Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session L13: Micro/nano Flow Devices: Complex Systems, Materials, and ApplicationsMicro
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Chair: Guy Ramon, Technion – Israel Institute of Technology Room: 506 |
Monday, November 20, 2017 4:05PM - 4:18PM |
L13.00001: Streaming current for particle-covered surfaces: simulations and experiments Jerzy Blawzdziewicz, Zbigniew Adamczyk, Maria L. Ekiel-Jezewska Developing \textit{in situ} methods for assessment of surface coverage by adsorbed nanoparticles is crucial for numerous technological processes, including controlling protein deposition and fabricating diverse microstructured materials (e.g., antibacterial coatings, catalytic surfaces, and particle-based optical systems). For charged surfaces and particles, promising techniques for evaluating surface coverage are based on measurements of the electrokinetic streaming current associated with ion convection in the double-layer region. We have investigated the dependence of the streaming current on the area fraction of adsorbed particles for equilibrium and random-sequential-adsorption (RSA) distributions of spherical particles, and for periodic square and hexagonal sphere arrays. The RSA results have been verified experimentally. Our numerical results indicate that the streaming current weakly depends on the microstructure of the particle monolayer. Combining simulations with the virial expansion, we provide convenient fitting formulas for the particle and surface contributions to the streaming current as functions of area fractions. For particles that have the same $\zeta$-potential as the surface, we find that surface roughness reduces the streaming current. [Preview Abstract] |
Monday, November 20, 2017 4:18PM - 4:31PM |
L13.00002: Modeling Microscale Electro-thermally Induced Vortex Flows Rajorshi Paul, Tian Tang, Aloke Kumar In presence of a high frequency alternating electric field and a laser induced heat source, vortex flows are generated inside micro-channels. Such electro-thermally influenced micro-vortices can be used for manipulating nano-particles, programming colloidal assemblies, trapping biological cells as well as for fabricating designed bacterial biofilms. In this study, a theoretical model is developed for microscale electro-thermally induced vortex flows with multiple heat sources. Semi-analytical solutions are obtained, using Hankel transformation and linear superposition, for the temperature, pressure and velocity fields. The effect of material properties such as electrical and thermal conductivities, as well as experimental parameters such as the frequency and strength of the alternating electric field, and the intensity and heating profile of the laser source, are systematically investigated. Resolution for a pair of laser sources is determined by analyzing the strength of the micro-vortices under the influence of two heating sources. Results from this work will provide useful insights into the design of efficient optical tweezers and Rapid Electrokinetic Patterning techniques. [Preview Abstract] |
Monday, November 20, 2017 4:31PM - 4:44PM |
L13.00003: Dynamics of Viscous Back-Flow from a Network of Elastic Microfluidic Channels Asaf Dana, Zhong Zheng, Gunnar G. Peng, Howard A. Stone, Herbert E. Huppert, Guy Z. Ramon We present a model for investigating the dynamics of back-flows caused by the elastic relaxation of a pre-strained medium. Namely, a model network with n bifurcated channel generations modeled as fluid confined between two rigid plates. The model uses a combination of lubrication theory for the flow and the equations of linear elasticity for the boundaries. The model assumes elastic deformation occurs only in channel aperture while the channel’s length remains constant throughout the process. The asymptotic results show good agreement with numerical calculations in early and late times, when the aperture and back-flow rate tend to $t^(-1/3)$ and $t^(-4/3)$, respectively. This work presents a case where the pressure gradient along the network is steepest near the outlet while the bulk of the network serves as a `reservoir’. In addition, an asymptotic solution is derived for late times and large n. For a fixed total length, networks with larger n are less efficient at evicting fluids, manifested through a longer time required for evicting a given fractional reduction of the initial volume. The model can be used to investigate the back-flow dynamics of fractured rocks e.g., in hydraulic fracturing operations. [Preview Abstract] |
Monday, November 20, 2017 4:44PM - 4:57PM |
L13.00004: Characterization of chaotic electroconvection near flat electrodes under oscillatory voltages Jeonglae Kim, Scott Davidson, Ali Mani Onset of hydrodynamic instability and chaotic electroconvection in aqueous systems are studied by directly solving the two-dimensional coupled Poisson--Nernst--Planck and Navier--Stokes equations. An aqueous binary electrolyte is bounded by two planar electrodes where time-harmonic voltage is applied at a constant oscillation frequency. The governing equations are solved using a fully-conservative second-order-accurate finite volume discretization and a second-order implicit Euler time advancement. At a sufficiently high amplitude of applied voltage, the system exhibits chaotic behaviors involving strong hydrodynamic mixing and enhanced electroconvection. The system responses are characterized as a function of oscillation frequency, voltage magnitude, and the ratio of diffusivities of two ion species. Our results indicate that electroconvection is most enhanced for frequencies on the order of inverse system RC time scale. We will discuss the dependence of this optimal frequency on the asymmetry of the diffusion coefficients of ionic species. [Preview Abstract] |
