Electro-osmotic flow in a configuration with a center stream flowing between two parallel sheath streams with mismatched electrical conductivities is known to exhibit an electrokinetic instability (EKI). This flow instability, with orthogonal conductivity gradient and electric field, is characterized by either wavy or pearl-necklace type structures depending upon the relative conductivities of center and sheath streams. In this paper, we propose a physical mechanism underlying such wavy and pearl-necklace type structures of the EKI. In order to verify the proposed mechanism, we perform EKI experiments in a cross-shaped microchannel at varying electric fields and for two flow configurations wherein the center stream has either higher or lower conductivity than the sheath streams. Using dynamic mode decomposition of time-resolved experimental data, we identify the spatio-temporal coherent structures that represent the dynamics of instability. These coherent structures provide a comprehensive validation of the proposed physical mechanism. In addition, experimentally observed coherent structures provide valuable insight into the dynamics and the spatio-temporal scales of the EKI.

Abstract

Field amplified sample stacking (FASS) uses differential electrophoretic velocity of analyte ions in the high‐conductivity background electrolyte zone and low conductivity sample zone for increasing the analyte concentration. The stacking rate of analyte ions in FASS is limited by molecular diffusion and convective dispersion due to nonuniform electroosmotic flow (EOF). We present a theoretical scaling analysis of stacking dynamics in FASS and its validation with a large set of on‐chip sample stacking experiments and numerical simulations. Through scaling analysis, we have identified two stacking regimes that are relevant for on‐chip FASS, depending upon whether the broadening of the stacked peak is dominated by axial diffusion or convective dispersion. We show that these two regimes are characterized by distinct length and time scales, based on which we obtain simplified nondimensional relations for the temporal growth of peak concentration and width in FASS. We first verify the theoretical scaling behavior in diffusion‐ and convection‐dominated regimes using numerical simulations. Thereafter, we show that the experimental data of temporal growth of peak concentration and width at varying electric fields, conductivity gradients, and EOF exhibit the theoretically predicted scaling behavior. The scaling behavior described in this work provides insights into the effect of varying experimental parameters, such as electric field, conductivity gradient, electroosmotic mobility, and electrophoretic mobility of the analyte on the dynamics of on‐chip FASS.

Zinc Sulfide nanoparticles were prepared by chemical rout i.e. co-precipitation method. X-ray diffraction profiles of ZnS have been conformed as single phase with hexagonal structure. And crystalline in nature. The lattice parameters of prepared material is a= 3.8314A0 c=6.2431A0 with space group P63mc. The particle size was determined by scherer formula and found to be 28 nm. The band gap energy of ZnS nanoparticles was determined by optical absorption experiment and found to be 3.68 eV at 300oK. Photoluminescence spectra ware recorded by luminescence spectrophotometer. All the plots contains two peak centered at 315 nm and 425 nm. The excitation wavelength was 250 nm. Appearance of broad peaks centered at 425 nm is attributed to the presence of sulphur vacancies in the lattice.

Current monitoring method for measurement of EOF in microchannels involves measurement of time-varying current while an electrolyte displaces another electrolyte having different conductivity due to EOF. The basic premise of the current monitoring method is that an axial gradient in conductivity of a binary electrolyte in a microchannel advects only due to EOF. In the current work, using theory and experiments, we show that this assumption is not valid for low concentration electrolytes and narrow microchannels wherein surface conduction is comparable with bulk conduction. We show that in presence of surface conduction, a gradient in conductivity of binary electrolyte not only advects with EOF but also undergoes electromigration. This electromigration phenomenon is nonlinear and is characterized by propagation of shock and rarefaction waves in ion concentrations. Consequently, in presence of surface conduction, the current-time relationships for forward and reverse displacement in the current monitoring method are asymmetric and the displacement time is also direction dependent. To quantify the effect of surface conduction, we present analytical expressions for current-time relationship in the regime when surface conduction is comparable to bulk conduction. We validate these relations with experimental data by performing a series of current monitoring experiments in a glass microfluidic chip at low electrolyte concentrations. The experimentally validated analytical expressions for current-time relationships presented in this work can be used to correctly estimate EOF using the current monitoring method when surface conduction is not negligible.