Adhesion in humid environments is fundamentally challenging because of the presence of interfacial bound water. Spiders often hunt in wet habitats and overcome this challenge using sticky aggregate glue droplets whose adhesion is resistant to interfacial failure under humid conditions. The mechanism by which spider aggregate glue avoids interfacial failure in humid environments is still unknown. Here, we investigate the mechanism of aggregate glue adhesion by using interface-sensitive spectroscopy in conjunction with infrared spectroscopy. We demonstrate that glycoproteins act as primary binding agents at the interface. As humidity increases, we observe reversible changes in the interfacial secondary structure of glycoproteins. Surprisingly, we do not observe liquid-like water at the interface, even though liquid-like water increases inside the bulk with increasing humidity. We hypothesize that the hygroscopic compounds in aggregate glue sequester interfacial water. Using hygroscopic compounds to sequester interfacial water provides a novel design principle for developing water-resistant synthetic adhesives.

Surface-sensitive spectroscopy and contact mechanics reveal ice-like confined water between surfactant-covered charged surfaces.

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Sum frequency generation spectroscopy (SFG) and attenuated-total-reflection IR (ATR-IR) were used to investigate polymer adsorption on solid surfaces in CCl 4 (neutral), CHCl 3 (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 (Al 2 O 3 ) surfaces were saturated by train segments at 1.3 × 10 −5 volume fraction for both CCl 4 and CHCl 3 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 address the question of how a surface of a glassy polymer reorganizes after coming in contact with water. Because contact angle hysteresis measurements are also affected by surface roughness and chemical heterogeneity, we have used surface-sensitive sum frequency generation spectroscopy (SFG) in conjunction with water contact angles to answer this question. To increase the magnitude of the surface reorganization, we have designed an amphiphilic polymer, poly­(α-hydroxymethyl-n-butyl acrylate) (PHNB), to study the changes in the structure of polar hydroxy groups and nonpolar (methyl and methylene) groups at the interface. The SFG and the water contact angles show that reorganization does occur for PHNB below T g. However, complete reorganization requires heating the sample above the bulk T g. These heating experiments were conducted by first heating the sample in the presence of water and then followed by cooling the sample to room temperature in the presence of water to lock the changes in the surface structure (we refer to this treatment as water annealing). The polar contribution to the total surface energy of PHNB, determined by Owens–Wendt–Rabel–Kaelble (OWRK) method at room temperature, increases after water annealing above T g. This is consistent with our SFG results that show an increase in concentration of polar hydroxy groups at room temperature after water annealing the PHNB film above T g. For PHNB, the contact angle hysteresis is higher for samples that are water annealed above T g. This is consistent with the surface energy and SFG results. For a low-T g polymer, poly­(n-butyl acrylate), which has the same nonpolar side group but lacks the hydroxyl group, surface reorganization takes place immediately after contact with water, and these changes are reversible.

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.

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Spider aggregate glue can absorb moisture from the atmosphere to reduce its viscosity and become tacky. The viscosity at which glue adhesion is maximized is remarkably similar across spider species, even though that viscosity is achieved at very different relative humidity (RH) values matching their diverse habitats. However, the molecular changes in the protein structure and the bonding state of water (both referred to here as molecular structure) with respect to the changes in RH are not known. We use attenuated total reflectance-infrared (ATR-IR) spectroscopy to probe the changes in the molecular structure of glue as a function of RH for three spider species from different habitats. We find that the glue retains bound water at lower RH and absorbs liquid-like water at higher RH. The absorption of liquid-like water at high RH plasticizes the glue and explains the decrease in glue viscosity. The changes to protein conformations as a function RH are either subtle or not detectable by IR spectroscopy. Importantly, the molecular changes are reversible over multiple cycles of RH change. Further, separation of glue constituents results in a different humidity response as compared to pristine glue, supporting the standing hypothesis that the glue constituents have a synergistic association that makes spider glue a functional adhesive. The results presented in this study provide further insights into the mechanism of the humidity-responsive adhesion of spider glue.