Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session L34: Micro/Nano Flows: Electrokinetics |
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Chair: Steffen Hardt, TU-Darmstadt Room: 242 |
Monday, November 21, 2022 8:00AM - 8:13AM |
L34.00001: Particle diffusiophoresis in viscous polymer solutions Viet Sang Doan, Tanja Riess, Sangwoo Shin We study the effect of polymer molecules on diffusiophoresis of polystyrene latex under salt gradients. For a given solution (macro)viscosity, we show that the particle diffusiophoresis is sensitive to the microviscosity of the solution within the Debye layer, which is determined by the polymer size and concentration. Steric exclusion of the polymer molecules from the Debye layer directly influences diffusioosmotic flow therein, and thus the particle diffusiophoresis, which we confirm experimentally by varying the polymer chain length and the thickness of the Debye layer. Our results offer useful insights into applications that involve controlling colloidal transport in viscous polymer solutions such as colloidal film deposition, painting, and drug delivery. |
Monday, November 21, 2022 8:13AM - 8:26AM |
L34.00002: Selective transport of alkali ions in vertically aligned sub-nanometer-diameter carbon-nanotube membranes Da-Chi N Yang, Richard J Castellano, Ricardo P Silva, Robert F Praino, Francesco Fornasiero, Jerry W Shan Normally, the ion conductance of different ions through a porous membrane is proportional to the ion mobility. For alkali ions in aqueous solution, the ion mobility increases as the hydrated ionic diameter decreases, e.g., with µK+ > µNa+ > µLi+. This phenomenon remains valid even for porous membranes with nanoscale pores, which typically are still larger than the size of hydrated ions. However, when the pore size shrinks to the angstrom scale, alkali ions can undergo partial or complete dehydration, in which water molecules around the ions are re-oriented or stripped away in order to enter the pores. Hence, the relative ordering of the ion-conductance values for alkali ions in angstrom-scale pores can become dependent on the size of the dehydrated ions, which is entirely opposite to that of hydrated alkali ions. This mechanism has been widely confirmed in biological ion channels, but not experimentally seen in carbon nanotubes (CNTs), to our knowledge. Here, we describe ion conductance in the first macroscopic membranes having sub-nanometer-diameter, vertically aligned carbon-nanotube (CNT) pores. In such membranes with 0.8-nm CNT pores, the ion conductances of aqueous alkali metal chloride solutions followed an order reversed from the bulk ion mobilities, indicating that the ions had their hydration shells significantly re-oriented or stripped. As a comparison, we demonstrated that the relative ion conductances of 3-nm CNT membranes remained in the order of bulk ionic mobility. These results not only verify the integrity of the scalably fabricated sub-nanometer-diameter CNT membranes, but provide an avenue to study fundamental ion-transport mechanisms in angstrom-scale 1-D pores, and for applications such as ion separation, rectification and gating. |
Monday, November 21, 2022 8:26AM - 8:39AM |
L34.00003: Probing the carbon particle motion in a microfluidic electrochemical flow capacitor Brandon K Stacks, Kelsey Hatzell, Deyu Li, Haoxiang Luo Electrochemical flow capacitors (EFCs) are promising energy storage devices due to their large storage capacity, low fatigue rates, and fast charge/discharge rates. These devices employ the electric double layer of high-surface-area carbon particles in a flowable slurry, i.e., a ‘flowable electrode’, to store ionic charges from an electrolyte solution. The flow characteristics of the carbon slurry and the particle interactions are therefore directly linked to the charging behaviors of the device. Despite this, there are very few studies that directly observe the particle motion inside the charging channels. To address this issue, we have developed a microfluidic electrochemical flow capacitor (MEFC) that is compatible with optical imaging. This platform allows us to investigate the influence of various slurry flow behaviors on the charging performance of the flowable electrode during the operation of the device. We have examined the effect of slurry flow rate on the particle clustering pattern and consequently the charging performance. We will report on the fabrication of the MEFC, the experimental setup, and the charging results under different slurry flow rates. |
Monday, November 21, 2022 8:39AM - 8:52AM |