Monday, November 20, 2017 4:57PM - 5:10PM |
L13.00005: Determination and characterization by numerical simulations of flow mixing due to electrokinetic instabilities in cross-shaped microchannels. Esteban Guerrero, Daming Chen, Logan Hageman, Amador Guzman This article describes a computational study of flow mixing in microchannels due to electrokinetic instabilities that are compared to experimental results obtained in a cross- microchannel with an ionic solution of potassium chloride with two different ionic concentrations, with the purpose of determining the parameter combinations to produce the onset of flow mixing and its characteristics. For the numerical simulation process carried out using a finite element method-based commercial code, we applied a typical zeta potential used in other articles as a boundary condition for the microchannel walls. For the experiments, we used a commercial silicon glass (Caliper NS95) microchannel. For determining a flow mixing regime, we use the concept of ``mixing index'' established by (Fu et al., 2005) for an electrical conductivity ratio range of 18 to 52 with an electric field range of 1100 to 1900 V/cm. From our numerical simulation results we have found a threshold for the electrical Rayleigh number for starting a flow mixing regime, and a minimum microchannel characteristic length for achieving a 90{\%} of flow mixing that will allow us to significantly reduce the mixing time. [Preview Abstract] |
Monday, November 20, 2017 5:10PM - 5:23PM |
L13.00006: A Sinusoidal Applied Electric Potential can Induce a Long-Range, Steady Electrophoretic Force Seyyed Hashemi Amrei, William D. Ristenpart, Greg R. Miller We use the standard electrokinetic model to numerically investigate the electric field in aqueous solutions between parallel electrodes under AC polarization. In contrast to prior work, we invoke no simplifying assumptions regarding the applied voltage, frequency, or mismatch in ionic mobilities. We find that the nonlinear electromigration terms significantly contribute to the overall shape of the electric potential vs. time, which at sufficiently high applied potentials develops multi-modal peaks. More surprisingly, we find that electrolytes with non-equal mobilities yield an electric field with non-zero time average at large distances from the electrodes. Our calculations indicate this long-range electric field suffices to levitate colloidal particles many microns away from the electrode against the gravitational field, in accord with experimental observations of such behavior (Woehl et al., PRX, 2015). Moreover, the results indicate that particles will aggregate laterally near electrodes in some electrolytes but separate in others, helping explain a longstanding but not well understood phenomenon. [Preview Abstract] |
Monday, November 20, 2017 5:23PM - 5:36PM |
L13.00007: DNA Concentration Due To Migration Under Parallel Fields Ryan Montes, Anthony Ladd, Jason Butler Pressure driven flow of DNA solutions through a microchannel (100-400 micron) in the presence of an opposing electric field, generates a net migration of DNA towards the walls. Over time a strongly inhomogeneous distribution of DNA develops in the channel with almost all the DNA localized within 10 micron of the walls. Near the walls the electrophoretic velocity drives a return flux of DNA towards the inlet. Thus despite an average fluid velocity that is at least 10 times the electrophoretic velocity, almost all the DNA remains trapped within the device. Experimental observations have provided unambiguous evidence that DNA migration occurs as outlined above, and that the migration is necessary for trapping. The mechanism inducing this DNA migration has been attributed to hydrodynamic interactions induced by the electric field. Migration due to viscoelasticity is another possibility, but a series of experiments systematically varying the ionic strength of the buffer solution verify that the electrically-induced flow, not the viscoelasticity, is the cause of the migration. This migration mechanism can be used to purify and concentrate DNA within micro-TAS devices. [Preview Abstract] |
Monday, November 20, 2017 5:36PM - 5:49PM |
L13.00008: Surface Charge Effects on the Electro-Orientation of Insulating Nanotubes in Aqueous Electrolytes. Semih CETINDAG, Bishnu Tiwari, Dongyan Zhang, Yoke Khin Yap, Sangil Kim, Jerry W. Shan While the alignment of electrically conductive nanowires and nanotubes by electric fields in liquid solution has been well studied, much less is known about the electro-orientation of insulating 1D particles, such as boron-nitride nanotubes (BNNTs). Here, we demonstrate for the first time the electro-orientation of individual insulating BNNTs in aqueous KCl solutions under AC fields. Comparison to theory indicates that the observed frequency response is not related to the crossover for Maxwell-Wagner interfacial polarization. Instead, the cross-over frequency in the low-frequency regime scales as the square root of solution conductivity, indicating that alignment is associated with the formation and motion of an electrical double layer (EDL), much like induced-charge electro-osmosis for a conducting particle. However, the mechanism for the formation of the EDL is presumably different for insulating particles like BNNTs as compared to conductors. By varying the surface charge of the particle by changing pH, we show that the alignment rate increases with increasing surface charge, and is likely a result of counter-ion migration and EDL polarization under the influence of applied electric field. Thus, particle surface charge (large Dukhin number) is believed to play a vital role in the electro-orientation of insulating particles in aqueous solutions. [Preview Abstract] |
