Pore-scale dispersion in electrokinetic flow through a random sphere packing.

Area of Science:

• Fluid Dynamics
• Computational Physics
• Colloid Science

Background:

• Understanding fluid flow and solute transport in porous media is crucial for various applications.
• Electrokinetic flow (EOF) phenomena are significant in microfluidic devices and subsurface hydrology.
• Previous studies often simplified porous structures or flow conditions, limiting applicability.

Purpose of the Study:

• To investigate the three-dimensional velocity field and hydrodynamic dispersion in electrokinetic flow through random sphere packings.
• To analyze the pore-scale velocity profiles and their impact on tracer advection-diffusion.
• To compare the dispersion characteristics of EOF with pressure-driven flow in similar porous structures.

Main Methods:

• Generation of a random sphere packing with a porosity of 0.38 using a collective-rearrangement algorithm.
• Calculation of the interparticle velocity field via the lattice-Boltzmann (LB) method.
• Modeling advection-diffusion of an inert tracer using a random-walk particle-tracking method within the LB velocity field.

Main Results:

• Demonstrated nonuniform pore-scale velocity profiles for EOF even under ideal conditions (negligible electrical double layer thickness, uniform surface potential, no pressure gradients).
• Identified nonuniform local electrical field strength in sphere packing as the cause of complex EOF dynamics.
• Observed significant hydrodynamic dispersion in EOF through sphere packing, exceeding that of plug-like flow in a single channel.

Conclusions:

• EOF in random sphere packings exhibits complex velocity distributions leading to substantial hydrodynamic dispersion.
• The dispersion behavior in EOF is influenced by packing microstructure and average velocity.
• EOF demonstrates superior hydrodynamic performance compared to pressure-driven flow due to smaller velocity fluctuations at microscopic and mesoscopic scales.