Session F03: Electrokinetic Flows I

Intermediate and Flow Reversal Regimes in Nanochannel Electroosmotic Flows: A Comparison Between Molecular Simulations and Continuum Models

Electroosmotic slip flows in the Debye–Hückel regime were previously investigated using molecular dynamics and continuum transport perspectives ( J. Phys. Chem. C 2018, 122, 9699). This continuing work focuses on distinct electrostatic coupling regimes, where the variations in electroosmotic flows are elucidated based on Poisson–Fermi and Stokes equations and molecular dynamics simulations. In particular, aqueous NaCl solution in silicon nanochannels are considered under realistic electrochemical conditions, exhibiting intermediate flow and flow reversal regimes with increased surface charge density. Electroosmotic flow exhibits plug flow behavior in the bulk region for channel heights as small as 5 nm. With increased surface charge density, constant bulk electroosmotic flow velocity first increases and then it begins to gradually decrease until flow reversal is observed. In order to capture the flow physics and discrete motions within electric double layer accurately, the continuum model includes overscreening and crowding effects as well as slip contribution and local variations of enhanced viscosity. After extraction of the continuum parameters based on molecular dynamics simulations, good agreement between simulation results and continuum predictions are obtained for surface charges as large as −0.37 C/m². For further details please see J. Phys. Chem. C 2019, 123, 22, 14024–14035.   

Charge translucency and ion transport in 1D SWCNT-BN van der Waals heterostructures

Atomically thin graphene is known to be optically transparent, as well as partially translucent to van der Waals and electrostatic interactions at interfaces, which suggests that the fluidic properties of graphitic nanochannels may be modulated by the environment around the channel.  Here, we describe aqueous ion-transport studies in membranes fabricated from 2nm-diameter nanotube heterostructures consisting of single-wall carbon nanotubes that are CVD-coated on the outside with boron nitride. In aqueous electrolytes, ion conductance studies reveal that the vertically aligned SWCNT-BN nanotube arrays showed enhanced cation selectivity compared to the original SWCNTs.  The results suggest that the graphene inner surface of the heterostructures is partially transparent to charge regulation at the BN-graphene-water interface, resulting in higher surface charge and thus ion selectivity in an aqueous solution.  The slip length in the SWCNTs may also be affected by the outer BN coating.  These vertically aligned SWCNT-BN membranes enable the fundamental study of the effects of atomic translucency on transport in 1D nanotube heterostructures, as well as the exploration of technological applications like osmotic energy harvesting and separations.   

Self-Pumping Nanoporous Membranes Driven by Asymmetric Electrocatalytic Reactions

When two dissimilar metals are electrically connected in the presence of an aqueous solution, they can generate spontaneous fluid flows that result in either self-propulsion of freely suspended bimetallic particles, or fluid pumping if the metals are immobilized onto a solid surface. In the latter case, this can lead to novel phenomena such as catalytic micropumps. We present a numerical model of a membrane containing nanoscale cylindrical pores with platinum (Pt) coated onto one surface and gold (Au) on the opposite surface. In the presence of hydrogen peroxide, electrochemical charge-transfer reactions occur on each metal that result in a concentration gradient of protons, which in turn generates an electric field that drives electroosmotic flows through the pores. Electric double layer overlap is shown to be detrimental to self-pumping. For large pore radii, such as 6 microns, electric double layer overlap is no longer a concern and the velocity of self-pumping can exceed 20 microns per second. This work highlights the potential of utilizing catalytic reactions to pump liquid via membranes without external power, enhances the understanding of the physics underlying self-pumping flow, and provides guidance on the designing the next generation of self-pumping devices.   

Ion transport in nanopores with highly overlapping electric double layers

Investigation of ion transport through nanopores with highly overlapping electric double layers is exceptionally challenging. This difficulty can be attributed to the non-linear Poisson-Boltzmann (PB) equation that governs the behavior of the electrical potential distribution. In this talk, we will show how to simplify PB to a simpler and more universal equation [1], from which we derive an asymptotic solution. We leverage this new solution to address several highly debated issues. We derive the equivalent of the Gouy-Chapman equation for systems with highly overlapping electric double layers. We utilize this new relationship between the surface charge density and the surface potential to determine the power-law scaling of nanopore conductance as a function of the bulk concentrations. We derive the transport coefficients for a pore with overlapping electric double layers and compare them to the renowned uniform potential model. We show that the uniform potential model is only an approximation for the exact solution for small surface charges. The findings presented in this talk are essential to help uncover additional hidden attributes of ion transport through nanopores[2].

[1] Schnitzer and Yariv, Phys. Rev. E 87, 054301 (2013).

[2] Green, J. Chem. Phys. 154, 084705 (2021).   

An investigation of bending and branching in electrospinning of polymer nanofibers

The interaction of fluids with electricity is the subject of immense interest among researchers for centuries owing to its broad range of industrial applications including the production of nanofibers. Electrospinning is an electrohydrodynamical phenomena produced polymer nanofibers with the cross-sectional diameter in the range of nanometers to a few microns. The process of bending, stretching and extensive thinning of the jet induced by the electrical forces leads its path and shape to become more complex, which ultimately converted into solid fibers after evaporation of the solvent. The electrospinning experiments are carried out using two ultra-high imaging cameras with the frame rate up to 200,000 fps. Particular attention is paid to the electrically driven bending instability and the emanation of multiple branching of the primary and secondary jets. The results obtained using high-frame rate videography provide new insights into the evolution of electrified jets of polymer solutions. A theoretical description of the bending and branching phenomena is also proposed in this work.   

Pinning of Electroconvective Vortices Using Spacer Structure inside Ion Selective Membrane

Approaches to control electroconvection in ion depletion zone have been actively developed to improve ion transport efficiency in electrochemical membrane systems. One of the representing researches presented a model with partially impermeable surface via surface patterning on an ion selective membrane. However, the model has a limitation that the internal structure of the membrane is different from a practical membrane. Membrane utilized in industrial sites usually includes a (ion-impermeable) support structure to maintain the physical frame of the membrane. Therefore, we present a micro/nanofluidic platform with an impermeable internal-spacer structure inside a cation selective membrane for capturing electroconvective vortices in an ion depletion zone. Simulation and experimental results show that the vortices were pinned depending on the spatial configuration of the spacer structure both in chaotic regime and limiting regime. Furthermore, we demonstrate that the spacer which is closer to the anode and wider in width results in the anchoring of the vortices. From these results, we conclude that the spacer structure inside the nanoporous membrane can enhance ion transport efficiency and stability in the ion depletion zone.