70KB)Download full-size image

Superhydrophobic surfaces are appealing as anti-icing surfaces, given their excellent water repellent performance. However, when water condenses on the surface due to high humidity, the water becomes pinned, and superhydrophobic surfaces fail to perform. Here we studied how the stability of the superhydrophobicity affected water condensation and frost formation. We created rough surfaces with the same surface structure, but with a variety of surface chemistries, and compared their antifrost properties as a function of intrinsic contact angle. Frost initiation was significantly delayed on surfaces with higher intrinsic contact angles. We coupled these macromeasurements with environmental scanning electron microscopy of water droplet initiation under high humidity conditions. These provide experimental evidence toward previous hypotheses that for a lower intrinsic-angle rough surface, Wenzel state is thermodynamically favorable, whereas the higher intrinsic-angle surface maintains a Cassie–Baxter state. Surfaces with a thermodynamically stable Cassie–Baxter state can then act both as antisteam and antifrost surfaces. This research could answer the persistent question of why superhydrophobic surfaces sometimes are not icephobic; anti-icing performance depends on the surface chemistry, which plays a critical role in the stability of the superhydrophobic surfaces.Keywords: freezing; hydrophobicity; ice; superhydrophobic; surface energy; surface modification; water;

Nature has evolved a fantastic gallery of structural colors, offering a source of inspiration for the development of artificial, multifunctional photonic devices. Inspired by the widespread use of melanin particles to produce structural colors in bird feathers, we recently demonstrated that synthetic melanin nanoparticles (SMNPs) offer the same rare, but critical combination of high refractive index and high absorption as natural melanin. Here, we show for the first time fast, significant, and reversible changes of structural coloration in self-assembled SMNP films in response to changes in humidity. This process is driven by the hygroscopic nature of the particles, leading to changes in the thickness of the SMNP layer that alter the interference color. This mechanistic explanation is supported by water absorption measurements, an optical model, and environmental scanning electron microscopy. Humidity-induced dynamic colors arising from SMNP films offer possible routes for synthetic melanin as an important material in sensors and coatings.

Previous studies have found that superhydrophobic surfaces are effective in delaying freezing of water droplets. However, the freezing process of water droplets on superhydrophobic surfaces depends on factors such as droplet size, surface area, roughness, and cooling rate. The role of surface energy, independent of any other parameters, in delaying freezing of water is not understood. Here, we have used infrared-visible sum frequency generation spectroscopy (SFG) to study the freezing of water next to solid substrates with water contact angles varying from 5° to 110°. We find that the freezing temperature of water decreases with increasing surface hydrophobicity only when the sample volume is small (∼10 μL). For a larger volume of water (∼300 μL), the freezing temperature is independent of surface energy. For water next to the surfaces with contact angle ≥54°, we observe a strong SFG peak associated with highly coordinated water. This research sheds new light on understanding the key factors in designing new anti-icing coatings.

Sum frequency generation spectroscopy (SFG) and attenuated-total-reflection IR (ATR-IR) were used to investigate polymer adsorption on solid surfaces in CCl4 (neutral), CHCl3 (acidic), and acetone (basic) solvents. Fowkes showed that the adsorbed amount of the polymer from acidic and basic solvents is less than that from a neutral solvent (Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 3–7). Here, we show that besides the differences in adsorbed amount, chains adsorbed from an acidic solvent adopted a flat conformation with a much smaller ratio of segments of loops and tails to trains compared to those adsorbed from a neutral solvent. Sapphire (Al2O3) surfaces were saturated by train segments at 1.3 × 10–5 volume fraction for both CCl4 and CHCl3 solutions, with a large fraction of the surface sites occupied by the PMMA segments, which was different from what was expected based on Fowkes’ experiment. In contrast, PMMA segments were not able to replace acetone molecules from the surface in a time period of 2 h. Surface interaction parameters alone were unable to predict the differences in conformation of chains adsorbed from acidic or neutral solvents.

