Surface Interaction Mechanisms between Deformable Bubbles and Hydrophobic Surfaces in Aqueous Solutions

Abstract

Deformable bubbles in complex fluidic systems are crucial components in a variety of traditional and emerging industrial processes, where their surface interactions with hydrophobic surfaces play an important role in realizing the targeted functionalities. In this project, a bubble probe atomic force microscopy (AFM) technique was applied to directly measure the interactions between air bubbles and various hydrophobic surfaces under different aqueous conditions, and the measured interaction forces were theoretically analyzed based on a Stokes-Reynolds-Young-Laplace model which incorporated the effect of disjoining pressure. By virtue of this methodology, the project systematically investigated the critical roles of the surface interactions including van der Waals (VDW), electrical double-layer (EDL) and hydrophobic (HB) interactions in the bubble-surface interaction processes. The VDW and EDL interactions could be well explained by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which tended to stabilize the thin water film and prevent the bubble attachment onto the hydrophobic surface. The non-DLVO HB interaction originating from the water structuring effect at the hydrophobic interface was responsible for the bubble attachment, which could be quantified using an exponentially decaying model derived from the thermodynamic consideration. Besides, the effects of a series of principal factors which could modulate the HB interactions between air bubbles and hydrophobic surfaces have been elucidated. Firstly, it was found that altering aqueous salinity and pH could hardly affect the HB interactions between air bubbles and hydrocarbon self-assembled monolayers (SAMs) composed of octadecyltrichlorosilane (OTS), with the decay length D0 remaining ~ 1.0 nm across all the investigated cases. However, for the cases involving hydrophobic polystyrene (PS) surfaces, interfacial nanobubbles (INBs) could spontaneously form on the PS surfaces and significantly affect the measurement of the bubble-PS interactions. As verified by the AFM imaging results, high salinity (e.g., 1000 mM NaCl) could sufficiently suppress the nanobubble formation and simplify the quantification of the intrinsic HB interaction between the air bubble and the pristine PS surface, and the decay length D0 was theoretically fitted to be ~ 0.75 nm. Moreover, the bubble-polymer HB interactions were found to be subject to ion specificity. The selective binding of the cations with low charge density (e.g., K+ and NH4+) to the aromatic benzene groups, the so-called âcation-Ï interactionâ could shorten the range of the bubble-PS HB interaction with D0 declining from ~ 0.75 nm to ~ 0.60 nm; while the preferential adsorption of heavy halide anion (e.g., I-) onto the poly (methyl methacrylate) (PMMA) and polydimethylsiloxane (PDMS) surfaces could reduce the corresponding D0 from ~ 0.63 nm and ~ 0.72 nm to ~ 0.50 nm and ~ 0.59 nm, respectively. Furthermore, the range of the HB interaction could also be evidently modulated by altering the surface properties. By coadsorbing two components of alkanethiols with different chain length or tail groups, surface mobility and nano-scaled chemistry heterogeneity could be introduced into the SAM surfaces, which could effectively relax the physical restriction that orderly rearranges the interfacial water molecules and shorten the range of the HB interaction. This project provides a useful method to quantitatively study the interactions between deformable droplets (e.g., air bubbles) and hydrophobic surfaces in complex aqueous media, and contributes to an improved understanding of the surface interaction mechanisms. The results of quantifying the HB interactions under different experimental conditions comprehensively illustrate the effects of the influencing factors such as water chemistry and surface properties, and sheds novel light on the physical mechanism underlying the HB interaction, which is of both fundamental and practical significance.