(485g) Electrokinetic Enhancement of Ion Transport in Microfluidic Desalination Systems

Fundamental understanding of ion transport processes in electrodialysis (ED) processes is of importance to enhance efficiency in these systems. In typical ED processes a potential is applied over a stack of alternating cation- and anion exchange membranes, yielding alternating compartments enriched and depleted in the electrolyte. ED can only be industrially operated at relatively mild potentials, as above a critical voltage no additional ions are transported as a result of diffusion limitations in the feed solution (“limiting current”). However, at the so-called “overlimiting current”, increased ion transport is observed. Electrokinetic flows induced by local concentration and electric field gradients are considered to be one of the mechanisms leading to this overlimiting current. Understanding of the origin of these electrokinetic flows in the vicinity of charge selective interfaces is of key importance in the enhancement of the efficiency of electrodialysis processes. Microfluidic investigations allow for the direct observation of the onset, development and manipulation of these flows. In this research, different microfluidic platforms containing a variety of charge-selective interfaces are employed for the investigation and possible enhancement of ion transport adjacent and through the interface.

Nanochannels with a characteristic size in the order of the Debye layer are charge-selective as a result of charge exclusion of co-ions based on Donnan exclusion (charged-walls exerting an electric field). We experimentally investigate the development of ion concentration polarization (ICP) and the resulting electrokinetic effects in microchannels that are connected by different arrays of nanochannels with a characteristic size of 10 nm [1]. Due to the nature of most microfluidic systems, the electric field is not necessarily distributed normally over the charge selective interface. As a result of the different nanochannel configurations, the effect of the local electric field on the charge transport can be investigated. Aqueous solutions of sodium chloride are used as a model system to study ion-transport phenomena, along with a negatively charged fluorescent dye and particles to estimate fluid velocity. In this way, we are able to observe both concentration profiles and hydrodynamics in the microchannels, while obtaining current-voltage characterization of the system. We observed, using Alexa 488 as the negatively charged fluorescent dye, wave-shaped depletion zones being formed in the vicinity of the nanochannels. As a result of these local concentration gradients, which induce local electric field disturbances, electrokinetic flows develop. Additionally, we observe the formation of hydrodynamic vortices near the nanochannels, with velocities up to 1500 µm/s. The formation of depletion zones and electrokinetic flows is highly dependent on the geometry of the array of nanochannels and their distance from the driving electrodes. In geometries consisting of multiple nanochannel patches, the vortices forming on the outer most patches closest to the electrodes are the largest and the fastest, while at the nanochannel patches further from the electrodes vortices are smaller and the speed inside the vortex is lower. At patches sufficiently far from the electrodes no vortices and depletion zones are observed, indicating an absence of strong polarization. The observed hydrodynamic phenomena are attributed to a combination of electro-osmotic flow (EOF) and electro-osmotic instabilities (EOI), which result from local concentration differences and their interplay with the distorted electric field lines.

In a second study, we experimentally investigate electrokinetic flows adjacent to charge selective hydrogels using a microfluidic ED-platform [2] consisting of alternating rows of positively and negatively charged hydrogels. The charge selective hydrogels can be patterned in PDMS chips allowing for access to different geometries and providing a platform for rapid investigation of topological effects. This approach is motivated by theoretical work relating to geometric patterning of membranes and spacer topologies to enhance ion-transport. Our experimental platform provides an excellent model system to investigate the influence of such geometric patterns as it allows for direct visualization of depletion/enrichment behavior simultaneous with electrical characterization. Experiments were conducted using salt solutions of different concentrations seeded with charged fluorescent dyes to visualize local ion concentration. By means of fluorescence microscopy using a negatively charged dye we visualized the formation of depletion zones near different hydrogels, both with and without cross flow. In order to quantify concentration profiles in the system, we used Fluorescence Lifetime Image Microscopy (FLIM) to measure local chloride ion concentrations near the hydrogels and compare these to the fluorescence microscopy results. For different geometries, we find that the development of these depletion zones is distinctly different as a result of the distribution of the electric field lines through the different geometries. Pinning of depletion zones occurs in heterogeneously patterned systems, and enhancement in the total transport is observed with increasing system heterogeneity as a result of electroosmotic contributions to the charge transport towards the selective interface [3]. Tangential components of the electric field are induced as a result of the geometric features of the system, yielding increased ion transport in all characteristic regimes, from Ohmic to overlimiting. Electrokinetic instabilities at the interface are observed at elevated potentials, increasing the total current through the hydrogels. This indicates that electroosmotic and electrokinetic contributions to the total charge transport can be enhanced by inducing non-uniform electric fields using membrane topology variations and spacers.

Both of these experimental studies yield important experimental information on the effect of geometry changes on the electrokinetic effects in ion transport processes. We show that the influence of the system geometry and the resulting electric-field distortions are of high importance for the development of hydrodynamic instabilities and the transport of charged species. A relatively small change in geometry can affect the resulting electrokinetic instabilities to an extremely large degree.

[1] Benneker, Anne M., et al. Scientific reports 6 (2016): 37236.

[2] Gumuscu, B. et al., Adv. Funct. Mater., (2016) 26: 8685–8693.

[3] Benneker, Anne M., et al. Lab on a Chip 18.11 (2018): 1652-1660.