Electroconvective viscous fingering in a single polyelectrolyte fluid on a charge selective surface

Area of Science:

• Fluid dynamics
• Electrochemistry
• Materials science

Background:

• Viscous fingering instability occurs when a low-viscosity fluid displaces a higher-viscosity fluid, creating finger-like patterns.
• This phenomenon is common in natural and engineered systems involving two immiscible fluids.

Purpose of the Study:

• To demonstrate a novel form of viscous fingering at a single fluid-solid interface.
• To investigate the underlying mechanisms and governing parameters of this electroconvective viscous fingering.

Main Methods:

• Utilizing a single polyelectrolyte fluid on a charge-selective surface.
• Inducing selective ion rejection to create viscosity gradients and electroconvective vortices.
• Analyzing the onset and shape of viscous fingering using dimensionless numbers.

Main Results:

• Selective ion rejection initiated stepwise viscosity gradients and electroconvective vortices.
• Vortex growth led to the displacement of the viscosity gradient boundary, causing viscous fingering.
• The viscosity ratio governed the onset, while other dimensionless numbers (electric Rayleigh, Schmidt, Deborah) controlled finger shape and characteristics.

Conclusions:

• Electroconvective viscous fingering is achievable at a single fluid-solid interface.
• This mechanism offers controllable onset and shape, presenting new avenues for manipulating ion transport and dendritic instabilities in electrochemical systems.