L34.00004: Characterization of electrothermal microfluidic tweezers (REP) in bio-relevant media Zhengwei Chen, Kshitiz Gupta, Nicolas G Green, Steven T Wereley Rapid electrokinetic patterning (REP) has proven to be a powerful microfluidic tweezer that generates spatially and temporally specified microflow via electrothermal vortices. The ultra-small trapping forces on the scale of femtoNewtons exerted by REP, due to its viscous drag of the vortical flow on the trapped particles, attracts many potential bio-manipulation applications. We report, for the first time, use of isotonic sugar-based media to simulate bio-relevant environment for flow manipulation through REP. A DC field was introduced, in addition to the typical AC field, to enhance the vortical flow induced particle trapping performance. In this work, we study the effects of the magnitude and sign of the DC offset on the vortex characteristics. Results show that addition of the DC component in the electric field has a significant impact on the electrothermal micro-vortex and the electric double layer of the electrodes and the trapped particles. We also observe that an abrupt change in the DC offset destabilized the vortex and washed away some particles as the trap equilibrium re-established. However, the trap remained stable when the step change in the DC offset was smaller than ~500 mV. |
Monday, November 21, 2022 8:52AM - 9:05AM |
L34.00005: Micro-meso-macroscale responses of non-colloidal suspensions undergoing dipolar interactions in an electric field: effects of confinement and shear flow Siamak Mirfendereski, Jae Sung Park The dipolar response of non-colloidal dispersions driven by an electric field has led to a class of smart material known as electrorheological fluids. Even though extensive effort has been made to quantify the macroscopic characterizations of such a system, a robust connection of its macroscale properties to the particle-level (micro) and cluster-level (meso) responses is largely unexplored. To this end, we use a large-scale Stokesian dynamics simulation to systematically draw this connection for a range of volume fractions up to a highly concentrated regime. First, we will quantify the volume fraction-dependence of the multiscale responses. Our results reveal different behaviors of structural formation for various concentration regimes with a transition at a specific volume fraction. Second, we will characterize the effect of confinement by applying different levels of confinement. Confinement seems to manipulate the structures primarily at the mesoscopic level, which could link the change in the macroscopic properties. Lastly, we will discuss the degradation of the particulate structures across the length scales due to an external shear flow. |
Monday, November 21, 2022 9:05AM - 9:18AM |
L34.00006: Chemically-Driven Particle Penetration into Bacterial Biofilms Ambika Somasundar, Boyang Qin, Suin Shim, Bonnie L Bassler, Howard A Stone Bacterial biofilms are detrimental to human health. Their defining feature is a three-dimensional extracellular polymer matrix that presents a mechanical barrier to the transport of chemicals, such as antimicrobials, making them robust and difficult to remove. Targeting the disruption of the extracellular polymer matrix for increasing the susceptibility of the biofilm to antimicrobials is a promising but relatively unexplored approach to biofilm control and management. In this regard, the development of micro- and nanoparticles has offered new ideas for the management and eradication of bacterial biofilms. Hence, controlling and enhancing the transport of micro- and nanoparticle carriers in a biofilm's extracellular polymer matrix may have significant implications for how to approach the management and eradication of biofilms. In this work, we investigate externally imposed chemical gradients as a mechanism of transport of polystyrene particles into bacterial biofilms. We show how it is possible to transport particles into a biofilm matrix through imposed chemical gradients and discuss the dynamics and complexity in motion that arise due to the presence of the biofilm matrix. Our results demonstrate the importance of chemical species in disrupting the biofilm matrix and regulating particle transport in macromolecularly crowded environments. The results suggest potential applications in the transport and delivery of particles in physiologically relevant and macromolecularly crowded environments. |
Monday, November 21, 2022 9:18AM - 9:31AM |
L34.00007: Thermoelectric energy conversion in nanochannels filled with ionic liquids Rajkumar Sarma, Satarupa Dutta, Steffen Hardt In the past few years, thermoelectric energy conversion in electrolyte-filled nanochannels has received increasing attention. The thermovoltage created with corresponding devices often exceeds that of solid-state thermoelectric devices. Usually, significant thermovoltages can only be generated in the case of overlapping electric double layers (EDLs) from opposing channel walls. Ionic liquids (ILs) can be regarded as molten salts and are fundamentally promising electrolytes because of their high density of primary charge carriers. For ILs, the standard scheme of computing the EDL thickness results in values in the range of Angstroms, which precludes EDL overlap for all practical purposes. However, recent experimental results [1] indicate that the effective charge carriers in confined ILs are actually pseudoparticles, i.e. clusters of many primary charge carriers that can partially