ELECTRO-OSMOTIC FLOW OF BILAYER FLUIDS IN A MICROCHANNEL WITH SURFACE HETEROGENEITIES

Fluidic interfaces disintegrate under sufficiently strong electric fields,\nleading to electrohydrodynamic (EHD) tip streaming. Taylor cones, which emit\ncharged droplets from the tip of a conical cusp, are among the most prominent\nand well-studied examples of EHD instabilities. In liquid-liquid systems, more\ncomplex interface deformation modes than simple Taylor cones can be observed,\nwith the interface being pushed away from the electrode, and additional cone\nstructures emerging from the rim of the dimple. In this article, we investigate\nthe mechanisms behind these deformation modes experimentally and numerically,\nand demonstrate that the presence of droplets triggers the dimple at the\ninterface. In order to characterize the underlying processes, we replace the\npin electrode by a hollow metallic needle with a prescribed electrolyte volume\nflow. The submerged electrospray introduces droplets of an aqueous KCl solution\nwith varying ion concentrations into silicone oils with varying viscosities. By\nmeasuring the corresponding electric current and by optical investigation of\nthe interface deformation, we study the system response to variations of the\nionic concentration, viscosity, applied voltage as well as flow rate. In\naddition to the experiments, we use a finite element solver and compute the\ncharge transport due to the droplets in the oil phase. Further, we compute the\nelectric potential distribution, flow field and interface deformation. After\ncalibration of our model with particle tracking velocimetry data of the flow\ninside the oil phase, we reproduce the experimentally observed dimple at the\nliquid-liquid interface. In summary, this work highlights the importance of\ncharged droplets for the complex dynamic modes observed when a liquid-liquid\ninterface is exposed to a local electric field.\n

Capillary forces between particles at a liquid interface: General theoretical approach and interactions between capillary multipoles

Chapter 4 - Single-Phase Electrokinetic Flow in Microchannels

We study the dynamics of the deformation of a soft liquid-liquid interface by the optical radiation pressure of a focused cw Gaussian laser beam. We measured the temporal evolution of both the hump height and the hump curvature by direct observation and by detecting the focusing effect of the hump acting as a lens. Extending the results of Yoshitake [J. Appl. Phys. 97, 024901 (2005)] to the case of liquid-liquid interfaces and to the Bo approximately =1 regime [Bo=(omega0/lc)2, , where omega0 is the beam waist and lc the capillary length], we show that, in the Bo<<1 and Bo approximately =1 ranges, the small-amplitude deformations are correctly described by a linear hydrodynamic theory predicting an overdamped dynamics. We also study the dynamics of the large-amplitude interface deformations at the onset of optohydrodynamic instability [Phys. Rev. Lett. 90, 144503 (2003)]. Using a simple, phenomenological model for the nonlinear evolution of the hump height, we interpret the observed interface dynamics at the instability onset as the signature of an imperfect subcritical instability.

Electroosmotic Flow in a Capillary Annulus with High Zeta Potentials

The shape of a liquid-air interface advancing on a heterogeneous surface was studied experimentally, together with the force induced by the pinning of the contact line to surface defects. Different surfaces were considered with circular defects introduced as arrays of cocoa butter patches or small circular holes. These heterogeneous surfaces were submerged in aqueous ethanol solutions while measuring the additional force arising from the deformation of the advancing contact line and characterizing the interface shape and its pinning on the defects. Initially, the submersion force is linear with submerged depth, suggesting a constant defect-induced stiffness. This regime ends when the contact line depins from the defects. A simple scaling is proposed to describe the depinning force and the depinning energy. As the defect separation increases, the interface stiffness is found to increase too, with a weak dependency on the defect radius. This interaction between defects cannot be captured by simple scaling but can be well predicted by a theory considering the interface deformation in the presence of a periodic arrays of holes. Creating a four-phase contact line by including solid defects (cocoa butter) reduced pinning forces. The radius of the defect had a nonlinear effect on the depinning depth. The four-phase contact line resulted in depinning before the defects were fully submerged. These experimental results and the associated theory help to understand quantitatively the extent to which surface heterogeneities can slow down wetting. This in turn paves the way to tailoring the design of heterogeneous surfaces toward desired wetting performances.

Simulation of an optically induced asymmetric deformation of a liquid–liquid interface

Flow-induced dynamic free-surface deformations are experimentally studied in a confined liquid volume of 5cSt silicone oil (Prandtl number Pr = 68). The geometry of the problem is a liquid column concentrically surrounded by an annular gas channel. A gas stream entering the duct from the top or bottom entrains the motionless liquid. The dynamic deformation of the gas–liquid interface is caused by a steady axisymmetric shear-driven flow. The experiments are performed in normal gravity conditions and the static deformation of a liquid bridge interface is unavoidable. The magnitude and shape of the dynamic surface deformation are analyzed using optical measurements with a comprehensive treatment of the images. The deviation of the free surface shape from the corresponding equilibrium profile is determined with an uncertainty of about 0.1 μm. The order of magnitude of the interface deformation is proportional to the capillary number, which is defined as the ratio of the viscous force per unit area to the capillary pressure. The study is performed for a large range of volumes and aspect ratios as well as for different gas velocities. As a general trend, the dynamic deformation grows with the gas velocity, which plays the role of a driving force, but a linear dependence is not observed for all volume ratios, despite the small Reynolds numbers, 280 &amp;lt; Reg &amp;lt; 560. The dynamic deformation displays a strong dependence on the liquid volume ratio and the direction of the gas stream parallel to the interface. When the gas flow is directed against gravity, the largest interface deformations are observed at the smallest volumes among the analyzed ones. In contrast, when the gas stream is aligned with gravity, then the deformations decrease with a decrease in the volume ratio, at a certain value attaining zero (vanishing) and then changing sign.

