Ionic Origin of Electro-osmotic Flow Hysteresis

Abstract

Electro-osmotic flow, the driving of fluid at nano- or micro- scales with electric field, has found numerous applications, ranging from pumping to chemical and biomedical analyses in micro-devices. Electro-osmotic flow exhibits a puzzling hysteretic behavior when two fluids with different concentrations displace one another. The flow rate is faster when a higher concentration solution displaces a lower concentration one as compared to the flow in the reverse direction. Although electro-osmotic flow is a surface phenomenon, rather counter intuitively we demonstrate that electro-osmotic flow hysteresis originates from the accumulation or depletion of pH-governing minority ions in the bulk of the fluid, due to the imbalance of electric-field-induced ion flux. The pH and flow velocity are changed, depending on the flow direction. The understanding of electro-osmotic flow hysteresis is critical for accurate fluid flow control in microfluidic devices, and maintaining of constant pH in chemical and biological systems under an electric field.

Sr2Fe1+xRe1−xO6 double perovskites: magnetoresistance and (magneto)thermopower

“…This can be done only if the Fe and Re oxidation states are known. The latter determined by electron energy-loss magnetic chiral dichroism were found to be Fe 3+ and Re 5+ for Sr 2 FeReO 6 synthetized by using a Re excess [18][19][20]. The Fe K-edge XANES spectra of our three samples show similar features (Fig.…”

“…Indeed, as this compound crystallizes in the I4/m space group, with a≈5.67Å and c≈7.90Å, that is characteristic of Ae 2 FeMO 6 double perovskites, its formula must be written as Sr [16]. In the Sr 2 Fe 1+x Re 1-x O 6 series, the degree of cationic ordering (the distribution of Fe and Re on 2a and 2b Wyckoff sites) has an effect on the magnetic properties, whatever x [17][18][19][20]. We report on the synthesis, structural characterization by X-ray powder diffraction (XRPD) and transmission electron microscopy (TEM) of polycrystalline samples of the Sr 2 Fe 1+x Re 1-x O 6 series.…”

“…For instance, high pressure is required [17] or a Re excess is needed at normal pressure [18][19][20]. In normal synthesis condition, the x=0 compound tends to form anti-site defects detrimental to the ferrimagnetic ordering, leading thus to a reduced saturation magnetization (M s ) and to a T C increase [18][19][20]. In our synthesis conditions, as an impurity is observed in the XRPD pattern of Sr 2 FeReO 6 consequently our efforts were put on the Sr 2 Fe 1.33 Re 0.67 O 6 (x=0.33) monophasic sample (Fig.…”

Electrokinetic instability in microchannel ferrofluid/water co-flows

“…Another circumstance involving fluid property variation is the electrokinetic flow of two fluids that are either displacing 31 32 or co-flowing 33 34 with each other in single microchannels. The electrokinetic displacement of fluids takes place in, for example, zeta potential measurement via electric current monitoring 35 and sample stacking via isotachophoresis 36 etc.…”

Electroosmotic Flow in Microchannel with Black Silicon Nanostructures

“…EOF of two solutions with dissimilar ionic species [ 65 , 66 ] or large concentration difference [ 67 , 68 , 69 ] causes accumulation/depletion of the main constituent ionic species or pH-governing minority ions, which induces pH changes and demonstrates hysteresis phenomenon, whereby the EOF flow rate for solution A displacing solution B is different from that of solution B displacing solution A. Moreover, electrolysis at the electrodes generates hydronium (H 3 O + ) and hydroxide (OH − ) ions that will alter the pH in the reservoirs [ 70 ], which in turn affect the EOF velocity.…”