Does the Roll-off Angle Depend on Work of Adhesion?

The roll-off angle of hexadecane droplets was measured on a family of nearly ideal surfaces having similar structures but different interfacial free energies. The variation in interfacial energy was reflected in the contact angles of hexadecane, which provide a measure of the work of adhesion. The hysteresis in the contact angle on these surfaces was low and approximately constant, thereby approximately removing it as a variable and allowing for an assessment of any dependence of the roll-off angle on work of adhesion directly. The results revealed no such dependence, consistent with the descriptions of early researchers in this area.

Byproduct-Free Route to Aminosiloxane Monolayers on Silicon/Silicon Dioxide.

The chemisorption of N-methyl-aza-2,2,4-trimethylsilacyclopentane from either the solution or the vapor phase produces monolayer films on silicon (oxide) substrates. The formation of a covalent siloxane linkage to the surface by this adsorbate is accompanied by ring opening, which produces no byproduct. The resulting secondary amine reacts with maleic anhydride to produce a carboxylic acid-terminated surface, accompanied by the formation of a stable amide bond. These reactions and their products were characterized by a combination of optical ellipsometry, contact-angle goniometry, and X-ray photoelectron spectroscopy.

Electrokinetics of polar liquids in contact with nonpolar surfaces.

Zeta potentials of several polar protic (water, ethylene glycol, and formamide) as well as polar aprotic (dimethyl sulfoxide) liquids were measured in contact with three nonpolar surfaces using closed-cell electroosmosis. The test surfaces were chemisorbed monolayers of alkyl siloxanes, fluoroalkyl siloxanes, and polydimethylsiloxanes (PDMS) grafted on glass slides. All these liquids exhibited substantial electrokinetics in contact with the nonpolar surfaces with these observations: the electrokinetic effect on the fluorocarbon-coated surface is the strongest and on a PDMS grafted surface, the effect is the weakest. Even though these hygroscopic liquids contain small amounts of water, the current models of charging based on the adsorption of hydroxide ions at the interface or the dissociation of pre-existing functionalities (e.g., silanol groups) appear to be insufficient to account for the various facets of the experimental observations. The results illustrate how ubiquitous the phenomenon of electrokinetics is with polar liquids contacting such apparently passive nonpolar surfaces. We hope that these results will inspire further experimental and theoretical studies in this important area of research that has potential practical implications.

Spatially selective formation of hydrocarbon, fluorocarbon, and hydroxyl-terminated monolayers on a microelectrode array.

A protection-deprotection strategy, using gold oxide as a passivating layer, was used to direct the self-assembly of monolayers (SAMs) selectively at individual gold microelectrodes in an array. This approach allowed the formation of hydroxyl-terminated monolayers, without side reactions, in addition to hydrocarbon and fluorocarbon SAMs. Fluorescence microscopy was used to visualize selective dewetting of hydrophobic monolayers by an aqueous dye solution, and spatially resolved X-ray photoelectron spectroscopy was used to demonstrate a lack of cross-contamination on neighboring microelectrodes in the array.

Gold oxide as a protecting group for regioselective surface chemistry.

Selective modification of electrode surfaces is a vital step in the development of many practical applications of self-assembled monolayers (SAMs). This paper describes a protection-deprotection strategy similar to that commonly utilized in organic synthesis, with gold oxide as a protecting layer, to direct self-assembly on one gold electrode in the presence of another.

Mechanism of spontaneous formation of monolayers on gold from alkyl thiosulfates.

The kinetics of adsorption of self-assembled monolayers (SAMs) from solutions of hexadecyl thiosulfate in tetrahydrofuran was studied under a variety of experimental conditions to elucidate the mechanism(s) important in this process. Monolayers did not form in the absence of water, which ruled out the direct addition of the alkyl thiosulfate to the gold surface and implicated hydrolysis as a route to surface-active adsorbates. The role of tetrafluoroborate ion, known to inhibit SAM formation in this system, was also examined. These studies, too, were consistent with hydrolysis as an intermediate step in the formation of monolayer films in this system.

Direct route to well-defined, chemically diverse electrode arrays.

