Session R3: Electrokinetics: Surface and Particle Induced Flows

Interplay of induced charge electroosmosis, electrothermal flow, and dielectrophoresis at insulating constrictions

We present a theoretical and experimental study on the combined influence of induced charge electroosmotic flow (ICEO) and electrothermal flow on particle motion in an insulator based dielectrophoretic (iDEP) device. Strong electric fields used for particle trapping induce charges on the channel wall of low, but finite permittivity [1], and also induce strong temperature gradients [2] because of Joule heating. Consequently, the background fluid flow near the constriction is a superposition of these two effects. Our analysis presents a hitherto unexplored interplay between these two effects and how they influence particles which also experience dielectrophoresis. From our analysis, we find that for channels of low surface permittivity and conductivity, electrothermal effects are stronger near the constriction compared to ICEO effects, while the opposite is true when the surface permittivity or conductivity (or both) are comparable to that of bulk fluid. The analysis also includes the pH and electrolyte concentration dependent contributions of the dynamic Stern layer on ICEO flow. \\[4pt] [1] Zhao, C.; Yang, C. AC Field Induced-Charge Electroosmosis over Leaky Dielectric Blocks Embedded in a Microchannel. Electrophoresis 2011, 32, 629--637.\\[0pt] [2] Hawkins, B. G.; Kirby, B. J. Electrothermal Flow Effects in Insulating (electrodeless) Dielectrophoresis Systems. Electrophoresis 2010, 31, 3622--3633.   

3D experimental investigation of the interplay between dielectrophoresis and induced-charge electroosmosis

It is well-known that the advent non-linear electrokinetic flows, such as induced-charge electroosmosis, are strongly dependent on the frequency of the applied field. However, to date, there exists no unifying theory which can exactly predict both the strength and frequency dispersion of such electrokinetic flows. Using microPIV and temperature sensitive dyes we demonstrate the presence of a number of competing non-linear effects including dielectrophoresis, electrothermal flow and wall effects which compete with induced-charge electrokinetic flow, potentially causing a distortion of both the strength and frequency dispersion predicted for pure induced-charge effects. In terms of the wall effects, we investigate the differences between channels in which the walls are conducting (the field is perpendicular to the wall) and insulating (the field is parallel to the wall). This work is of both fundamental and practical importance and may be used to further refine non-linear electrokinetic theory and optimize the flow parameters of electroosmotic pumps and the mobility of electrokinetically driven micromotors or carriers in lab-on-a-chip analysis systems.   

Induced- and alternating-current electro-osmotic control of the diffusion layer growth in a microchannel-membrane interface device

The passage of an electric current through an ionic permselective medium under an applied electric field is characterized by the formation of ionic concentration gradients, which result in regions of depleted and enriched ionic concentration at opposite ends of the medium. Induced-current electro-osmosis (ICEO) and alternating-current-electro-osmosis (ACEO) are shown to control the growth of the diffusion layer (DL) which, in turn, controls the diffusion limited ion transport through the microchannel-membrane system. We fabricated and tested devices made of a Nafion membrane connecting two opposite PDMS microchannels. An interdigitated electrode array was embedded within the microchannel with various distances from the microchannel-membrane interface. The induced ICEO (floating electrodes) / ACEO (active electrodes) vortices formed at the electrode array stir the fluid and thereby suppress the growth of the DL. The intensity of the ACEO vortices is controlled by either varying the voltage amplitude or the frequency, each having its own unique effect. Enhancement of the limiting current by on-demand control of the diffusion length is of importance in on-chip electro-dialysis, desalination and preconcentration of analytes.   