We have conducted studies on the freezing of water molecules next to charged surfaces to elucidate the effect of water orientation on the structure of ice using sum frequency generation spectroscopy. We observed that when water is frozen next to a positively charged sapphire surface, the signal intensity of ice is higher than that of liquid water as expected from previous theoretical studies. However, when water is frozen next to a negatively charged sapphire surface (using NaOH as pH adjuster), the signal intensity decreases. The same signal attenuation upon freezing is obtained when cesium hydroxide (CsOH) and tetramethylammonium hydroxide (N(CH3)4OH) are used as pH adjusters. Since Na+, Cs+, and N(CH3)4+ ions have different hydration properties, the cation specific effect for this attenuation in signal intensity for ice is ruled out. Experiments on a mica surface (inherently negatively charged) also showed similar attenuation in signal intensity for ice as negatively charged sapphire surface. We conclude that the orientation of the water molecules next to a surface plays an important role in the structure of ice. These results have important implications in understanding the strength of ice nucleation and strength of ice adhesion next to charged surfaces.

The contact of two hydrophobic surfaces in water is of importance in biology, catalysis, material science, and geology. A tenet of hydrophobic attraction is the release of an ordered water layer, leading to a dry contact between two hydrophobic surfaces. Although the water-free contact has been inferred from numerous experimental and theoretical studies, this has not been directly measured. Here, we use surface sensitive sum frequency generation spectroscopy to directly probe the contact interface between hydrophobic poly(dimethylsiloxane) (PDMS) and two hydrophobic surfaces (a self-assembled monolayer, OTS, and a polymer coating, PVNODC). We show that the interfacial structures for OTS and PVNODC are identical in dry contact but that they differ dramatically in wet contact. In water, the PVNODC surface partially rearranges at grain boundaries, trapping water at the contact interface leading to a 50% reduction in adhesion energy compared to OTS–PDMS contact. The Young–Dupré equation, used extensively to calculate the thermodynamic work of adhesion, predicts no differences between the adhesion energy for these two hydrophobic surfaces, indicating a failure of this well-known equation when there is a heterogeneous contact. This study exemplifies the importance of interstitial water in controlling adhesion and wetting.

The aggregation of surfactants around oppositely charged polyelectrolytes brings about a peculiar bulk phase behavior of the complex, known as coacervation, and can control the extent of adsorption of the polyelectrolyte at an aqueous–solid interface. Adsorption kinetics from turbid premixed polyelectrolyte–surfactant mixtures have been difficult to measure using optical techniques such as ellipsometry and reflectometry, thus limiting the correlation between bulk phases and interfacial adsorption. Here, we investigated the adsorption from premixed solutions of a cationic polysaccharide (PQ10) and the anionic surfactant sodium dodecyl sulfate (SDS) on an amphoteric alumina surface using quartz crystal microbalance with dissipation (QCMD). The surface charge on the alumina was tuned by changing the pH of the premixed solutions, allowing us to assess the role of electrostatic interactions by studying the adsorption on both negatively and positively charged surfaces. We observed a maximum extent of adsorption on both negatively and positively charged surfaces from a solution corresponding to the maximum turbidity. Enhanced adsorption upon diluting the redissolved complexes at a high SDS concentration was seen only on the negatively charged surface, and not on the positively charged one, confirming the importance of electrostatic interactions in controlling the adsorption on a hydrophilic charged surface. Using the Voight based viscoelastic model, QCMD also provided information on the effective viscosity, effective shear modulus, and thickness of the adsorbed polymeric complex. The findings of viscoelastic analysis, corroborated by atomic force microscopy measurements, suggest that PQ10 by itself forms a flat, uniform layer, rigidly attached to the surface. The PQ10–SDS complex shows a heterogeneous surface structure, where the underlayer is relatively compact and tightly attached and the top is a loosely bound diffused overlayer, accounting for most of the adsorbate, which gets washed away upon rinsing. Understanding of the surface structure will have important implications toward understanding lubrication.

