Ping Yuan Hsu
Published: 2011
Total Pages: 197
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The molecular structures of soft or fluid-like surfaces during contact in aqueous media play an important role in understanding adhesion and wetting between colloidal and biological interfaces. For example, it has been suggested that the presence of bound water (hydration layer) is crucial in controlling the fusion of lipid bilayers and the adsorption of proteins to bio-materials. Interfacial force measurements have revealed the importance of interfacial molecular structures on the viscosity, lubricity and adhesion acting between two surfaces. However, force measurements cannot provide direct information of the molecular structure after contact. Due to the limitations of experimental techniques, the understanding about the molecular structures of soft contact interfaces is limited. In this dissertation, we have developed an experimental approach to study the contact interface between a liquid and a solid substrate in an environment where they are surrounded by water. The surface sensitive infrared-visible sum frequency generation spectroscopy (SFG) provides information on the chemical groups, concentration and orientation of the molecules at the interface. We have studied the interface between hexadecane and sapphire surface using this technique. The adhesion between hexadecane droplets and the sapphire surface are determined by pH and the isoelectric point of sapphire substrate. Also, the SFG results suggest that the oil does not come in direct contact with the sapphire surface but is separated by a thin layer of water, even though the oil droplet sticks to the sapphire surface. The presence of the surfactant generates heterogeneous patchy contact between the oil and the sapphire, where the methyl groups of hexadecane are in direct contact with the surface hydroxyl groups of the sapphire surface. We have also used this design to study the contact interface between surfactant (stearyl trimethyl ammonium bromide, STAB) monolayers to mimic lipid bilayer contact. We have taken advantage of the adsorption of STAB on polystyrene and on hexadecane to create a contact interface with surfactant molecules on both sides. At conditions when both the surfaces were saturated with the surfactant molecules, it was impossible to drain the water away and the spectral signature of water did not change. This indicated that the double layer forces were strong enough to prevent any drainage of water at the fluid-like interface. In addition, the structure of water remained the same which is consistent with the expectations from force measurements that water structure is only affected under confinement and between two rigid and flat substrates. We also studied soft contact interface between elastomeric poly-dimethyl siloxane lenses and sapphire in water by using SFG. The confined spectra showed peaks related to PDMS as well as water, suggesting formation of water puddles in the contact area. The presence of the peak at 3690 cm-1 suggests the contact of surface hydroxyl groups with PDMS, supporting our hypothesis that the contact is heterogeneous. This heterogenous picture provides insight into the higher friction for a rubber sliding on sapphire surface in the presence of water. By using the established experimental protocols of SFG and the matrix free nanoassisted laser desorption-ionization (NALDI) mass spectroscopy, the actual adhesive contact interface between the soft gecko toe pad and the sapphire substrate was determined. A gecko's stickiness derives from van der Waals interactions between proteinaceous hairs called setae and the substrate. However, the molecular structure of the immediate contact at the adhesive interface is unknown. The SFG experiments demonstrate that there is a high representation of C-H bonds at the interface during gecko/sapphire contact, but the signatures of O-H bonding (e.g. water) and aromatic groups (e.g. amino acids/proteins) are entirely absent. Our discovery and analysis of gecko footprints have led to a surprising finding that geckos left behind a distinct trace of phospholipid molecules, a material that has never been considered in papers that deal with gecko adhesion. Particularly interesting ramifications include the previously unexplained sensitivity of gecko shear adhesion to variation in humidity, and the observation that setae show little if any wear. In the former case we find that an increase in the surface exposure of methylene groups is correlated with exposure of setae to water. In the latter case, it may be that sacrificial lipid-like molecules prevent damage to the rigid setae made of [Beta]-keratin. Our analysis of gecko footprints and the toe pad/substrate interface has significant consequences for models of gecko adhesion and by extension, the design of synthetic mimics.
