Session P09: Electrokinetic Flows II

Electrokinetic Pumping Flow Model in a Microchannel with Squeezing Walls

The nonlinear Poisson–Boltzmann equation coupled with lubrication theory are used to model the flow motion in a microchannel with squeezing walls. The analysis is also used to describe the distribution of the electric potential across the electric double layer. A microfluidic pumping flow model driven by electroosmosis phenomena is developed to analyze the flow behavior of aqueous electrolytes. The solution is obtained without the classical use of the Debye–Hückel linearization technique and has been kept general. The effects of Debye length, zeta potential, and electric field on the pressure distribution, velocity profiles, shear stress, potential distribution, and net flow rate are presented. The results have shown that the electrokinetic pumping phenomena can possibly be used to understand microscale flow transport in various physiological systems.   

Possibility of Obtaining Two Orders of Magnitude Larger Electrokinetic Streaming Potential through Liquid Infiltrated Surfaces

Electrokinetic flow emerges as a promising method for liquid energy harvesting. However, the generated streaming potential on a smooth surface is still too low for practical applications. To solve this problem, we employed slippery liquid-filled surface to introduce charged slipping interfaces for enhanced electrokinetic effects. It is shown that the magnitude of the streaming potential can be significantly enhanced from ∼0.02 V to as much as ∼1.6 V, in electrokinetic flows through microchannels. This was done through flows on liquid-filled surfaces, where the grooves were filled with oils of viscosity in the range 30–3000 mPa·s. The presence of immiscible oils and the improved slip are both factors that could significantly increase the streaming potential. The analytical relationship between streaming potential and filled lubricant viscosity was derived and verified through corresponding experimental results. The work yields novel insights into complex electrolyte flows and indicates avenues for more efficient energy harvesting.   

A simple electrokinetic nanorotor

The phenomenon of dielectric particles tumbling spontaneously in low-conductivity fluids under strong DC electric fields is known as Quincke rotation. For this to happen, the charge relaxation time of the particle must typically be longer than that of the liquid, which normally requires that the particle be less conductive than the surrounding medium. Here, we show that semiconducting nanowires in deionized water, when subject to high-frequency AC field, can also exhibit Quincke rotation. We explain the tumbling by analyzing the torque balance between the moments of the electroosmotic flows, the electric force on the induced dipole, and the Stokes drag. In addition, we suggest that this Quincke rotation starts at a critical field strength and frequency. This work demonstrates a simple concept for a continuous nanorotor in aqueous solution using a stationary, linearly polarized electric field.   

A Discrete Ion Stochastic Continuum Overdamped Solvent Algorithm for Modeling Electrolytes

In this talk we present a methodology for the mesoscale simulation of strong electrolytes. This is an extension of the Fluctuating Immersed Boundary (FIB) approach that treats a solute as discrete Lagrangian particles that interact with Eulerian hydrodynamic and electrostatic fields. In both cases the Immersed Boundary (IB) method of Peskin is used for particle-field coupling. Hydrodynamic interactions are taken to be overdamped, with thermal noise incorporated using the fluctuating Stokes equation, including a "dry diffusion" Brownian motion to account for scales not resolved by the coarse-grained model of the solvent. Long range electrostatic interactions are computed by solving the Poisson equation, with short range corrections included using a novel immersed-boundary variant of the classical Particle-Particle Particle-Mesh (P3M) technique. Also included is a short range repulsive force based on the Weeks-Chandler-Andersen (WCA) potential. The new methodology is validated by comparison to theory for ion-ion pair correlation functions, for conductivity (including the Wien effect for strong electric fields), and by comparison to theory and existing numerical results for electro-osmotic flows.   

Modeling Induced-Charge Electroosmosis using a meso-scale approach

We apply a recently developed method for simulating electrolytes at the mesoscale, the Discrete Ion Stochastic Continuum Overdamped Solvent (DISCOS) algorithm, to study the complex electrokinetic effect Induced Charge Electro-Osmosis (ICEO). Electrokinetic flows are important in many applications such as microfluidic pumping and desalination, and in understanding a range of biological processes. ICEO is specifically interesting because an applied field can be used generate bulk flows in an electroneutral system. One crucial aspect of ICEO is the development of electric double layers. Thermal fluctuations play a crucial role in this process, which cannot be captured at this scale by a purely continuum model. While molecular dynamics can faithfully model the double layer, high computational cost makes it challenging to apply this approach to real applications. To overcome these challenges we employ DISCOS, where the solvent is modeled using continuum fluctuating hydrodynamics, and ions are treated discretely using the immersed boundary method. We compare our results to experiments and theories using simulations with a ζ-potential below and on the order of the thermal voltage, and explore applied electric fields outside of this parameter space.   

Fluid elasticity enhanced insulator-based dielectrophoresis in a very dilute polymer solution

Insulator-based dielectrophoresis (iDEP) is an emerging technique that has been increasingly used for particle and cell manipulation in microfluidic devices. However, the majority of the previous studies on iDEP is based upon Newtonian fluids. We demonstrate in this work that the addition of a very small amount of polyethylene oxide (PEO) polymer (~50 ppm) into a buffer solution can significantly enhance the iDEP focusing of particles in a constriction microchannel. We also perform a parametric study of the effects of polymer concentration, polymer molecular weight, and particle size on such fluid elasticity enhanced iDEP (E-iDEP).   