Adhesion in humid conditions is a fundamental challenge to both natural and synthetic adhesives. Yet, glue from most spider species becomes stickier as humidity increases. We find the adhesion of spider glue, from five diverse spider species, maximizes at very different humidities that matches their foraging habitats. By using high-speed imaging and spreading power law, we find that the glue viscosity varies over 5 orders of magnitude with humidity for each species, yet the viscosity at maximal adhesion for each species is nearly identical, 105–106 cP. Many natural systems take advantage of viscosity to improve functional response, but spider glue’s humidity responsiveness is a novel adaptation that makes the glue stickiest in each species’ preferred habitat. This tuning is achieved by a combination of proteins and hygroscopic organic salts that determines water uptake in the glue. We therefore anticipate that manipulation of polymer–salts interaction to control viscosity can provide a simple mechanism to design humidity responsive smart adhesives.Keywords: adhesion; aggregate protein; humidity; orb web; spider silk; viscid glue; viscosity;

Structural colors arising from interactions of light with submicron scale periodic structures have been found in many species across all taxa, serving multiple biological functions including sexual signaling, camouflage, and aposematism. Directly inspired by the extensive use of self-assembled melanosomes to produce colors in avian feathers, we set out to synthesize and assemble polydopamine-based synthetic melanin nanoparticles in an effort to fabricate colored films. We have quantitatively demonstrated that synthetic melanin nanoparticles have a high refractive index and broad absorption spanning across the UV–visible range, similar to natural melanins. Utilizing a thin-film interference model, we demonstrated the coloration mechanism of deposited films and showed that the unique optical properties of synthetic melanin nanoparticles provide advantages for structural colors over other polymeric nanoparticles (i.e., polystyrene colloidal particles).Keywords: bio-inspired; biomimicry; melanin; polydopamine; structural colors;

Infrared–visible sum frequency generation spectroscopy (SFG) was used to directly probe water between polyurethane (PU) and sapphire substrates after exposing samples to liquid water and water vapor. For liquid water, the observation of SFG peaks associated with H2O bands (3000–3400 cm–1) and D2O bands (2300–2600 cm–1) indicated water molecules diffused to the buried interface and existed in the form of a hydrogen-bonded water network. The water layer disrupted interactions between polyurethane and sapphire. When PU films were exposed to water vapor, the SFG peak intensities of PU hydrocarbon and sapphire hydroxyl groups changed significantly, which suggested water molecules had reached the interface. However, no hydrogen-bonded water bands were present; instead, the H2O peak at 3550 cm–1 and D2O peaks (2600–2700 cm–1) were observed. We assigned these peaks to low-coordination water molecules or hydroxyl groups hydrogen bonded with carboxyl groups of PU at the interface. The water molecules did not form a uniform layer at the interface and as a consequence did not completely disrupt the PU/sapphire interactions. These results provide important implications for understanding interfacial adhesion, coatings, and corrosion.

The chemical composition and molecular structure of polymeric surfaces are important in understanding wetting, adhesion, and friction. Here, we combine interface-sensitive sum frequency generation spectroscopy (SFG), all-atom molecular dynamics (MD) simulations, and ab initio calculations to understand the composition and the orientation of chemical groups on poly(methyl methacrylate) (PMMA) surface as a function of tacticity and temperature. The SFG spectral features for isotactic and syndiotactic PMMA surfaces are similar, and the dominant peak in the spectra corresponds to the ester-methyl groups. The SFG spectra for solid and melt states are very similar for both syndiotactic and isotactic PMMA. In comparison, the MD simulation results show that both the ester-methyl and the α-methyl groups of syndiotactic-PMMA are ordered and tilted toward the surface normal. For the isotactic-PMMA, the α-methyl groups are less ordered compared to their ester-methyl groups. The backbone methylene groups have a broad angular distribution and are disordered, independent of tacticity and temperature. We have compared the SFG results with theoretical spectra calculated using MD simulations and ab initio calculations. Our analysis shows that the weaker intensity of α-methyl groups in SFG spectra is due to a combination of smaller molecular hyperpolarizability, lower ordering, and lower surface number density. This work highlights the importance of combining SFG spectroscopy with MD simulations and ab initio calculations in understanding polymer surfaces.

The evolutionary origin of modern viscid silk orb webs from ancient cribellate silk ancestors is associated with a 95% increase in diversity of orb-weaving spiders, and their dominance as predators of flying insects, yet the transition’s mechanistic basis is an evolutionary puzzle. Ancient cribellate silk is a dry adhesive that functions through van der Waals interactions. Viscid threads adhere more effectively than cribellate threads because of the high extensibility of their axial silk fibers, recruitment of multiple glue droplets, and firm adhesion of the viscid glue droplets. Viscid silk’s extensibility is permitted by the glue’s high water content, so that organic and inorganic salts present in viscid glue droplets play an essential role in contributing to adhesion by sequestering the atmospheric water that plasticizes the axial silk fibers. Here, we provide direct molecular and macro-scale evidence to show that salts also cause adhesion by directly solvating the glycoproteins, regardless of water content, thus imparting viscoelasticity and allowing the glue droplets to establish good contact. This “dual role” of salts, plasticizing the axial silk indirectly through water sequestration and directly solvating the glycoproteins, provides a crucial link to the evolutionary transition from cribellate silk to viscid silk. In addition, salts also provide a simple mechanism for adhering even at the extremes of relative humidity, a feat eluding most synthetic adhesives.

