[Paper Review] Charged Obstacles Augment Electrokinetic Energy Conversion Efficiency

Kahali, Tanoy, Antarip Poddar+2 more|arXiv (Cornell University)|Dec 4, 2017

Nanopore and Nanochannel Transport Studies2 references3 citations

TL;DR

This study demonstrates that introducing charged obstacles in narrow fluidic channels significantly enhances electrokinetic energy conversion efficiency by manipulating streaming potential, with optimal performance achieved using regular patterns of obstacles having a radius >10% of channel height and charge density ratio ≥9, yielding up to 18% efficiency—surpassing planar channels by ~14 percentage points when slip and finite ion size effects are included.

ABSTRACT

In the present study, we delineate the effect of introducing flow obstructions on streaming potential and energy conversion efficiency in a narrow fluidic confinement taking into consideration the wall hydrodynamic slip, finite ionic size, and local permittivity variation effects. We consider two types of the geometric pattern of flow obstacle between which regular pattern turns out to be more effective. It is observed that, implementing structured flow obstacles of radii greater than 10% of channel height and charge density ratio (charge density of flow obstacle surface to that of channel wall) greater than or equal to 9 renders significant enhancement of power generation efficiency. On the other hand, it is effective to operate at a charge density ratio~1 for obstacles radii less than or equal to 10% of channel height. An effective normalized pitch length of magnitude 0.6 or above has to be maintained in order to obtain optimum energy conversion efficiency. We found that implementation of charged flow obstacle causes a significant enhancement of energy conversion efficiency (~18% with regular pattern) comparing to planar slit channel (~4%) without considering wall hydrodynamic slip and steric effects. Introducing finite ionic size and wall hydrodynamic slip effect, pertinent to flows in narrow confinement, leads to further enhancement in electrokinetic energy conversion efficiency.

Motivation & Objective

• To investigate how flow obstacles influence electrokinetic energy conversion in nanoscale fluidic systems.
• To analyze the impact of wall hydrodynamic slip, finite ionic size, and local permittivity variations on energy conversion efficiency.
• To determine optimal geometric and electrostatic parameters for maximizing efficiency using structured obstacles.
• To compare the effectiveness of regular versus irregular obstacle patterns in enhancing streaming potential and power output.

Proposed method

• Modeling electrokinetic flows in a slit channel with charged obstacles using Navier-Stokes and Poisson-Nernst-Planck equations.
• Incorporating wall hydrodynamic slip via the Navier slip boundary condition to reflect nanoscale confinement effects.
• Accounting for finite ionic size through the modified Poisson-Boltzmann equation with steric effects.
• Using computational simulations to evaluate streaming potential and energy conversion efficiency across varying obstacle geometries and charge densities.
• Defining normalized pitch length as obstacle spacing relative to channel height to assess periodicity effects.
• Comparing performance of regular and irregular obstacle patterns under identical flow and electrostatic conditions.

Experimental results

Research questions

• RQ1 How does the introduction of charged obstacles affect streaming potential and energy conversion efficiency in narrow fluidic channels?
• RQ2 What is the optimal obstacle size and charge density ratio for maximizing electrokinetic energy conversion efficiency?
• RQ3 How do wall hydrodynamic slip and finite ionic size influence the efficiency gains from obstacle integration?
• RQ4 Does the periodicity (pitch length) of obstacle arrays significantly affect energy conversion performance?
• RQ5 Which obstacle pattern—regular or irregular—yields superior efficiency enhancement?

Key findings

• A regular pattern of charged obstacles increases electrokinetic energy conversion efficiency by up to 18%, compared to 4% in a planar slit channel.
• Obstacles with radius >10% of channel height and charge density ratio ≥9 yield the highest efficiency enhancement.
• For obstacles with radius ≤10% of channel height, an optimal charge density ratio of ~1 maximizes efficiency.
• A normalized pitch length of 0.6 or greater is required to achieve peak energy conversion efficiency.
• Incorporating wall hydrodynamic slip and finite ionic size effects further enhances efficiency beyond predictions from idealized models.
• The combination of slip, steric effects, and structured obstacles leads to a cumulative improvement in energy conversion performance.