Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session G6: Microfluids: Particles II - Electrokinetically Induced Flow |
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Chair: German Drazer, Rutgers University Room: 328 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G6.00001: Direct numerical simulations (DNS) of particles in spatially varying electric fields E. Amah, M. Janjua, I.S. Fischer, P. Singh We have developed a direct numerical simulation (DNS) scheme to simulate the motion of dielectric particles suspended in a dielectric liquid in nonuniform electric fields. The motion of particles is tracked using a distributed Lagrange multiplier method (DLM) and the electric forces acting on the particles are calculated by an efficient scheme in which the Maxwell stress tensor (MST) is integrated over the surfaces of the particles to obtain the force. The code is validated by performing a convergence study and by comparing the particle trajectories in a dielectrophoretic cage with those given by the point-dipole method. We also show that the trajectories of the two or more interacting particles given by the MST method can be different from those obtained using the point-dipole method since the latter does not consider particle-particle interactions. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G6.00002: The DC Force Exerted on a Charged Microparticle by an AC Electric Field Dennis C. Prieve, Christopher L. Wirth, Paul J. Sides In 0.15 mM solution of KOH or NaHCO3 in water, a single negatively charged 6-micron polystyrene sphere is levitated about 200 nm above a negatively charged planar ITO electrode by double-layer repulsion. The potential energy profile of forces acting on the sphere is determined by monitoring the distribution of elevations sampled by Brownian motion of the sphere and measured using total internal-reflection microscopy, which can detect changes in elevation as small as 1 nm. Application of a 10 kV/m electric field oscillating at 10 kHz produced oscillations in elevation which were completely swamped by Brownian motion. Nonetheless an unexpected steady attractive force was detected which was comparable in magnitude to the net weight of the sphere (0.05 pN). This additional force was proportional to the square of the electric field amplitude and is about a factor of 2 stronger in KOH compared to NaHCO3. The DC force appears to be dielectrophoretic attraction resulting between aligned dipoles induced in the sphere and the planar electrode by the electric field [Dietz, J. Appl. Phys. 48, 1036 (1977)]. A similar force causes ``necklaces'' of colloidal particles to form. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G6.00003: Analysis of eletrectrohydrodynamic jetting using multifunctional and three-dimensional tomography Han Seo Ko, Xuan Hung Nguyen, Soo-Hong Lee, Young Hyun Kim Three-dimensional optical tomography technique was developed to reconstruct three-dimensional flow fields using a set of two-dimensional shadowgraphic images and normal gray images. From three high speed cameras, which were positioned at an offset angle of 45$^{\circ}$ relative to one another, number, size and location of electrohydrodynamic jets with respect to the nozzle position were analyzed using shadowgraphic tomography employing a multiplicative algebraic reconstruction technique (MART). Additionally, a flow field inside cone-shaped liquid (Taylor cone) which was induced under electric field was also observed using a simultaneous multiplicative algebraic reconstruction technique (SMART) for reconstructing intensities of particle light and combining with a three-dimensional cross correlation. Various velocity fields of a circulating flow inside the cone-shaped liquid due to different physico-chemical properties of liquid and applied voltages were also investigated. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G6.00004: Electrokinetic filtration and separation of particles by size in single-spiral microchannels. John DuBose, Nathaniel Tupper, John Stonaker, Saurin Patel, Xiangchun Xuan In this talk we demonstrate the utility of single-spiral microchannels for the continuous filtration and separation of particles by size. The negative dielectrophoretic force used in manipulating particle trajectories arises from the continual non-uniformity of the imposed electric field within the curved channel. When subjected to an externally imposed direct current power supply to electrokinetically drive the flow, 5, 10, and 15 micrometer polystyrene beads in 1 mM phosphate buffer solutions are independently focused. The various experimentally determined voltages needed for complete particle focusing differ depending upon the diameter of the separate particle species, which allows for the possibility of the continuous filtration and separation of binary particle mixtures at the outlet of the single-spiral microchannel. We also demonstrate an effective separation of a ternary particle mixture by size in a single-spiral microchannel with multiple outlet reservoirs. [Preview Abstract] |
Monday, November 25, 2013 8:52AM - 9:05AM |
G6.00005: Ion correlation and ion steric effects on electrophoresis of a colloidal particle Robert Stout, Aditya Khair We revisit the classic problem of electrophoresis of a spherical colloid, using modified PNP equations that account for: (i) steric repulsion between finite sized ions through Bikerman's model [1]; and (ii) electrostatic correlations between ions via a modified Poisson equation recently proposed by Bazant et al. [2]. At low particle zeta potentials, we derive an analytical formula for the electrophoretic mobility accounting for ion correlations, which predicts mobility reversals at sufficiently large ion correlation lengths. Next, we develop an asymptotic scheme for thin Debye layers to compute the mobility for larger zeta potentials, where ion steric effects impose a limit on the counter-ion density in the diffuse Debye layer. Our calculations are compared to experiments on electrophoresis in concentrated multivalent electrolytes. \\[4pt] [1] J. J. Bikerman, Philos. Mag. 33, 384 (1942) \\[0pt] [2] M. Z. Bazant, B. D. Storey, and A. A. Kornyshev, Phys. Rev. Lett. 106, 046102 (2011). [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G6.00006: Frequency dispersion in dipolophoresis of metallodielectric Janus spheres Alicia Boymelgreen, Gilad Yossifon, Touvia Miloh Dipolophoresis (DIP) is an umbrella term for the two non-linear electrokinetic phenomenon of induced-charge electrophoresis (ICEP) and dielectrophoresis (DEP). It has previously been shown that this effect is responsible for the obtainment of a finite velocity by a metallodielectric (comprised of one conducting and one dielectric hemisphere) Janus spheres, even under the application of a uniform AC field. At low frequencies, this mobility is dominated by induced-charge effects, wherein the stronger induced-charge electroosmotic flow around the polarizable hemisphere propels the particle perpendicular to the electric field in the direction of its dielectric end. Surprisingly, it was observed that this motion is at a maximum for applied frequencies in the range of 1kHz beyond which the effect decays. Here we examine the effect of varying experimental conditions including electrolyte concentration and particle size on this limit. Additionally, we present for the first time an analytical solution which is capable of predicting this optimum based on our previous formulation which is uniquely valid for arbitrary electric double layer length. This work is of both fundamental and practical importance and may be used to optimize the behavior of Janus micromotors in lab-on-a-chip systems. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G6.00007: Bifurcation in the equililbrium height of colloidal particles over an electrode in low frequency electric fields Taylor Woehl, Cari Dutcher, Nicholas Talken, Bing Jie Chen, William Ristenpart Colloidal particles are known to change their equilibrium height above an electrode in response to an applied AC electric field, partially due to a lift force caused by electrohydrodynamic (EHD) flow generated around each particle. Here we report the existence of an unexpected bifurcation in the equilibrium particle height in response to low frequency ($\sim$100 Hz) fields. Optical and confocal microscopy observations reveal that upon application of the field 40\% of the particles rapidly move several particle diameters up from the electrode, while the remaining 60\% move slightly down. Statistics compiled from repeated trials demonstrate that the probability of any particle moving up follows a binomial distribution, indicating that particle lift up is random and does not result from membership in a distinct subpopulation of particles. The observations provide strong evidence for the existence of a tertiary minimum in the interaction potential at a surprisingly large distance from the electrode. We present scaling arguments for the interaction potential in terms a balance between colloidal forces, EHD flow, dipole image attraction, and gravity, yielding a predicted interaction potential with a tertiary minimum that is qualitatively consistent with the observed bifurcation. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G6.00008: Nonlinear electrophoresis of ideally polarizable particles Bruno Figliuzzi, Wai Hong Ronald Chan, Cullen R. Buie We focus in this presentation on the nonlinear electrophoresis of ideally polarizable particles. At high applied voltages, significant ionic exchanges occur between the EDL which surrounders the particle and the bulk solution. In this situation, the velocity field, the electric potential and the ionic concentration at the immediate vicinity of the particle are described by a complicated set of coupled nonlinear partial differential equations. These equations are classically considered in the limit of a weak applied field, which enables further analytical progress (Khair and Squires, Phys. Fluids, 2010). However, in the general case, the equation governing the electrophoretic motion of the particle must be solved numerically. In this study, we rely on a numerical approach to determine the electric potential, ionic concentration and velocity field in the bulk solution surrounding the particle. The numerical simulations use a pseudo-spectral which was used successfully by Chu and Bazant to determine the electric potential and the ionic concentration around an ideally polarizable metallic sphere (Physical Review E, 2006). Our numerical model also incorporates the steric model developed by Kilic \textit{et al.} in 2007 to account for crowding effects in the electric double layer. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G6.00009: Thermal dielectrophoretic force on a dielectric particle Barukyah Shaparenko, Howard Hu, Haim Bau A particle immersed in a fluid subjected simultaneously to electric and thermal fields experiences an electrostatic force given by not only classical dielectrophoresis~(DEP), but also an additional force, which we term thermal DEP. Assuming the change in the background electric field across the particle and the relative change of temperature-dependent electric properties across the particle are both small, we develop a linearized model to solve the electric field analytically and integrate the Maxwell stress tensor to find an expression for the thermal DEP force for aligned electric and thermal fields. This thermal DEP force is proportional to the temperature gradient, the square of the electric field strength, and the particle's volume. We compute the fully-coupled system in COMSOL to determine a range of validity for our linearized model and show a practical way to superimpose the classical DEP and thermal DEP forces to find the total electrostatic force on the particle relative to the fluid. Additionally, we examine the thermal DEP force and torque on the particle for unaligned fields. Due to the high electrical conductivity of common biological buffers, the thermal DEP force can play an important role when an electric field is used to control and manipulate cells or bacteria. [Preview Abstract] |
Monday, November 25, 2013 9:57AM - 10:10AM |
G6.00010: Deterministic separation of particles by electrophoresis: e-DLD Srinivas Hanasoge, Raghavendra Devendra, Francisco J. Diez, German Drazer A suspension of particles of different size driven through an array of posts, by either gravity or flow, results into streams of particles moving in different directions, which can be used for the continuous separation of different species. In this work, we explore a spatially uniform electric field as a driving force. Specifically, we apply an electric field to a quiescent aqueous suspension (constant pH) of spherical particles of different size, across an isotropic two dimensional array of cylindrical posts at different angles with respect to the principal directions of the array and track the motion of particles as they move through the array. In general, the results are in agreement with the existence of deterministic lateral displacement and directional locking, as with the other driving fields. We discuss characterization and separation experiments as well as the advantages of using electric field to separate particles. [Preview Abstract] |
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