Ice formation next to solid surfaces is important in many biological, materials, and geological phenomena and may be a factor in how they impact various technologies. We have used sum frequency generation (SFG) spectroscopy to study the structure of ice as well as the freezing and melting transition temperatures of water in contact with sapphire substrates. We have observed that the structure of ice and water are a function of pH and the surface charge of the sapphire substrate. At low pH, we observed an increase in the SFG signal subsequent to ice formation. Contrary to expectations, at pH 9.8, corresponding to a negatively charged surface, the intensity of the ice SFG signal is about 10 times lower than that of water. Recent simulation studies have suggested that charge transfer is important for the high intensity of the ice peak at the ice–air interface. We believe that the segregation of sodium ions next to the negatively charged sapphire substrate may be responsible for disrupting the charge transfer and stitching bilayer at high pH, providing a plausible explanation for the experimental observations. Even though the structure of water and ice are affected by pH, the freezing and melting transition temperatures are independent of the surface charge. This report offers a unique insight on how ions next to solid surfaces could influence the structure of ice.

Understanding the freezing of salt solutions near solid surfaces is important in many scientific fields. Here we use sum frequency generation (SFG) spectroscopy to study the freezing of a NaCl solution next to a sapphire substrate. During cooling we observe two transitions. The first corresponds to segregation of concentrated brine next to the sapphire surface as we cool the system down to the region where ice and brine phases coexist. At this transition, the intensity of the ice-like peak decreases, suggesting the disruption of hydrogen-bonding by sodium ions. The second transition corresponds to the formation of NaCl hydrates with abrupt changes in both the SFG intensity and the sharpness of spectral peaks. The similarity in the position of the SFG peaks with those observed using IR and Raman spectroscopy indicates the formation of NaCl·2H2O crystals next to the sapphire substrate. The melting temperatures of the hydrates are very similar to those reported for bulk NaCl·2H2O. This study enhances our understanding of nucleation and freezing of salt solutions on solid surfaces and the effects of salt ions on the structure of interfacial ice.

A control over wetting properties of a surface can be achieved by tuning surface roughness and surface chemistry. In this study, we formed single layer roughness and dual levels of hierarchical roughness with hexagonal non-contiguously close packed (HNCP) patterns of spherical particles using colloidal lithography. Surface chemistry was controlled using plasma-enhanced chemical vapour deposition (PECVD). A hexagonal unit cell model, which is representative of the HNCP pattern, was used to predict the contact angles. The predictions of this model were in good agreement with experimentally measured contact angles and also provided thermodynamic stability of different wetting state. The systematic thermodynamic analysis of wetting properties is important when using structured surfaces at different hydrostatic pressures, relative humidity, temperature fluctuations or prolonged exposure to water.

Exciton delocalization has been proposed to have a strong impact on the performance of organic solar cells. For example, large exciton delocalization estimates have promoted the theory of long-range charge transfer as a mechanism for efficient charge separation. Here, two new computational modeling techniques for analyzing femtosecond transient absorption spectroscopy experiments are developed in order to estimate the magnitude of exciton delocalization in semiconducting polymers. The developed techniques are then used to analyze previously published experimental data for regioregular poly(3-hexylthiophene) (P3HT). Based on modeling both the exciton–exciton annihilation behavior in a pure P3HT film and the exciton dissociation dynamics in a P3HT:PCBM blend film, the exciton delocalization radius in regioregular P3HT is estimated to be in the range of 1–2 nm, which is significantly smaller than estimated in a number of previous studies. These results suggest that exciton delocalization is not likely to be a significant contributing factor to efficient charge separation.

