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Nonionic triblock copolymers, surfactant Pluronic F68 (PEO76-PPO29-PEO76), are widely used in industrial processes, such as foaming, emulsification, and stabilization. The behaviors of triblock copolymers such as the salt-dependent self-assembly in bulk solution and the irreversible adsorption at the oil/water interface are mainly focused to explore their effects on the interaction forces between nano-spacing interfaces of oil droplets. In this study, the atomic force microscopy (AFM) technique was employed to measure the drop interaction forces with different F68 bulk concentrations. All selected bulk concentrations (≥100 μM) of copolymers can ensure the formation of a stable layer structure of stretched polymer chains ("brush") at the oil/water interface, which behaved as a mechanical barrier at the interface. This study quantified the forces caused by the space hindrance of F68 copolymers both in the bulk phase and at the interface of oil/F68 aqueous solution during drop interaction. The effects of monovalent electrolyte (NaCl)-induced self-assembly behavior of triblock copolymers F68 in bulk solution on drop interaction forces were measured through the AFM technique.

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Creating surfaces with controlled superoleophobicity is of real significance for the manipulation, transportation, and self-cleaning of organic fluids of strategic importance. In the present work, particular attention is given to elucidate the superoleophobicity mechanism of surfaces coated with polyelectrolyte multilayers (PEMs) assembled by layer-by-layer deposition of the cationic polyelectrolyte polydiallyldimethylammonium chloride (PDDA) and the anionic polyelectrolyte poly(styrenesulfonate) (PSS) in 1.0 M NaCl solution. The atomic force microscopy (AFM) drop probe technique was used to measure the salinity-specific adhesion forces between the oil droplet and underwater polyelectrolyte PEM-coated surface. Meanwhile, topographic images of the fabricated PEMs in solutions with different ionic strengths were also obtained and compared. The results show that the fabricated (PDDA/ PSS) 1.5 , (PDDA/PSS) 3.5 , and (PDDA/PSS) 4.0 PEMs exhibit excellent oil repellency upon immersion into aqueous solutions with high salinity. Additionally, the underwater superoleophobicity of the prepared PEM films was well-demonstrated through AFM adhesion force measurements. It is proven in our study that the underwater superoleophobicity of the fabricated surface has a strong relationship with the salinity-specific behavior of deposited polyelectrolytes.

The heterointeraction between liquid drops and air bubbles dispersed in another immiscible liquid is studied with the application of the atomic force microscopy (AFM) probe techniques. The tetradecane drops and air bubbles readily coalescence to form a lens-like structure in 100 mM sodium chloride aqueous solution, demonstrating strong hydrophobic (HB) attraction. The interaction range and strength of this hydrophobic attraction between oil drops and air bubbles is investigated by fine control of electrical double layer thicknesses related to specific electrolyte concentrations, and a midrange term in combination with a short-range term is found to present a proper characterization of this hydrophobic attraction. A further step is taken by introducing a triblock copolymer (Pluronic F68) into the aqueous solution, with results indicating that a relatively long-range steric hindrance (SH) furnished by a polymer “brush” surmounts the hydrophobic attraction. Finally, the interaction between a water drop and an air bubble in tetradecane is also measured as a comparison. The repelling action between a hydrophobic body (air bubble) and water drop indicates a strong repulsion. The present results show an interesting understanding of hydrophobic interactions between drops and bubbles, which is of potential application in controlling dispersion stability.