Stabilization of thin liquid films by repulsive van der Waals force

Area of Science:

• Fluid dynamics
• Colloid and surface science
• Interfacial phenomena

Background:

• Understanding thin liquid film stability is crucial in various industrial and natural processes.
• The role of intermolecular forces, specifically van der Waals (vdW) interactions, in film stability is complex and depends on the fluid phases involved.

Purpose of the Study:

• To investigate the stability of thin liquid films formed between a rising bubble and a liquid-liquid interface.
• To determine if van der Waals interactions, predicted by refractive indices, can accurately forecast film stability.
• To elucidate the underlying mechanisms governing the rupture or stability of these thin films.

Main Methods:

• High-speed video recording of bubble rise experiments in immiscible oil-water systems.
• Systematic variation of nonpolar oils (perfluorohexane, tetradecane) and water with different refractive indices.
• Analysis of film stability in relation to attractive and repulsive van der Waals forces.

Main Results:

• For oil films, stability was predictable by vdW forces: repulsive forces led to stable films, attractive forces to rupture.
• Aqueous electrolyte films between air bubbles and oil phases consistently ruptured, even with predicted repulsive vdW interactions.
• These findings suggest an additional destabilizing factor for water films.

Conclusions:

• Van der Waals interactions accurately predict thin film stability when the film-forming phase is oil.
• A hydrophobic attraction, not accounted for by standard vdW calculations, likely causes the rupture of thin water films.
• This hydrophobic attraction plays a critical role in the stability of water films at air-oil interfaces.