The selective placement of molecules of interest at specific locations on surfaces is a keystone for the bridge between interfacial science and technology. One approach to this problem is the use of electrochemistry to direct interfacial reactions that immobilize species from solution onto surfaces. In this study, sets of individually functionalized gold electrodes were formed by the selective formation of monolayers from four different alkyl thiosulfates. Analysis of the arrays using spatially resolved X-ray photoelectron spectroscopy (XPS) revealed each type of functionality exclusively on the electrode to which it was directed. The wetting behavior of these surfaces was also consistent with homogeneous monolayers placed selectively on each electrode. The flexibility of this method provides the ability to produce a wide variety of chemical patterns at interfaces of interest for a range of technological applications.

Compatibility of omega-functionality in the electrochemically directed self-assembly of monolayers on gold from alkyl thiosulfates.

Self-assembled monolayers were formed on gold electrochemically from omega-functionalized alkyl thiosulfates (Bunte salts). The resulting SAMs were characterized using X-ray photoelectron spectroscopy (XPS), contact-angle goniometry, and ellipsometry. A range of terminal functionality was examined, including CH(3), perfluoroalkyl, CO(2)H, CO(2)CH(3), CONH(2), CH(2)OH, and vinyl groups. Side-reactions involving some of these functional groups were consistent with intermediates proposed in our earlier publications and begin to define the scope of this method for building chemical structures at interfaces.

Temperature-actuated changes in wettability at elastomer/water interfaces.

Surface modification of 1,4-polybutadiene and cis-1,4-polyisoprene to introduce polar functional groups provided surfaces that reconstructed reversibly against water as a function of temperature. These surfaces became hydrophobic in contact with hot water, but their original hydrophilicity returned upon equilibration against cold water. Repeated cycling between hot and cold water, however, led to a damping of this reversibility. A series of parallel experiments on both the interfacial and bulk behavior of these elastomers strongly indicated that this damping was due to the alignment of extended interfacial chains during temperature cycling and to a decay of the restoring force on the interfacial chains under extension. These studies thus demonstrate that the interfacial behavior of elastomers can display close analogies to the bulk viscoelastic properties of the solid.

A method for imaging single clay platelets by scanning electron microscopy.

A method for preparing and observing clay platelets for size and shape analysis using scanning electron microscopy (SEM) was developed. Samples of the clay platelets were prepared by polyelectrolyte-assisted adsorption onto a pyrolytic graphite surface. The use of graphite as a substrate was advantageous because of the low number of secondary electrons emitted from it during imaging by SEM. The resulting low background noise allowed the emission from the approximately 1 nm thick clay sheets to be clearly visualized. Images of centrifuged montmorillonite showed large exfoliated platelets with lateral dimensions between 200 and 600 nm. In contrast, uncentrifuged montmorillonite appeared to contain a large amount of unexfoliated clusters. Although it was not possible to obtain high-quality images of the smaller sheets of Laponite RD, the images of this material did contain size features comparable to the approximately 30 nm2 size reported previously using light scattering, as well as transmission electron and atomic force microscopies.

Preparation of thin silica films with controlled thickness and tunable refractive index.

Silica films with controlled thickness and refractive index have been formed by the sequential adsorption of a cationic polyelectrolyte and silica sols. The conditions used to prepare the sol were varied, and allowed films with refractive indices as low as 1.16 to be obtained. The sequential adsorption technique allows the thickness of these films to be controlled in increments of 5-10 nm, depending on the desired refractive index. Scanning electron microscopy revealed that a low packing density of constituent silica particles was responsible for the low indices of these films. The as-adsorbed films are thermally robust; calcination at 500 degrees C resulted in only very small decreases in film thickness (by < or =1.8%) and refractive index (to as low as 1.14). After calcination, the silica films remained hydrophilic and sorbed water vapor from the atmosphere. As a result, the refractive indices of these films increased with increasing relative humidity (RH). The dependence of the refractive index on RH was eliminated by treating the calcined films with trimethylchlorosilane.

A smart adhesive joint: entropic control of adhesion at a polymer/metal interface.

The study of adhesion has a long and rich history, with theory, experiments, and applications bridging numerous disciplines, including physics, chemistry, engineering, and medicine. This diverse interest has led to the development of a large number of methods for both enhancing and inhibiting adhesion at specific interfaces of interest. We report herein "smart" adhesion at a polymer/metal (oxide) interface that responds reversibly to changes in temperature by increasing or decreasing in magnitude. The temperature dependence in this system arises from the rubber elasticity of the polymer, 1,4-polybutadiene, and mirrors the interfacial behavior of the same polymer against water. Such systems offer unique opportunities for designing responsive materials whose properties can be actively controlled.