Domain shape relaxation and local viscosity in stratifying foam films

Area of Science:

• Colloid and Surface Science
• Soft Matter Physics
• Rheology

Background:

• Foam films thin via stepwise domain formation, expansion, and coalescence.
• Understanding domain dynamics is key to controlling foam stability and properties.

Purpose of the Study:

• To analyze the dynamics of domain shape relaxation in a polymer-surfactant foam film.
• To differentiate between 2D and 3D dissipation effects in foam film thinning.
• To determine the film viscosity and line tension of the investigated system.

Main Methods:

• Studied domain shape relaxation in a stratifying foam film (anionic polymer/cationic surfactant).
• Analyzed circular domain coalescence governed by 2D dissipation.
• Modeled stripe withdrawal using a moving disc under external force, considering 3D dissipation (Stokes paradox).

Main Results:

• Coalescing domain relaxation is governed solely by 2D dissipation, allowing line tension to film viscosity ratio determination.
• Stripe withdrawal involves 3D dissipation, with equilibrium velocity logarithmically dependent on air viscosity.
• Film viscosity is at least 30 times greater than bulk viscosity due to molecular ordering and surface interactions.

Conclusions:

• The study provides a quantitative method to determine film viscosity and line tension from relaxation dynamics.
• Observed high film viscosity suggests significant molecular ordering and surface interactions within the foam film.
• Findings offer insights into the fundamental processes governing foam film stability and evolution.