Confinement-dependent diffusiophoretic transport of nanoparticles in collagen hydrogels

The transport of nanoparticles in biological hydrogels is often hindered by the strong confinement of the media, thus limiting important applications such as drug delivery and disinfection. Here, we investigate nanoparticle transport in collagen hydrogels driven by diffusiophoresis. Contrary to common expectations for boundary confinement effects where the confinement hinders diffusiophoresis, we observe a non-monotonic behavior in which maximum diffusiophoretic mobility is observed at intermediate confinement. We find that such behavior is a consequence of the interplay be- tween multiple size-dependent effects. Our results display the utility of diffusiophoresis for enhanced nanoparticle transport in physiologically relevant conditions under tight confinement, suggesting a potential strategy for drug delivery in compressed tissues.   

Particle and Fluid Motion in Microchannels under Transverse Oscillatory Fields

Asymmetric rectified electric fields (AREFs) have recently been established to induce a steady electric field in response to an applied oscillatory electric field, provided the ions present have unequal mobilities. Here we examine the behavior of colloidal particles suspended in a thin flat microchannel between parallel electrodes. For unimodal fields, we demonstrate that the pseudo-steady particle distribution is consistent with AREFs acting upon both the particles and the channel surfaces. Theoretical calculations of the 2D fluid flow field inside the channel with an electroosmotic slip boundary condition induced by the AREF provides predictions for the net drag forces acting upon the particles that accord with experimentally observed particle distributions. These findings point to application of oscillatory fields for tunable particle manipulation and micron-scale assembly processes.   

Numerical simulations of electrolyte jets in an electric field

When an electrolyte jet is injected through a grounded nozzle into a region with an electric field, we observe non-axisymmetric whipping instabilities in the jet. These instabilities are characterized by large scale violent, chaotic and quick whips of the jet. This system is numerically modeled using an electrohydrodynamic formulation that includes the Nernst-Planck model for ion transport with an aim to investigate the origin and propagation of the instabilities in the jet. Numerical results are being verified with experimental studies performed at the Multiphase & Cardiovascular Flow Laboratory (MCFL) of the University of Washington where such instabilities are observed in a sodium chloride jet. In this talk, we discuss the formulation and modeling of electrolyte jets using a Poisson–Nernst–Planck (PNP) model. Ion advection, electrodiffusion and electromigration have been tested qualitatively and quantitatively. Simulating this process will help gain an in-depth insight into the complex physical phenomena that occur in electrolytic multiphase flows with applications in fields such as biophysics, electrochemistry, nanofluidics and solid-state physics.   

Non-antiperiodic electric fields with zero time-average yield net electrophoresis

We report that multimodal periodic electric fields with time asymmetry but zero time average induce controllable net electrophoretic motion of colloids. Net motion is observed both experimentally and numerically if the electric field has two frequency modes that are the ratio of odd and even numbers (e.g., 3 Hz and 2 Hz). Furthermore, the direction of motion is reversed by changing the sign of the applied waveform, for example by swapping which electrode is powered and grounded. We provide generalized theoretical arguments indicating that non-antiperiodic force excitations will yield net motion in isotropic media, provided there is a nonlinear component in the equation of motion, and we show how similar ratchet-like responses occur in other mechanical systems involving nonlinearities.   

Electroosmotic flow in a thin microchannel when the slippage condition and the viscosity of the electrolytic solution depend on the temperature

Based on a perturbative scheme, we treat in the present work, a non-isothermal electroosmotic pattern flow in a rectangular microchannel. For this purpose, we consider that a non-uniform slip regime prevails, controlling simultaneously the main characteristics of the velocity field induced externally by a uniform electric field. Taking into account the proper limitations of the asymptotic analysis, we add full numerical solutions of the governing equations, which basically are composed of the continuity, momentum, and energy equations for the electrolyte flow, and the related with the electrical phenomenon: the equation of Poisson-Boltzmann. A direct comparison between both asymptotic and numerical solutions shows favorable results for those particular cases when the viscosity of the fluid and the slip condition depend on the temperature. The impact of the temperature is readily appreciated through the well-known Joule heating effect, which is yielded by the external electric field on the electrolyte solution. Due to that, the previous effect heats the electrolyte solution in the entire domain, and to avoid overheating for the electroosmotic flow, we impose on the boundaries of the microchannel, a cooling convective condition. Therefore, the full set of the dimensionless equations depends on several parameters that determine between them the different scenarios for the induced volumetric flow rate. In addition, we complete the results by adding the corresponding velocity, pressure, and temperature fields. The results show clearly that the variable effect of slippage favors notably the increment of the volumetric flow rate.   

A novel phenomena of EHD vortices as a promising technology for air purification application

Oscillations of a pair of sub-milli scale water droplets in air, created using non-invasive electric force, is studied using experimental, numerical and analytical methods. Nonlinear features of oscillations such as bifurcation between two critical states of drop deformations and nonlinearly beating oscillations of drops were quantified upon processing high speed imaging results. Such electrically triggered oscillations of drops create vortices around the droplets, which are engineered into novel phenomena we report as "Electrohydrodynamic vortices (EHDV)". EHDV contains unique spatio-temporal features to capture particulate matter from ambient air into the droplets. We report a particle size dependent scaling of optimal EHD vortex strength for maximizing the particle capture. Probability of electrostatic particle capture of a 0.3micron airborne particle improves from 3% to above 90% in a span of 100ms, by employing EHD vortices. With the versatile capabilities to capture airborne particulate matter, disinfect airborne pathogens, absorb gaseous pollutants and especially the ability to dispose of the pollutants captured into the droplets, EHDV enters the scientific community as a strong contestant to be the next generation air purification technology in future.