Thin electroosmotic flow (EOF) micropumps can generate flow in confined spaces such as lab-on-a-chip microsystems and implantable drug delivery devices. However, status quo methods for quantifying flow and other important parameters in EOF micropumps depend on microfluidic interconnects or fluorescent particle tracking: methods that can be complex and error-prone. Here, we present a novel connected droplet shape analysis (CDSA) technique that simplifies flow rate and zeta potential quantification in thin EOF micropumps. We also show that a pair of droplets connected by an EOF pump can function as a tunable convex lens system (TCLS). We developed a biocompatible and all polymer EOF micropump with an SU-8 substrate and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrodes. We microdrilled a channel through the electrode/SU-8/electrode layers to realize a monolithic EOF micropump. Then, we deposited a pinned droplet on each end of the microchannel so that it connected them. By controlling the EOF between the droplets and measuring the corresponding change in their shape, we quantified the nanoliter EOF rate and zeta potential at the interface of SU-8 with two liquids (deionized water and a l-glutamate neurotransmitter solution). When the droplet pair and pump were used as a TCLS, CDSA successfully predicted how the focal length would change when the pump drove fluid from one droplet to another. In summary, CDSA is a simple low-cost technique for EOF rate and zeta potential measurement, and a pair of droplets connected by an EOF micropump can function as a TCLS without any moving parts.

Thickness-averaged model for numerical simulation of electroosmotic flow in three-dimensional microfluidic chips

Electroosmotic flow in microchannels with arbitrary geometry and arbitrary distribution of wall charge

Electroosmotic Flow in Heterogeneous Microchannels

The electrohydrodynamic and stress instability of the interface between two viscous fluids with different electrical properties under tangential electric fields in a microchannel is analytically and experimentally investigated. In the analytical model, the two-layer system is subjected to a tangential electric field. There is no assumption on the magnitude of the ratio of fluid to electric time scales, and thus the linear Poisson-Boltzmann equation are solved using separation of variable method for densities of bulk charge and surface charge; the electric field and fluid dynamic are coupled only at the interface through the stress balance equations. Under constant flowrates, the fractions of the fluids are calculated for different parameters. Using the calculated fractions, the stability of the system can be determined according to the linear perturbation theory. In the experiments, two immiscible fluids, aqueous NaHCO3 (conducting fluid) and silicone oil (non-conducting fluid) are pumped into a PMMA microchannel. The tangential electric field is added to the aqueous NaHCO3 using a high voltage power supply. The results are recorded using a CCD camera. The results show that the electric field can have either destabilizing or stabilizing effect depending on the ratios of viscosity of the two fluids. The flowrates, zeta potential, and dimension of the microchannel affect the growth rate of the perturbation. Both experimental and analytical results show a good agreement. Keywords: Interface, constant flowrate, linear instability, microchannel, TAS, conducting fluid, electroosmotic flow, VALIDATION, Viscosity

Purpose The characteristics of fluid motions in micro-channel are strong fluid-wall surface interactions, high surface to volume ratio, extremely low Reynolds number laminar flow, surface roughness and wall surface or zeta potential. Due to zeta potential, an electrical double layer (EDL) is formed in the vicinity of the wall surface, namely, the stern layer (layer of immobile ions) and diffuse layer (layer of mobile ions). Hence, its competent designs demand more efficient micro-scale mixing mechanisms. This paper aims to therefore carry out numerical investigations of electro osmotic flow and mixing in a constricted microchannel by modifying the existing immersed boundary method. Design/methodology/approach The numerical solution of electro-osmotic flow is obtained by linking Navier–Stokes equation with Poisson and Nernst–Planck equation for electric field and transportation of ion, respectively. Fluids with different concentrations enter the microchannel and its mixing along its way is simulated by solving the governing equation specified for the concentration field. Both the electro-osmotic effects and channel constriction constitute a hybrid mixing technique, a combination of passive and active methods. In microchannels, the chief factors affecting the mixing efficiency were studied efficiently from results obtained numerically. Findings The results indicate that the mixing efficiency is influenced with a change in zeta potential (ζ), number of triangular obstacles, EDL thickness (λ). Mixing efficiency decreases with an increment in external electric field strength (Ex), Peclet number (Pe) and Reynolds number (Re). Mixing efficiency is increased from 28.2 to 50.2% with an increase in the number of triangular obstacles from 1 to 5. As the value of Re and Pe is decreased, the overall percentage increase in the mixing efficiency is 56.4% for the case of a mixing micro-channel constricted with five triangular obstacles. It is also vivid that as the EDL overlaps in the micro-channel, the mixing efficiency is 52.7% for the given zeta potential, Re and Pe values. The findings of this study may be useful in biomedical, biotechnological, drug delivery applications, cooling of microchips and deoxyribonucleic acid hybridization. Originality/value The process of mixing in microchannels is widely studied due to its application in various microfluidic devices like micro electromechanical systems and lab-on-a-chip devices. Hence, its competent designs demand more efficient micro-scale mixing mechanisms. The present study carries out numerical investigations by modifying the existing immersed boundary method, on pressure-driven electro osmotic flow and mixing in a constricted microchannel using the varied number of triangular obstacles by using a modified immersed boundary method. In microchannels, the theory of EDL combined with pressure-driven flow elucidates the electro-osmotic flow.

The rotating movement of three immiscible fluids—a benchmark problem