Fingering instabilities of a reactive micellar interface

Area of Science:

• Fluid Dynamics
• Soft Matter Physics
• Interface Science

Background:

• Hele-Shaw cells are used to study fluid instabilities.
• Wormlike micellar fluids exhibit complex rheological properties.
• Surfactant-laden interfaces can form gel-like structures.

Purpose of the Study:

• To experimentally investigate fingering instabilities in a Hele-Shaw cell.
• To characterize the formation of gel-like interfaces between surfactant solutions.
• To explore novel fingering regimes and their underlying mechanisms.

Main Methods:

• Utilizing a Hele-Shaw cell to confine fluid flow.
• Preparing aqueous solutions of cetyltrimethylammonium bromide (a cationic surfactant) and salicylic acid (an organic salt).
• Varying injection rates to control front velocity and observing resulting fingering patterns.

Main Results:

• Observed diverse fingering instabilities dependent on injection velocity and fluid configuration.
• Identified a regime of nonconfined stationary or wavy fingers.
• Demonstrated width selection in nonconfined fingers, deviating from Saffman-Taylor conditions.

Conclusions:

• The study reveals new fingering phenomena in Hele-Shaw cells with elastic interfaces.
• Nonconfined finger width selection is possible, challenging previous assumptions.
• Observed mechanisms may relate to natural phenomena like lava flows and biofilm growth.