Electroosmotic flow: From microfluidics to nanofluidics

Abstract

Electroosmotic flow (EOF), a consequence of an imposed electric field onto an electrolyte solution in the tangential direction of a charged surface, has emerged as an important phenomenon in electrokinetic transport at the micro/nanoscale. Because of their ability to efficiently pump liquids in miniaturized systems without incorporating any mechanical parts, electroosmotic methods for fluid pumping have been adopted in versatile applications—from biotechnology to environmental science. To understand the electrokinetic pumping mechanism, it is crucial to identify the role of an ionically polarized layer, the so‐called electrical double layer (EDL), which forms in the vicinity of a charged solid–liquid interface, as well as the characteristic length scale of the conducting media. Therefore, in this tutorial review, we summarize the development of electrical double layer models from a historical point of view to elucidate the interplay and configuration of water molecules and ions in the vicinity of a solid–liquid interface. Moreover, we discuss the physicochemical phenomena owing to the interaction of electrical double layer when the characteristic length of the conducting media is decreased from the microscale to the nanoscale. Finally, we highlight the pioneering studies and the most recent works on electro osmotic flow devoted to both theoretical and experimental aspects.

A theoretical understanding of ionic current through a nanochannel driven by a viscosity gradient

“…This behavior of the ionic current can be attributed to the competition of the EDL thickness and ρ ave , where by increasing n b , the absolute value of ρ ave increases and then decreases. In other words, when we increase the bulk ionic concentration, the surface charge density at the nanochannel walls increases [7,27,39]. However, for higher amount of n b , leading to thinner electrical double layers, the major part of the nanochannel is electro-neutral (ρ e = 0) which does not contribute into the average net charge density.…”

“…Hereinafter, we will present the non-dimensional parameters with over-bar. Ionic species are typically moving in an isothermal domain owing to the three transport phenomena: advection, diffusion, and electromigration [7,36]. Since in Wiener and Stein's experiments [20] there is no applied external electric field, pressure gradient, and ionic concentration gradient, we can simply relate the drift velocity to the advection of ionic species which gives rise to…”

“…Migration of ionic species is triggered by asymmetrical system conditions such as gradients of concentration [1,2], pressure [3], temperature [4][5][6], and electrical potential [7]. These methods of ion transport phenomena have found numerous applications in energy harvesting [8][9][10][11], water treatment [12][13][14][15], or logical part of nanofluidic chips [4,[16][17][18][19].…”

Soft hydraulics: from Newtonian to complex fluid flows through compliant conduits

“…A general introduction to electrokinetic actuation of microscale flows is provided by Chakraborty and Chakraborty [214, section 1.4.5], while Ghosal [215] covers the mathematical modeling of charge transport during electroosmotic flows. Alizadeh et al [216] provide an updated tutorial review on electroosmotic flows in micro-and nanofluidic applications, including a historical overview and discussion of applications of confined flows through micro-and nanoporous media. Electrohydrodynamics is a broad field, and only a few key results related to non-Newtonian flows and flows in compliant conduits are summarized here.…”

A simple yet efficient approach for electrokinetic mixing of viscoelastic fluids in a straight microchannel

“…In doing so, an electroosmotic flow (EOF) technique is often used wherein the fluid motion is created due to the interaction between the electrical double layer (EDL) developed along a charged surface and an externally applied electric field 9 . From the past several decades, a significant body of literature, comprising of both theoretical (analytical and numerical) and experimental investigations, is present on how different factors, such as wall zeta potential, applied voltage, patterned surface, rotating or non-rotating surface, system size and structure, electric field type, i.e., AC or DC, presence of obstacles, etc., can influence this electrokinetic transport of various simple Newtonian as well as complex non-Newtonian fluids 10 – 16 .…”