Criterion for Fingering Instabilities in Colloidal Gels

Area of Science:

• Rheology and Soft Matter Physics
• Fluid Dynamics
• Material Science

Background:

• Colloidal gels are complex fluids with unique mechanical properties.
• Understanding their flow behavior under stress is crucial for various applications.
• Viscous fingering is a known instability in fluid dynamics.

Purpose of the Study:

• To investigate the flow behavior of colloidal gels under tensile stress.
• To identify the conditions leading to unstable flow and air fingering.
• To establish a criterion for the onset of fingering in gels.

Main Methods:

• Tensile testing of colloidal gel sandwiched between parallel plates.
• Inducing radial flow by lifting the upper plate at a constant velocity.
• Analyzing stable and unstable flow regimes, including air finger growth.

Main Results:

• Two distinct flow regimes were observed: stable flow and unstable flow with air fingering.
• Unstable fingering initiated beyond a critical energy input, independent of macroscopic yield stress.
• The instability exhibited characteristic scalings of classical viscous fingering.

Conclusions:

• A local fluidization at the tip of air fingers drives the instability.
• The onset of fingering is governed by local shear-induced yielding, aligning with delayed failure frameworks.
• This study provides a quantitative criterion for predicting fingering in colloidal gels.