dissociate in a thermally activated process. Based on the coupled Poisson-Nernst-Planck, Navier-Stokes and heat transport equations, we study thermovoltage generation in an IL-filled nanochannel with different temperatures applied to the two ends of the channel. In that context, the number density of effective charge carriers is obtained from an Arrhenius equation. Next to the numerical model, we have developed an analytical model based on the long-wavelength approximation. The results indicate that for the same degree of EDL overlap, the thermovoltage obtained with ILs is significantly higher than that obtained with aqueous electrolytes, by about a factor of five at a wall zeta potential of 25 mV. The predictions of the analytical model agree very well with the numerical results. In total, our theoretical studies indicate that confined ILs bear a significant potential for thermoelectric energy conversion. |
Monday, November 21, 2022 9:31AM - 9:44AM |
L34.00008: Electrokinetic microflow of non-Newtonian polyelectrolyte solutions in brush-grafted channels Myung-Suk Chun We investigated electrokinetic flow of polyelectrolyte (PE) solutions in PE brush-grafted microchannels by extending our previous studies concerning electrokinetic Newtonian microfluidics [Lim and Chun, Phys. Fluids, 2011]. In our model framework, the PE brush-layer can be represented by the Alexander-de Gennes model, and the Poisson-Nernst-Planck equations are solved for electrostatic field, where each ion concentration is estimated by multi-species ion balance. With the Brinkman hydrodynamic friction inside the brush, Bird-Carreau constitutive model is adopted in the momentum equation to describe the non-Newtonian PE solution of anionic polyacrylic acid (PAA). This presentation reports the new results regarding the potential profile due to space charge and the retardation of flow velocity in terms of PAA-brush height, grafting density, concentration of dispersed PAA, and bulk pH. The Donnan potential shows several times higher than the surface potential in the bare channel, whereas it becomes lower with increasing PAA concentration. As the PAA concentration increases, the flow rate slows down due to viscosity, and further slows down at higher pH due to the PAA swelling, interplaying between non-Newtonian effect and flow retardation inside the brush-layer. |
Monday, November 21, 2022 9:44AM - 9:57AM |
L34.00009: Shear banding flow behavior around the periodic triangle-shaped pillars in microchannel Yoshiyasu Ichikawa, Masahiro Motosuke It is known that the shear banding is occurred by using a semi-dilute polymer solution due to the non-uniform shear rate. In this study, we investigated the flow behavior of semi-dilute polymer solution in a microchannel having a triangle-shaped pillar array by µPIV. Using this pillar array, the effect of continuous contraction and expansion flow which shows the periodic non-uniform shear rate on flow behavior, was evaluated. In the experiment, we visualized that the dead zone (DZ) was formed around the pillar due to the shear banding, and its shape changes depending on the Weissenberg number (Wi) and concentration of the solution. Based on the velocity measurement results, both the viscosity and flow type parameter distribution were calculated. It was revealed that the various viscosity distribution appeared inside the DZ, resulting in the complex flow behavior containing both extension and rotation behavior. Moreover, the dominated flow behavior around the pillar array depending on Wi was also investigated. |
Monday, November 21, 2022 9:57AM - 10:10AM |
L34.00010: Electroneutrality breakdown in charged, nanofluidic channels Pragati Shaw, Tian Tang, Peter Berg Charged, nanofluidic channels containing electrolyte solution are building blocks of many biological and technological systems. Due to their geometrical confinement and the dominance of surface forces, unique phenomena are observed such as slip flow enhancement. To understand these phenomena, it is necessary to (re)examine several fundamental physical mechanisms and associated assumptions. In particular, the assumption of complete screening of the fixed surface charges by counter-ions (i.e. electroneutrality) may not hold in a nanochannel. Recent work by Levy et al. (2020) provides evidence for electroneutrality breakdown in a single nanopore for zero-size ions, measured both locally in cross sections and globally for the entire channel. Our study goes beyond this initial investigation and employs the Poisson-Boltzmann equation to examine the electroneutrality in the presence of finite-size ions for a single nanopore as well as a periodic array of cylindrical nanopores. The effects of system dimensions, charge density on the channel surface, dielectric constants of system domains, and size of ions are explored. Additionally, an attempt is made to model the material surrounding the nanopore as a metal or a semiconductor so as to expand the technological applications of our model. |
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