A combination of atomic force microscopy (AFM) and reflection interference contrast microscopy (RICM) was used to measure simultaneously the interaction force and the spatiotemporal evolution of the thin water film between a bubble in water and mica surfaces with varying degrees of hydrophobicity. Stable films, supported by the repulsive van der WaalsÀCasimirÀLifshitz force were always observed between air bubble and hydrophilic mica surfaces (water contact angle, θ w < 5°) whereas bubble attachment occurred on hydrophobized mica surfaces. A theoretical model, based on the Reynolds lubrication theory and the augmented YoungÀLaplace equation including the effects of disjoining pressure, provided excellent agreement with experiment results, indicating the essential physics involved in the interaction between air bubble and solid surfaces can be elucidated. A hydrophobic interaction free energy per unit area of the form: W H (h) = Àγ(1 À cos θ w )exp(Àh/D H ) can be used to quantify the attraction between bubble and hydrophobized solid substrate at separation, h, with γ being the surface tension of water. For surfaces with water contact angle in the range 45°< θ w < 90°, the decay length D H varied between 0.8 and 1.0 nm. This study quantified the hydrophobic interaction in asymmetric system between air bubble and hydrophobic surfaces, and provided a feasible method for synchronous measurements of the interaction forces with sub-nN resolution and the drainage dynamics of thin films down to nm thickness. KEYWORDS: bubble . hydrodynamic force . hydrophobic interaction . AFM . mica . thin film drainage ARTICLE SHI ET AL.

The directed assembly of block copolymer nanostructures with large periods exceeding 100 nm remains challenging because the translational ordering of long-chained block copolymer is hindered by its very low chain mobility. Using a solvent-vapor annealing process with a neutral solvent, which was sequentially combined with a thermal annealing process, we demonstrate the rapid evolution of a perpendicularly oriented lamellar morphology in high molecular weight block copolymer films on neutral substrate. The synergy with the topographically patterned substrate facilitated unidirectionally structural development of ultrahigh molecular weight block copolymer thin films-even for the structures with a large period of 200 nm-leading to perfectly guided, parallel, and highly ordered line-arrays of perpendicularly oriented lamellae in the trenched confinement. This breakthrough strategy, which is applicable to nanolithographic pattern transfer to target substrates, can be a simple and efficient route to satisfy the demand for block copolymer assemblies with larger feature sizes on hundreds of nanometers scale.

Adsorption of interfacially active components at the water/oil interface plays critical roles in determining the properties and behaviors of emulsion droplets. In this study, the droplet probe atomic force microscopy (AFM) technique was applied, for the first time, to quantitatively study the interaction mechanism between water-in-oil (W/O) emulsion droplets with interfacially adsorbed asphaltenes. The behaviors and stability of W/O emulsion droplets were demonstrated to be significantly influenced by the asphaltene concentration of organic solution where the emulsions were aged, aging time, force load, contact time, and solvent type. Bare water droplets could readily coalesce with each other in oil (i.e., toluene), while interfacially adsorbed asphaltenes could sterically inhibit droplet coalescence and induce interfacial adhesion during separation of the water droplets. For low asphaltene concentration cases, the adhesion increased with increasing asphaltene concentration (≤100 mg/L), but it significantly decreased at relatively high asphaltene concentration (e.g., 500 mg/L). Experiments in Heptol (i.e., mixture of toluene and heptane) showed that the addition of a poor solvent for asphaltenes (e.g., heptane) could enhance the interfacial adhesion between emulsion droplets at relatively low asphaltene concentration but could weaken the adhesion at relatively high asphaltene concentration. This work has quantified the interactions between W/O emulsion droplets with interfacially adsorbed asphaltenes, and the results provide useful implications into the stabilization mechanisms of W/O emulsions in oil production. The methodology in this work can be readily extended to other W/O emulsion systems with interfacially active components.

Emulsions with interface-active components at the oil/water interface have long been of fundamental and practical interest in many fields. In this work, the interaction forces between two oil droplets in water in the absence/presence of asphaltenes were directly measured using droplet probe atomic force microscopy (AFM) and analyzed using a theoretical model based on Reynolds lubrication theory and the augmented Young-Laplace equation by including the effects of disjoining pressure. It was revealed that the interaction forces measured between two pristine oil droplets (i.e., toluene) could be well described by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, while an additional steric interaction should be included in the presence of asphaltenes in the oil. The surface interaction and the stability of oil droplets in aqueous solution were demonstrated to be significantly influenced by the asphaltenes concentration in oil, salt concentration, pH, and presence of divalent ions (Ca(2+)) in water. Adsorbed asphaltenes at the oil/water interface led to more negative surface potential of the oil/water interface and also induced steric repulsion between oil droplets, inhibiting the drop coalescence and stabilizing the oil-in-water emulsion. Lower pH of aqueous solution could lead to less negative surface potential and weaken the repulsion between oil droplets. Addition of divalent ions (Ca(2+)) was found to disrupt the protecting effects of adsorbed asphaltenes at oil/water interface and induce coalescence of oil droplets. Our results provide a useful methodology for quantifying the interaction forces and investigating the properties of asphaltenes at the oil/water interfaces and provide insights into the stabilization mechanism of oil-in-water emulsions due to asphaltenes in oil production and water treatment.

The outstanding water wettability and the capability of polyelectrolyte surfaces to spontaneously clean oil fouling are determined by their wetting mechanism in the surrounding medium. Here, we have quantified the nanomechanics between three types of polyelectrolyte surfaces (i.e. zwitterionic, cationic, and anionic) and water or oil drops using an atomic force microscope (AFM) drop probe technique, and elucidated the intrinsic wetting mechanisms of the polyelectrolyte surfaces in oil and water media. The measured forces between oil drops and polyelectrolyte surfaces in water can be described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Surprisingly, strong long-range attraction was discovered between polyelectrolyte surfaces and water drops in oil, and the strongest interaction was measured for the polyzwitterion. This unexpected long-range "hydrophilic" attraction in oil could be attributed to a strong dipolar interaction because of the large dipole moment of the polyelectrolytes. Our results provide new nanomechanical insights into the development of novel polyelectrolyte-based materials and coatings for a wide range of engineering, bioengineering, and environmental applications.