Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q35: Electrokinetic Flows: Preconcentration, Separations and Mixing |
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Chair: Juan Santiago, Stanford University Room: 617 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q35.00001: Rapid and multiplexed enrichment of specific DNA sequences using isotachophoresis Ashwin Ramachandran, Nobuyuki Futai, Catherine Hogan, Kanagavel Murugesan, Niaz Banaei, Juan G. Santiago We use on-chip isotachophoresis (ITP) to create electric-field-driven shock waves of ion concentration, which are formed at the interface between a high mobility leading electrolyte (LE) and a low mobility trailing electrolyte (TE). Ionic species with mobilities bracketed by these electrolyte species focus at the LE-to-TE interface. For trace sample concentrations, multiple species co-focus, pre-concentrate by 10,000x and react inside a single, order 10 um wide zone. We apply ITP to extract and purify DNA targets from complex biological samples and to immediately co-focus these with synthetic DNA/RNA probes. We complete in 30 min hybridization reactions which would normally take several days, and then separate reacted from unreacted DNA. We will present our work toward rapid and sequence-specific enrichment of rare DNA targets for two applications. First, we enrich human genomic DNA regions associated with cancer using a large library of synthetic RNA probes. Second, we enrich pathogenic cell-free DNA from infected human plasma samples, followed by PCR to achieve highly sensitive and rapid detection of tuberculosis. The techniques presented have potential to streamline new diagnoses and discovery work in a wide range of areas including oncology and infectious diseases. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q35.00002: Fast and homogenous mixing in a coaxial capillary device with two sheath flows Diego A. Huyke, Ashwin Ramachandran, Thomas Kroll, Daniel P. DePonte, Juan G. Santiago We have developed a novel microfluidic mixer with order 10 microsecond mixing times, and sample consumption of 1 to 500 nL/s. Importantly, the mixer achieves flow-area-weighted residence time distribution with a standard deviation width of 140 microsecond (for a 2.8 ms center-streamline residence time). In our mixer, the low flow rate sample stream which exits an inner capillary is hydrodynamically focused to a sub-micron radius by the high flow rate sheath stream within a tapered middle capillary. The mixed stream subsequently enters a third, outer capillary wherein its area (normal to the flow direction) is expanded 200 times to increase the sample detection volume. The latter expansion of the mixed stream decouples the upstream mixing region from the downstream probing region and increases the signal-to-noise (SNR) for line-of-sight integration techniques. The outermost capillary is constructed from glass or polyimide for, respectively, optical or hard X-ray sample detection. Analytical and numerical convection-diffusion models, design, and experimental validation of the mixer will be presented. The models will explore tradeoffs between mixing rate and homogeneity for different flow conditions. We validated the models and experimentally studied mixing performance using epifluorescence imaging of fluorescein-iodide quenching. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q35.00003: Joule heating-enabled nanoparticle enrichment in insulator-based dielectrophoretic microdevices. Amirreza Malekanfard, Zhijian Liu, Xiangchun Xuan Dielectrophoresis has become a popular method for particle manipulation in microfluidic devices. It requires electric field gradients that can be created using in-channel hurdles and posts etc. However, these insulating structures amplify the Joule heating effect, leading to a non-uniform fluid temperature and in turn gradients in fluid conductivity and permittivity (as well as viscosity). The action of electric field on the Joule heating-induced fluid property gradients causes a regional fluid flow in the form of vortices. This so-called electrothermal flow has been previously found to reduce the particle focusing and trapping performance in insulator-based dielectrophoretic microdevices. We demonstrate in this work that the electrothermal flow vortices can be utilized to entrain nanoparticles for a localized enrichment near the insulating tips of a ratchet microchannel. We also develop a depth-averaged numerical model to simulate the fluid, heat, charge and mass transport involved in the process. A good agreement is obtained for each of the various parametric studies. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q35.00004: Rapid mixing utilizing microscale magnets based on microfluidics device Ran Zhou, Marcel Mejulu Micromixing has been widely used for a myriad of applications in industries ranging from biological engineering to chemical engineering. Almost every chemical assay requires mixing of two or more fluids or reagents with a sample. Recently, to enhance the mixing efficiency using ferrofluid has been widely preferred in biological applications because its mixing efficiency is very high, and it does not require long channels or increased time of contact to achieve homogenous and complete mixing. The ferrofluid can be controlled by external permanent magnets and it does not generate heat to the sample as electromagnet. However, the traditional magnetic method relies on bulky permanent magnet, so it is difficult to control the mixing performance precisely. In this paper, we propose a miniaturized and integrated microfluidic device that can achieve the rapid mixing of ferrofluid and distilled water. To accomplish this, high-gradient microscale magnet was fabricated and integrated at one side of a microfluidic channel by a simple fabrication technique. The microscale magnet was fabricated by injecting and curing a mixture of neodymium powder in a structural channels, and subsequent permanent magnetization. This study further investigates the effect of the total flow rate and ferrofluid concentration on the mixing performance. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q35.00005: Electrical Double Layers: Predicting Overcharging and Layering of Ions using Continuum Models~ Ankur Gupta, Ananth Govind Rajan, Emily Carter, Howard Stone Electrical double layers (EDLs) form the basis for several phenomena such as electrophoresis, supercapacitor charging and discharging, and desalination. However, existing continuum models for the EDL are unable to predict experimentally-observed phenomena such as ion layering and EDL overcharging. To overcome these limitations, atomistic methods like classical density functional theory and molecular dynamics simulations have been employed to elucidate the nanoscale structure of EDLs. In this work, we bridge the gap between the continuum and atomistic approaches by proposing a modified continuum model. Our model predicts a near-wall stratified structure in EDLs wherein the cations and anions are arranged in~\textit{layers}, even in the dilute limit. Furthermore, we predict that for trivalent ions, the double layer can locally possess a net charge larger than the surface, a phenomenon commonly known as \textit{overcharging}. Our model does not require any fitting parameters or additional boundary conditions, and only depends on the ion properties. Since our approach is at the continuum scale, we envision that it can be readily extended to out-of-equilibrium systems such as electrophoresis and dynamics of supercapacitors.~ [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q35.00006: Electroosmotic flow in small-scale channels induced by surface-acoustic waves. Mathias Dietzel, Steffen Hardt Apart from acoustic streaming as the primary effect, surface-acoustic waves (SAWs) go along with time-periodic changes of the electrostatic potential at a solid-liquid interface. This surface potential polarizes the liquid in its vicinity, i.e. it induces a dynamic Debye layer. Alternating current electroosmosis (ACEO) relies on a similar principle. We study the flow field due to the interaction of the dynamic Debye layer with the electric field of the SAW. For this purpose, we consider parallel-plates channels less than 500 nm wide, characterized by an inverse RC time of the Debye layer similar to the characteristic SAW frequencies in the MHz range. By means of numerical simulations of the unsteady Stokes, Poisson, and Nernst-Planck equations, parametric studies are conducted, varying the SAW frequency and amplitude, ionic strength, Debye parameter, as well as the phase shift between two SAWs traveling along opposite channel walls. We demonstrate that for typical SAW amplitudes sizable (time-averaged) net velocities up to 1 mm/s can be obtained, especially if the phase shift equals 180 deg to maximize the periodic electric current between the walls. This significantly exceeds the velocities induced by acoustic streaming in narrow channels. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q35.00007: Stern compact layer in ionic conductor liquid charging at high voltages Farzad Mashayek, Babak Kashir, Anthony E. Perri, Alexander L. Yarin The stern compact layer at the conducting electrodes is studied theoretically and numerically. These electrodes are subjected to high voltages and sustain electric current. A novel approach is developed based on the Brunauer-Emmet-Teller (BET) mechanism to predict the thickness of the Stern compact layer. Non-specific (non-electric) adsorption is responsible for the formation of this layer on the oxide islands or impurities at the conducting electrodes. Concurrently, kinetics-limited faradaic reactions occur at the unscreened parts of the metallic conducting electrodes. The electron transfer occurs through the faradaic reactions characterized by the Frumkin-Butler-Volmer kinetics. The model relates the thickness of the Stern compact layer to the potential drop across it. Realistic values of the applied electrode voltage and sustained electric current density in electrostatic atomization are considered to predict the Stern compact layer properties. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q35.00008: A non-contact solution-based method to measure the electric dipoles of p-n silicon nanowire diodes Minh Thang Hoang, Gozde Tutuncuoglu, Amar Mohabir, Leonard Feldman, Michael Filler, Jerry Shan Dispersions of nanowires with p-n junctions are emerging as next-generation electronic colloidal materials. Developing the ability to efficiently manipulate such nanostructures for both characterization and separation based on electronic properties is crucial for scientific understanding and technological applications. Here we present a method for measuring the permanent and induced dipoles of p-n doped silicon nanowires by observing their rotation in fluid suspension under external electric fields. The permanent dipoles are formed by the opposite charges in the depletion layer at the p-n junction, while the induced dipoles are formed by nanowire polarization under the external field. The measured electrical torque exerted on the p-n nanowires due to applied DC and AC fields yields the permanent and induced dipoles. We then show how the dipoles change for nanowires of various doping concentrations and surface passivations. Numerical results also suggest a relation between the permanent dipole and the depletion width of the nanowire diode. This non-contact measurement method offers a new approach to efficiently determine the electronic properties of p-n nanowires, and is a significant step toward the high-throughput characterization and separation of complete nanowire devices. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q35.00009: Diffusiophoresis, Batchelor scale and effective Peclet numbers Florence Raynal, Romain Volk We study the joint mixing of colloids and salt released together in a stagnation point or in a globally chaotic flow. In the presence of salt inhomogeneities, the mixing time is strongly modified depending on the sign of the diffusiophoretic coefficient $D_\mathrm{dp}$. Mixing is delayed when $D_\mathrm{dp}>0$ (salt-attracting configuration), or faster when $D_\mathrm{dp}<0$ (salt-repelling configuration). In both configurations, as for molecular diffusion alone, large scales are barely affected in the dilating direction while the Batchelor scale for the colloids, $\ell_{c,\mathrm{diff}}$, is strongly modified by diffusiophoresis. We propose here to measure a global effect of diffusiophoresis in the mixing process through an effective P\'eclet number built on this modified Batchelor scale. Whilst this small scale is obtained analytically for the stagnation point, in the case of chaotic advection, we derive it using the equation of gradients of concentration, following Raynal \& Gence (1997). Comparing to numerical simulations, we show that the mixing time can be predicted by using the same function as in absence of salt, but as a function of the effective P\'eclet numbers computed for each configuration. [Preview Abstract] |
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