Fluidic Dielectrophoresis of Aqueous Electrical Interfaces

To date, alternating current (AC) electric fields have been exploited to dielectrophoretically manipulate bubbles, liquid drops, particles, biomolecules and cells. Research and applications in this area, however, has been primarily limited to the interfaces formed between two immiscible metal-liquid, particle-liquid, or gas-liquid surfaces on particles. The influence of AC electric fields across aqueous liquid-liquid interfaces remains relatively unexplored. Fundamentally, many electrokinetic phenomena arise from discontinuities in ionic flux and charge accumulation at electrical interfaces, and here I explore the influence of AC electric fields on the electrical interface created between two aqueous liquids with disparaging electrical properties Using a microfluidic channel with embedded electrodes, two fluid streams - one with a greater electrical conductivity, the other a greater dielectric constant - were made to flow side-by-side. An AC electric field was applied across the flow channel and fluid was observed to displace across the phase interface. The displacement direction is AC frequency dependent, and is attributed to the Maxwell-Wagner interfacial polarization at the liquid-liquid electrical interface. At low AC frequency, below the interfacial charge relaxation time, the high conductive stream is observed to displace into the high dielectric stream. Above this frequency, the direction of liquid injection reverses, and the high dielectric stream injects into the high conductivity stream. An analytical model is presented for this liquid crossover frequency, and applied towards biosensing applications.   

Bifurcation in the equilibrium height of colloidal particles near an electrode in oscillatory electric fields

Application of an oscillatory electric field is known to alter the equilibrium separation distance between micron-scale colloidal particles and an adjacent electrode. This behavior is believed to be partially due to a lift force caused by electrohydrodynamic (EHD) flow generated around each particle, with previous work focused on identifying a single equilibrium height of the individual particles over the electrode. Here we report the existence of a pronounced bifurcation in the equilibrium particle height in response to low frequency electric fields. Optical and confocal microscopy observations reveal that application of a $\sim$100 Hz field induces some of the particles to rapidly move several particle diameters up from the electrode, while the others move closer to the electrode. The fraction of particles that exhibit this ``extreme levitation'' increases with increased applied potential and decreased frequency, in a fashion qualitatively consistent with an energy landscape predicated on competition between EHD flow, colloidal interactions, and gravity. Taken together, the results provide evidence for the existence of a deep tertiary minimum in the electrode-particle interaction potential at a surprisingly large distance from the electrode.   

The influence of ionic strength on electrohydrodynamic aggregation of colloidal particles

Colloidal particles suspended in various electrolytes have been widely observed to aggregate near electrodes in response to oscillatory electric fields, a phenomenon believed to result from electrohydrodynamic flows induced around the particles. Most work has focused on elucidating the effects of the applied field strength, frequency, and electrolyte type on the aggregation rate, with less attention paid to the ionic strength. Here we demonstrate that the ionic strength of the electrolyte strongly affects both the aggregate morphology and aggregation dynamics. Optical microscopy observations reveal that an applied field causes micron-scale colloids in aqueous NaCl to rapidly aggregate over a wide range of ionic strengths, but with significant differences in aggregate morphology: at higher ionic strengths ($\sim$1 mM), particles arrange as hexagonal close packed (HCP) crystals, but at lower ionic strengths($\sim$0.2 mM), the particle aggregates are randomly closed packed (RCP). We interpret these results in terms of the effect of the ionic strength on the height of the particles over the electrode and their corresponding diffusivity, and we discuss preliminary modeling efforts of the effect of ionic strength on the electrohydrodynamic driving force for aggregation.   

Wall-Induced Non-inertial Lift in Electrophoresis for Continuous Particle Separation

We present in this talk a novel continuous-flow electrokinetic method for particle separation based on intrinsic properties which may include size, surface charge, shape and potentially deformability. This method utilizes the wall-induced non-inertial lift force to deflect a sheath flow-focused particle mixture to property-dependent positions in a laminar flow through a straight microchannel. It is demonstrated through both a binary and a ternary separation of polymer particles by size. A numerical model is also developed to understand this separation and to study the parametric effects on it. The numerical predictions are found to agree reasonably with the experimental observations.   

