Fluid electro-osmotic flow

The movement of acid-front to the cathode is due to migration (electric potentials), diffusion (chemical potentials), and advection (hydraulic potential) and causes desorption of heavy metals from clay surfaces and transports them into the pore fluid. Electro-osmotic flow and its associated phenomena constitute the mechanisms for removing heavy metals from soils. 

Solution-. The observed effect is the sum of two contributions, one of which is the electro-osmotic flow of the medium through the cell. The latter has its maximum value at the center since the layer of fluid adjacent to the walls is stationary. The particles tracked at the center of the cell therefore possess the maximum increment in velocity due to electroosmotic flow. Since the cell is a closed compartment, the liquid displaced by electroosmosis along the walls must circulate by a backflow down the center of the tube. Since the total liquid flow in a closed cell must be zero, the appropriate value from Figure 12.10a to use for the velocity is the average of observations made at all depths. ... 

In order to guarantee well-defined hydrodynamic conditions, usually closed cells are used, thus avoiding inadvertent variations of the external pressure. However, contrary to what one might expect, closing the cell does not imply immobilization of the continuous phase. The reason is that the (transparent) cell walls usually carry a charge so that an electro-osmotic fluid displacement is created, as in fig. 4.7. The electrophoretic displacement is superimposed on this electro-osmotic flow and to subtract the latter the hydrodynamics have to be considered in some detail. 

Poison N. A., M. A. Hayes, 2000, Electro-osmotic flow control of fluids on a capillary electrophoresis microdevice using an applied external voltage. Anal. Chem. 72, 1088-1092. 

Electro-osmosis refers to the movement of the liquid adjacent to a charged surface, in contact with a polar liquid, under the influence of an electric field applied parallel to the solid-liquid interface. The bulk fluid of liquid originated by this electrokinetic process is termed electro-osmotic flow (EOF). It may be produced both in open and in packed capillary tubes, as well as in planar electrophoretic systems employing a variety of supports, such as paper or hydrophilic polymers. 

The hrst pTAS devices were based on GC. Soon after, HPLC-based pTAS platforms began to appear and then CE followed. In chip CE, an injection was performed and an electrophoretic separation of a sample mixture (different fluorescent dyes) and all liquid handling was achieved using electro-osmotic flow. Since then, electrophoresis and electrokinetic fluid handling have been the cornerstones of many miniaturised analytical devices. 

CE CGE CHOL CIP CPG CTAB CZE dA, dG, dC DBU DEAE DMF DMT DNP DOPE DOTMA EDTA EM EOF ESI-MS Fmoc FPE ICAM-1 Capillary electrophoresis Capillary gel electrophoresis Cholesterol Cahn-Ingold-Prelog nomenclature system for absolute configuration Controlled pore glass Hexadecyltrimethylammonium bromide Capillary zone electrophoresis Deoxyadenosine, deoxyguanosine, deoxycytosine l,8-Diazabicyc o[5.4.0]undec-7-en Diethylaminoethyl- Dimethylformamide Bis(4-methoxyphenyl)phenylmethyl-, (syn. Dimethoxytrityl-) 2,4-DinitrophenyI- Dioleylphosphatidylethanolamine N-[l-(2,3-dioleyloxy)propyl]-N, N, N-trimethylammonium chloride Ethylenediamine tetra-acetic acid Electrophoretic mobility Electro-osmotic flow Electrospray ionization mass spectrometry 9-Fluorenylmethoxycarbonyl-Fluid phase endocytosis Intracellular Adhesion Molecule-1... 

As seen from Equation 1.7, the electro-osmotic flow depends on the dielectric constant and viscosity of pore fluid, as well as the surface charge of the solid matrix represented by the zeta potential (the electric potential at the junction between the fixed and mobile parts in the double layer). The zeta potential is a function of many parameters, including the types of clay minerals and ionic species that are present, as well as the pH, ionic strength, and temperature. If the cations and anions are evenly distributed, an equal and opposite flow occurs, causing the net flow to be zero. However, when the momentum transferred to the fluid in one direction exceeds the momentum of the fluid traveling in the other direction, electro-osmotic flow is produced. 

Ghosal, S., Lubrication theory for electro-osmotic flow in a microfluidic channel of slowly varying cross-section and wall charge. J. Fluid Mech, 2002, 459 103-128. 

Can the phenomenon of H" "-flux mediated electro-osmosis provide the biodynamic principle which we have been looking for It is clear that cellular transport processes such as axonal transport and cytoplasmic streaming could directly result from proto-osmosis if the ATPases inject protons into the filamentous systems. Ciliary movements could possibly result from a periodic proto-osmotic insurge of fluid in the cilia. A relative (contractile) movement of two sets of filaments could be caused by a proto-osmotic loop flow between the pairs of the filaments by means of the associated visous drag couple. But the question of the relevance of the effect can be solved only if it can be shown that the magnitude of the effect is sufficient, quantitatively, to explain at least one biodynamic phenomenon. The best-studied system in this context, from structural and biochemical aspects, is that of muscles. The necessary quantitative data for the comparision of theoretical result with experiments is available, but it has to be first confirmed that the conditions for the proposed electro-osmotic flow obtain in muscles. 

Sadr R. Yoda M. Zheng Z, Conlisk AT (2004) An experimental study of electro-osmotic flow in a rectangular microchannel. J Fluid Mech 506 357-367... 

Much work has been done using MD to investigate the transport laws or mechanisms of electro-osmotic flows (EOF) in smooth nanochannels. Qiao et al. reported the fluid properties near walls, such as viscosity and mobility, derived from the bulk values consequently they modified the Poisson-Boltzmann (PB) equation by introducing an electrochemical potential correction extracted from the ion distribution in a smaller channel using MD simulations, which predicted the ion distribution in larger channel widths with good accuracies [6]. Later on, a numerical evidence was reported for the self-similarity of ion distribution near walls within certain chaimel height limit. 

Computation of shear viscosity of hard spheres has been attempted using NEMD [11], Modified non-equilibrium molecular dynamics methods have also been developed for study of fluid flows with energy conservation [12], NEMD simulations have also been recently performed to compare and contrast the Poiseuille and Electro-osmotic flow situations. Viscosity profiles obtained from the two types of flows are found to be in good mutual agreement at all locations. The simulation results show that both type of flows conform to continuum transport theories except in the first monolayer of the fluid at the pore wall. The simulations further confirm the existence of enhanced transport rates in the first layer of the fluid in both the cases [13, 14]. 

Reproducible sample introduction is a crucial factor in p-chip-based electrophoretic separations. Various p-chip sample introduction schemes are illustrated in Fig. 2. Of the many proposed injection methods, electrokinetic injection based on electro-osmotic flow (EOF) is most commonly encountered on chips, because electrically driven fluid flow is easier to generate and control than pressure-driven flow. Electrokinetic sample injection is generally... 

S. Bhattacharyya, Z. Zheng, A.T. Conlisk, Electro-osmotic flow in two-dimensional charged micro and nanocharmels,/. Fluid Mech., 2005, 540, 247-267. 

Fig. 5. Electro-osmotic Flow. Flow profile of a fluid medium in an open tube with electro-osmotic flow, where is a velocity of the medium, d is the diameter of the tube, and dj the thickness of the double layer.

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