Viscous friction of squeezed bubbly liquid layers

Area of Science:

• Fluid dynamics
• Rheology
• Multiphase flow

Background:

• Shear viscosity in bubbly liquids depends on gas fraction and capillary number.
• Confinement effects on bubbly liquid rheology are not fully understood.

Purpose of the Study:

• Investigate the impact of confinement on the shear viscosity of semi-dilute bubbly liquid layers.
• Analyze bubble deformation and its relation to viscosity under confinement.

Main Methods:

• Experimental study of bubbly liquid layers confined between two plates.
• Varying confinement ratios (bubble diameter to layer thickness >= 1).
• Measurements at different capillary numbers (0.001-0.1 and >0.1).

Main Results:

• Viscosity decreases below the suspending liquid's viscosity for capillary numbers > 0.1 due to bubble stretching.
• At high capillary numbers, viscosity approaches unconfined values.
• At lower capillary numbers (0.001-0.1), viscosity shows a non-monotonic trend with a peak at a confinement ratio of ~1.8.

Conclusions:

• Confinement significantly modifies bubbly liquid rheology, especially at lower capillary numbers.
• A viscosity maximum at specific confinement ratios results from combined bulk and wall drag forces.
• Bubble stretching is key to viscosity reduction under high shear confinement.