Effect of the electric field ratio on electroosmotic flow patterns in cross-shaped microchannels by the lattice-Boltzmann Method

The electroosmotic flow (EOF) pattern characteristics in cross-shaped microchannels flow are important features when either suppressing or enhancing flow features for injection and separation or mixing of multiple species are the wanted objectives. There are situations in EOF in cross-shaped microchannels where the fluid flows toward unexpected and unwanted directions under a given external electric field that depends of both the applied electric field and lengths of the different channels. This article describes the effect of the electric field ratio, defined as the ratio between longitudinal nominal electric field E$_{Long}=$(V$_{E}$-V$_{W})$/(L$_{W}+$L$_{E})$ and the nominal electric field E$_{a}=$(V$_{S}$-V$_{E})$/(V$_{S}+$V$_{E})$, where E, S and W define the east, south and west directions of the cross-shaped microchannel; V is the externally applied voltage and L is the length, on the EOF characteristics in a cross-shaped microchannel. We use the lattice-Boltzmann method (LBM) for solving the discretized Boltzmann Transport Equation (BTE) describing the coupled processes of hydrodynamics and electrodynamic. Our numerical simulations allow us to determine the EOF pattern for a wide range of the electric field ratio and E$_{a}$ such that inverted flow features are captured and described, which are very important to determine for flow separation or mixing.   

Electrical Power Generation by Mechanically Modulating Electrical Double Layers

Since Michael Faraday and Joseph Henry made their great discovery of electromagnetic induction, there have been continuous developments in electrical power generation. Most people today get electricity from thermal, hydroelectric, or nuclear power generation systems, which use this electromagnetic induction phenomenon. Here we propose a new method for electrical power generation, without using electromagnetic induction, by mechanically modulating the electrical double layers at the interfacial areas of a water bridge between two conducting plates. We find that when the height of the water bridge is mechanically modulated, the electrical double layer capacitors formed on the two interfacial areas are continuously charged and discharged at different phases from each other, thus generating an AC electric current across the plates. We use a resistor-capacitor circuit model to explain the results of this experiment[1]. This observation could be useful for constructing a micro-fluidic power generation system and for understanding the interfacial charge distribution in solid-liquid interfaces in the near future.\\[4pt] [1] J. K. Moon, J. Jeong, D. Lee, and H. K. Pak, Nature Communications, 4,1487 doi: 10.1038/ncomms2485   

Magnetically Tuned Porous Electrode Formation in Electrochemical Flow Capacitor

In electrochemical flow capacitors (EFCs), high surface area, conducting, porous particles suspended in an electrolyte solution flow from one storage tank to another through a charging/discharging device between collecting electrodes (collector). In the collector, the particles quickly aggregate to form percolated, electrically conducting networks that facilitate electron flow. To achieve a highly conductive and rapidly assembling network, a high concentration suspension is needed. To facilitate easy pumping, a low concentration suspension is desired. To speed up the network formation process and overcome these conflicting requirements, it is possible to use magnetizable colloids. The particles will acquire a magnetic moment in the presence of an external magnetic field. The magnetic moment will reversibly disappear as soon as the magnetic field is removed. The magnetic field will be applied during the charge and discharge phases to accelerate the formation of electrically connected networks when desired and will be removed when it is time to flow the slurry and refresh the contents in the collector. In this study, we explore the network assembly process, and estimate the network connectivity and electric properties.   

Shape Oscillation of a Sessile Drop Under the Effect of Amplitude-Modulated High Frequency Magnetic Field

The shape oscillation of a sessile mercury drop under the effect of high frequency amplitude-modulated magnetic field (AMMF) is investigated experimentally. It is a new method to excite shape oscillation of a liquid metal sessile drop, which is different from the case in the presence of a low-frequency magnetic field. The high frequency AMMF is generated by a solenoid inductor fed by a specially designed alternating electric current. The surface contour of the sessile drop is observed by a digital camera. At a given frequency and magnetic flux density of the high frequency AMMF, the edge deformations of the drop with an azimuthal wave numbers (modes n $=$2, 3, 4, 5, 6) were excited. A stability diagram of the shape oscillation of the drop is obtained by analysis of the experimental data. It is found that the same oscillation mode is excited in different frequency range, and the corresponding frequencies have a ratio of 2. This is a typical character of Mathieu-type parametric instability of a liquid drop.