We have measured the melting and freezing behavior of linear alkanes confined between cross-linked poly(dimethylsiloxane) (PDMS) elastomers and solid sapphire substrates. Small molecules are often used as lubricants to reduce friction or as plasticizers, but very little is directly known about the migration or changes in physical properties of these small molecules at interfaces, particularly the changes in transition temperatures upon confinement. Our previous studies highlighted striking differences between the crystal structure of confined and unconfined pentadecane crystals in contact with sapphire substrates. Here, we have used surface-sensitive infrared–visible sum-frequency-generation spectroscopy (SFG) to study the melting temperatures (Tm) of alkanes in nanometer thick interfacial regions between swollen PDMS elastomers in contact with sapphire substrate. We find that confined alkanes show depression in Tm compared to the melting temperature of unconfined bulk alkanes. The depression in Tm is a function of chain length, and these differences were smallest for shorter alkanes and largest for 19 unit long alkanes. In comparison, the DSC results for swollen PDMS elastomer show a broad distribution of melting points corresponding to different sizes of crystals formed within the network. The Tm for confined alkanes has been modeled using the combination of Flory–Rehner and Gibbs–Thomson models, and the depression in Tm is related to the thickness of the confined alkanes. These findings have important implications in understanding friction and adhesion of soft elastomeric materials and also the effects of confinement between two solid materials.

Infrared-visible sum frequency generation spectroscopy (SFG) was used to measure the interfacial concentrations of poly(methyl methacrylate) (PMMA)/polystyrene (PS) blends next to a sapphire substrate. The acid–base interactions of carbonyl groups of PMMA with the hydroxyl groups on the sapphire drive the interfacial segregation of PMMA next to the sapphire substrate. Using the shift of sapphire surface OH peaks, we have determined the difference in interfacial energy between the PMMA/sapphire and the PS/sapphire to be ∼44–45 mJ/m2. These results highlight the importance of acid–base interactions and their role in controlling the interfacial segregation next to solid substrates in polymer blends.

We employ the adhesive web building strategy used by modern orb-weaving spiders to produce functional microthreads that are similar in structure (beads-on-a-string (BOAS) morphology) and adhesive properties to the capture-silk threads of the spider web. The diameter and spacing of droplets (beads) are controlled by varying the viscosity, velocity, and surface tension of the coating fluid. Using these functional threads, we also describe the behavior of the BOAS morphology during contact (mimicking the collision of an insect with the web) and during separation (mimicking insect rescue from the web). Our results show that the BOAS structure performs better than a cylindrical structure for adhesion, which may explain why this morphology is so prevalent in spider webs despite the cost of increasing the visibility of the web.

We have studied the interface between hexadecane droplets and sapphire substrates in water using infrared–visible sum frequency generation spectroscopy (SFG). At high pH and above the isoelectric point of the sapphire substrate, the hexadecane drop is repelled due to electrostatic forces. The SFG measurements are consistent with the observation that a thick layer of water is present between the oil and the sapphire substrate. Below the isoelectric point of the sapphire substrate, the hexadecane drops stick to the sapphire surface. Surprisingly, the SFG results show the presence of a thin layer of water between hexadecane drop and the sapphire substrate. At this contact interface, we observe contributions to the SFG signal from both the hexadecane/water and water/sapphire interfaces. The reasons for the presence of a thin water layer with adhesive contact can be explained due to weaker repulsive double layer and the attractive van der Waals interactions.

The wetting behavior of a surface under steam condensation depends on its intrinsic wettability and micrometer or nanoscale surface roughness. A typical superhydrophobic surface may not be suitable as a steamphobic surface because of the nucleation and growth of water inside the valleys and thus the failure to form an air–liquid–solid composite interface. Here, we present the results of steam condensation on chemically modified nanostructured carbon nanotube (CNT) mats. We used a plasma-enhanced chemical vapor deposition (PECVD) process to modify the intrinsic wettability of nanostructured CNT mats. The combination of low surface energy achieved by PECVD and the nanoroughness of the surface provides a mechanism to retain the superhydrophobicity of the CNT mats under steam condensation. The ability to withstand steam temperature and pressure for as long as 10 h implies the remarkably improved stability of the superhydrophobic state of the surface. The thermodynamic calculations carried out using a unit cell model clearly explain the steamphobic wetting behavior of the surface.