Monday, November 20, 2017 5:49PM - 6:02PM |
L13.00009: Prediction of Osmotic Pressure of Ionic Liquids Inside a Nanoslit by MD Simulation and Continuum Approach Gi Jong Moon, Yu Dong Yang, Jung Min Oh, In Seok Kang Osmotic pressure plays an important role in the processes of charging and discharging of lithium batteries. In this work, osmotic pressure of the ionic liquids confined inside a nanoslit is calculated by using both MD simulation and continuum approach. In the case of MD simulation, an ionic liquid is modeled as singly charged spheres with a short-ranged repulsive Lennard-Jones potential. The radii of the spheres are 0.5nm, reflecting the symmetry of ion sizes for simplicity. The simulation box size is 11nm×11nm×7.5nm with 1050 ion pairs. The concentration of ionic liquid is about 1.922mol/L, and the total charge on an individual wall varies from ±60e(7.944$\mu $m/cm$^{\mathrm{2}})$ to \textpm 600e(79.44$\mu $m/cm$^{\mathrm{2}})$. In the case of continuum approach, we classify the problems according to the correlation length and steric factor, and considered the four separate cases: 1) zero correlation length and zero steric factor, 2) zero correlation length and non-zero steric factor, 3) non-zero correlation length and zero steric factor, and 4) non-zero correlation and non-zero steric factor. Better understanding of the osmotic pressure of ionic liquids confined inside a nanoslit can be achieved by comparing the results of MD simulation and continuum approach. [Preview Abstract] |
Monday, November 20, 2017 6:02PM - 6:15PM |
L13.00010: Efficient Energy Conversion by Grafting Nanochannels with End-charged Stimuli-responsive Polyelectrolyte Brush Guang Chen, Siddhartha Das Polyelectrolyte (PE) brushes have aroused increasing attention in applications in energy conversion and chemical sensing due to the environmentally-responsive and designable nature. PE brushes are charged polymer chains densely grafted on solid-liquid interfaces. By designing copolymeric systems, one can localize the ionizable sites at the brush tip in order to get end-charged PE brushes. Such brushes demonstrate anomalous shrinking/swelling behaviors with tunable environmental parameters such as pH and salt concentration. In this study, we probe the conformation and electrostatics of such PE brush systems with various size, grafting density and charge distribution, and exploit the electrochemomechanical energy conversion capabilities of nanochannels grafted with such PE brush systems. Our results indicate that the presence of the end-charged PE brush layer can massively enhance the streaming potential mediated energy conversion efficiency, and the improvement is more significant in strongly ionic solution. [Preview Abstract] |
Monday, November 20, 2017 6:15PM - 6:28PM |
L13.00011: Superhydrophobic nanofluidic channels for enhanced electrokinetic conversion Antonio Checco, Aktaruzzaman Al Hossain, Amir Rahmani, Charles Black, Gregory Doerk, Carlos Colosqui We present current efforts in the development of novel slit nanofluidic channels with superhydrophobic nanostructured surfaces designed to enhance hydrodynamic conductivity and improve selective transport and electrokinetic energy conversion efficiencies (mechanical-electrical energy conversion). The nanochannels are fabricated on silicon wafers using UV lithography, and their internal surface is patterned with conical nanostructures (feature size and spacing \textasciitilde 30 nm) defined by block copolymer self-assembly and plasma etching. These nanostructures are rendered superhydrophobic by passivation with a hydrophobic silane monolayer. We experimentally characterize hydrodynamic conductivity, effective zeta potentials, and eletrokinetic flows for the patterned nanochannels, comparing against control channels with bare surfaces. Experimental observations are rationalized using both continuum-based modeling and molecular dynamics simulations. Scientific and technical knowledge produced by this work is particularly relevant for sustainable energy conversion and storage, separation processes and water treatment using nanoporous materials. [Preview Abstract] |
Monday, November 20, 2017 6:28PM - 6:41PM |
L13.00012: Spontaneous pulse generation in a steady channel flow of a colloidal suspension -- the role of dissolved gas Suin Shim, Orest Shardt, Howard A. Stone We introduce a phenomenon that is observed when deionized (DI) water with suspended charged particles flows through a single microfluidic channel. When an aqueous suspension of amine-modified, positively charged polystyrene particles (volume fraction $= 0.01$) flows steadily through a serpentine polydimethylsiloxane (PDMS) channel, a pulse of particles is generated, which then flows through the channel at a slower speed than the mean flow velocity. We quantify the results and rationalize the observations by considering the diffusiophoresis of charged particles driven by gas leakage through the permeable PDMS walls. A mathematical model will be compared with the experimental observations. [Preview Abstract] |
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