Liquid film thinning-thickening anomaly in electrolyte solutions

HYPOTHESIS: The distribution of dissolved ions depends largely on the position and shape of air-water interfaces. Following the interfacial interaction, both the surface deformation and reduced separation distance may induce different ion concentrations in liquid film and bulk solutions. The consequent Marangoni effect should change the flow dynamics inside the liquid films and thus influence the film evolution process. EXPERIMENTS: In this study, a home-made interferometer is employed to obtain the spatiotemporal evolution of film profiles during the bubble-solid surface interaction. The quantification of film thickness enables the determination of micro/nanoscale fluid flow from liquid films. FINDINGS: Unlike the common cases of continuous film drainage behaviors, the film thickness at the rim exhibits a counterintuitive increase, causing the previously approaching bubble to retract from solid surfaces. It is revealed how the inward flow can surpass the drainage outflow and reverse the normal film evolution. Theoretical developments involving the electrolyte type, ion concentration and film deformation are provided to describe this competition and address the film thinning-thickening anomaly. Our finding enriches the understanding of the microscopic fluid flow in highly confined and deformed regions, and the research outcome creates the possibility of modulating near-wall bubble dynamics as required.