Shear induced drainage in foamy yield-stress fluids

Area of Science:

• Rheology and Material Science
• Fluid Dynamics
• Foam Physics

Background:

• Yield-stress fluids exhibit a critical stress before flow initiation.
• Foam stability is governed by interstitial fluid properties and bubble interactions.
• Drainage in foams is crucial for understanding their structural evolution and properties.

Purpose of the Study:

• To investigate shear-induced drainage mechanisms in foamy yield-stress fluids.
• To elucidate the role of interstitial fluid rheology under shear.
• To understand the influence of bubble size and foam films on drainage dynamics.

Main Methods:

• Magnetic Resonance Imaging (MRI) techniques were employed to visualize and quantify drainage.
• Controlled horizontal shear was applied to the foamy yield-stress fluid.
• Experiments were conducted with varying bubble sizes to assess their impact.

Main Results:

• At rest, yield stress stabilizes the foam; under shear, the interstitial fluid behaves viscously.
• Effective viscosity is shear-dependent, influenced by transient foam films.
• Observed drainage behavior deviates from classical R2 scaling and shows an unexpected arrest.

Conclusions:

• Shear transforms the interstitial fluid, enabling rapid drainage.
• Foam films play a critical role in arresting drainage, leading to irreversible liquid trapping.
• The findings challenge existing models for foam drainage, particularly concerning bubble size effects.