Infrared-visible sum frequency generation spectroscopy (SFG) in conjunction with total internal reflection geometry (TIR) has been demonstrated as a powerful technique to study buried polymer interfaces. We have developed a theoretical model using linear and nonlinear boundary conditions to calculate the SFG signals as a function of incident angles and thickness of the polymer films. The validity of this model is tested using a polystyrene film (PS) coated on a sapphire prism. This PS film is exposed to heneicosane (C21H44) above and below its melting temperature. At temperatures greater than Tm, the SFG contributions from both interfaces (PS/sapphire and alkane/PS) are comparable and we observe strong interference effects. At temperatures below Tm, the SFG signals are dominated by the methyl signals of all-trans heneicosane molecules at the alkane/PS interface. The theoretical model is able to accurately capture the angle and thickness dependence of the SFG signal and provides a valuable tool to accurately determine the interference effects in multilayer samples using SFG in total internal reflection geometry. The model also provides physical parameters (i.e., film thickness, incident angle and substrate index of refraction) needed to suppress or enhance SFG signals generated at a particular interface.

The adhesion and friction behavior of soft materials, including compliant brushes and hairs, depends on the temporal and spatial evolution of the interfaces in contact. For compliant nanofibrous materials, the actual contact area individual fibers make with surfaces depends on the preload applied upon contact. Using in situ microscopy observations of preloaded nanotube hairs, we show how nanotubes make cooperative contact with a surface by buckling and conforming to the surface topography. The overall adhesion of compliant nanohairs increases with increasing preload as nanotubes deform and continuously add new side-wall contacts with the surface. Electrical resistance measurements indicate significant hysteresis in the relative contact area. Contact area increases with preload (or stress) and decreases suddenly during unloading, consistent with strong adhesion observed for these complaint nanohairs.

We have studied acid−base interactions at solid−liquid and solid−solid interfaces using interface-sensitive sum frequency generation (SFG) spectroscopy. The shift of the sapphire hydroxyl peak in contact with several polar and nonpolar liquids and polymers was used to determine the interaction energy. The trend in the interaction energies cannot be explained by measuring only water contact angles. Molecular rearrangements at the sapphire interface, to maximize the interaction of the acid−base groups, play a dominant role, and these effects are not accounted for in the current theoretical models. These results provide important insights into understanding adhesion, friction, and wetting on solid interfaces.

We have observed an unusual increase in adhesion hysteresis and frictional forces for poly(dimethylsiloxane) (PDMS) lenses sliding on smooth glassy surfaces after a period of aging in a laboratory environment. X-ray photoelectron spectroscopy, contact angle, and in situ surface-sensitive sum frequency generation spectroscopy (SFG) measurements show no differences between an aged and unaged lens, indicating that these changes in tribological properties cannot be due to surface contaminant or degradation. Instead, we observed that the SFG intensity of the PDMS Si−CH3 symmetric band is 3 orders of magnitude higher after sliding the aged lenses. Such a large increase in the SFG signal can only arise from very well-ordered PDMS molecules induced by sliding and has important consequences in understanding adhesion hysteresis and friction.

We report the synthesis of superhydrophobic coatings for steel using carbon nanotube (CNT)-mesh structures. The CNT coating maintains its structural integrity and superhydrophobicity even after exposure to extreme thermal stresses and has excellent thermal and electrical properties. The coating can also be reinforced by optimally impregnating the CNT-mesh structure with cross-linked polymers without significantly compromising on superhydrophobicity and electrical conductivity. These superhydrophobic conductive coatings on steel, which is an important structural material, open up possibilities for many new applications in the areas of heat transfer, solar panels, transport of fluids, nonwetting and nonfouling surfaces, temperature resilient coatings, composites, water-walking robots, and naval applications.

We report on the first in-situ sum frequency generation (SFG) spectroscopy characterization of polyacrylonitrile (PAN) interfacial interactions with air, sapphire, water, and heptane. Using the shift in the resonance frequency of CN at various interfaces, we demonstrated that PAN interacts with the surface hydroxyl of sapphire substrate through the lone pair orbital of nitrogen in the “end-on” configuration (σ-H bond). We also demonstrated that the CN−CN interaction is the main reason for the superior chemical resistance property of PAN. At room temperature the interaction between the polymer chains is much stronger than the interaction between the polymer and solvent molecules including water and heptane. At high temperatures, however, the interaction between the nitrile groups of the polymer weakens, making interaction possible between the nitrile groups and the surface hydroxyls of the substrate and water. These results provide an important insight as to why acrylonitrile when copolymerized with butadiene to form nitrile rubber results in one of the best known synthetic oil-resistant rubber.

