When is a surface foam-phobic or foam-philic?

Area of Science:

• Fluid dynamics
• Surface science
• Materials science

Background:

• Sessile bubbles are often assumed to fully wet their supporting surfaces.
• The degree of wetting influences how well foams adhere to surfaces.
• Foam stability is critical in various industrial and natural processes.

Purpose of the Study:

• To investigate the equilibrium shapes of vertical soap films spanning horizontal substrates.
• To determine the conditions under which Plateau borders, essential for foam structure, can form.
• To analyze the influence of substrate wettability on foam-substrate interactions.

Main Methods:

• Quasi-analytical solutions of the Young-Laplace equation for a simplified geometry.
• Investigation of the (θc,Bo) space, relating contact angle and Bond number.
• Comparison with experimental and numerical (Surface Evolver) data for validation.

Main Results:

• Plateau borders, where foam liquid accumulates, exist only within specific domains of contact angle (θc) and Bond number (Bo).
• Substrates supporting stable films are termed 'foam-philic,' while others are 'foam-phobic.'
• The size of Plateau borders is constrained by wettability, with top borders generally smaller than bottom ones and unable to sustain steady states above 90° contact angle.

Conclusions:

• Substrate wettability dictates the existence and size of Plateau borders, thereby controlling foam stability.
• The study provides critical parameters for foam-substrate interactions, relevant for applications involving foam in channels.
• The findings are expected to extend to more complex, non-planar soap films and bubbles.