Infrared-visible sum-frequency-generation spectroscopy (SFG) is used to investigate the interfacial structure of hexadecanol (C16H33OH) and heneicosane (C21H44) in contact with polystyrene films (PS) spin coated on a sapphire substrate. The interfacial structure of hexadecanol is very different from heneicosane in contact with PS. In the crystalline state, the hexadecanol molecules are oriented with the C−C−C axis parallel to the surface plane in contact with PS. For the crystalline heneicosane/PS interface, the SFG spectra are very similar to those observed for molecules oriented with the symmetry axis of the methyl groups parallel to the surface normal. The structure of both hexadecanol (or heneicosane) and the phenyl groups changes sharply at the melting temperature of hexadecanol (or heneicosane). Upon heating the hexadecanol/PS sample above the glass transition temperature (Tg) of PS, the hexadecanol molecules penetrate through the PS film and adsorb on the sapphire substrate. The adsorbed hexadecanol molecules are oriented with the symmetry axis of the methyl groups parallel to the surface normal. The structure of the PS molecules at the sapphire interface is different because the PS phenyl groups are now in contact with the hydrophobic tails of the hexadecanol molecules, rather than the hydrophilic sapphire substrate. The adsorbed hexadecanol molecules do not disorder at the bulk melting temperature of hexadecanol. In comparison, no adsorption of heneicosane molecules next to sapphire interface upon annealing was observed. The differences between the adsorption of hexadecanol and heneicosane can be explained by the preferential interactions between the hydroxyl groups of the alcohol and hydrophilic sapphire substrate.

The design of reversible adhesives requires both stickiness and the ability to remain clean from dust and other contaminants. Inspired by gecko feet, we demonstrate the self-cleaning ability of carbon nanotube-based flexible gecko tapes.

We have developed a synthetic gecko tape by transferring micropatterned carbon nanotube arrays onto flexible polymer tape based on the hierarchical structure found on the foot of a gecko lizard. The gecko tape can support a shear stress (36 N/cm2) nearly four times higher than the gecko foot and sticks to a variety of surfaces, including Teflon. Both the micrometer-size setae (replicated by nanotube bundles) and nanometer-size spatulas (individual nanotubes) are necessary to achieve macroscopic shear adhesion and to translate the weak van der Waals interactions into high shear forces. We have demonstrated for the first time a macroscopic flexible patch that can be used repeatedly with peeling and adhesive properties better than the natural gecko foot. The carbon nanotube-based tape offers an excellent synthetic option as a dry conductive reversible adhesive in microelectronics, robotics, and space applications.

We report a fabrication process for constructing polymer surfaces with multiwalled carbon nanotube hairs, with strong nanometer-level adhesion forces that are 200 times higher than those observed for gecko foot-hairs.

Considering the importance of salt and water on earth, the crystallization of salt hydrates next to solid surfaces has important implications in physical and biological sciences. Heterogeneous nucleation is driven by surface interactions, but our understanding of hydrate formation near surfaces is limited. Here, we have studied the hydrate formation of three commonly prevalent salts, MgCl2, CaCl2, and NaCl, next to a sapphire substrate using surface sensitive infrared-visible sum frequency generation (SFG) spectroscopy. SFG spectroscopy can detect the crystallization and melting of salt hydrates at the interface by observing the changes in the intensity and the location of the cocrystallized water hydroxyl peaks (3200–3600 cm–1). The results indicate that the surface crystal structures of these three hydrates are similar to those in the bulk. For the NaCl solution, the brine solution is segregated next to the sapphire substrate after the formation of the ice phase. In contrast, the MgCl2 and CaCl2 surface hydrate crystals are interdispersed with nanometer-size ice crystals. The nanosize ice crystals melt at much lower temperatures than bulk ice crystals. For NaCl and MgCl2 solution, the NaCl hydrates prefer to crystallize next to the sapphire substrate instead of the ice crystals and MgCl2 hydrates.