Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session A6: Electrokinetics I |
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Chair: William Ristenpart, University of California, Davis Room: 24B |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A6.00001: Modeling the Transport of Colloids to Electrode Strips During Electrophoretic Deposition Andrew Pascall, Kyle Sullivan, Joshua Kuntz Electrophoretic deposition (EPD) is an industrially relevant process in which colloidal particles suspended in a liquid are forced to deposit on a electrode under an applied electric field. Studies of the formation of deposits by EPD have generally focused on electrode geometries that yield analytical solutions, such as infinite parallel planes and concentric cylinders. Here, we focus on an experimentally relevant geometry that has not yielded analytical solutions---the planar strip electrode. We present a finite element model for the transport of material onto a planar strip electrode which shows excellent qualitative agreement to experimental results in a similar system. Notably, we demonstrate that the presence of the edges of the electrode lead to a singularity in the electric field that significantly effects the morphology of the deposit at short times or for thin deposits. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A6.00002: Electrohydrodynamic interactions of spherical particles under Quincke rotation Debasish Das, David Saintillan Quincke rotation denotes the spontaneous rotation of dielectric particles immersed in a slightly dielectric liquid when subjected to a high enough DC electric field. It occurs when the charge relaxation time of the particles is greater than that of the fluid medium, causing the particles to become polarized in a direction opposite to that of the electric field and therefore giving rise to an unstable equilibrium position. When slightly perturbed, the particles start to rotate, and if the electric field exceeds a critical value the perturbations do not decay and the particle rotations reach a steady state with a constant angular velocity. We use a combination of numerical simulations and asymptotic theory to study the effect of electrohydrodynamic interactions between particles under Quincke rotation. We study the prototypical case of two equally charged spheres carrying no net charge and interacting with each other both hydrodynamically and electrically. The case of spherical particles free to roll on a horizontal grounded electrode is also described. We show that Quincke rotation results in self-propulsion of the particles in the plane of the electrode, and interactions between a pair of such ``rollers'' are analyzed. [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A6.00003: Electro-orientation as a facile way to characterize the electrical properties of nanowires Cevat Akin, Jingang Yi, Jerry Shan, Qi Chen, Weihe Xu, Yong Shi The electrical conductivity and/or permittivity of nanowires are often poorly known, heterogeneous, and difficult to measure by traditional direct-characterization methods such as 4-point-probes. Electro-orientation, the rotation of nanowires in liquid suspension into alignment with an external electric field, offers a potential alternative measurement technique that is simple and also compatible with further solution-based sorting and processing of nanowires. We present experimental results obtained by optical microscopy on the alignment rate of nanowires under spatially uniform AC electric fields of different frequency. Silicon nanowires of known conductivity were fabricated using metal-assisted chemical etching and tested to determine how the cross-over frequency for electro-orientation varies with particle conductivity and aspect ratio. We compare our experimental results with theoretically obtained values, and assess the potential of electro-orientation as a quantitative method of characterizing the electrical properties of large-aspect-ratio particles in liquid suspension. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A6.00004: Accurate predictions of dielectrophoretic force and torque on particles with strong mutual field, particle, and wall interactions Qianlong Liu, Kenneth Reifsnider The basis of dielectrophoresis (DEP) is the prediction of the force and torque on particles. The classical approach to the prediction is based on the effective moment method, which, however, is an approximate approach, assumes infinitesimal particles. Therefore, it is well-known that for finite-sized particles, the DEP approximation is inaccurate as the mutual field, particle, wall interactions become strong, a situation presently attracting extensive research for practical significant applications. In the present talk, we provide accurate calculations of the force and torque on the particles from first principles, by directly resolving the local geometry and properties and accurately accounting for the mutual interactions for finite-sized particles with both dielectric polarization and conduction in a sinusoidally steady-state electric field. Since the approach has a significant advantage, compared to other numerical methods, to efficiently simulate many closely packed particles, it provides an important, unique, and accurate technique to investigate complex DEP phenomena, for example heterogeneous mixtures containing particle chains, nanoparticle assembly, biological cells, non-spherical effects, etc. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A6.00005: Role of Solution Conductivity in Reaction Induced Charge Auto-Electrophoresis Jeffrey Moran, Jonathan Posner The self-propelled motion of bimetallic particles in hydrogen peroxide solutions has been widely investigated. Multiple studies have predicted or reported that the swimming speed of these particles scales inversely with solution conductivity. We use scaling analyses and simulations to investigate the physical mechanism for the conductivity-induced deceleration. In particular, we focus on the interaction between dipolar charge density in the fluid surrounding the rod and the electric field it generates, which is weakened by the addition of electrolyte. The simulations show good agreement both with experimental data and with previous analytical treatments of the conductivity dependence. [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A6.00006: Reservoir-based dielectrophoresis (rDEP) for continuous separation of particles based upon surface charge Xiangchun Xuan, Saurin Patel, Shizhi Qian Separating particles from complex mixtures is important to many applications. We develop a continuous-flow microfluidic approach to separating 3 $\mu $m fluorescent and non-fluorescent particles by charge inside a reservoir under DC-biased AC electric fields. This separation exploits the reservoir-based dielectrophoresis (rDEP), which is induced by the inherent electric field gradient formed at the reservoir-microchannel junction, to continuously isolate the trapped fluorescent particles from the streaming non-fluorescent particles. The obtained particle images agree closely with the predicted particle trajectories from a 2D numerical model. It is, however, found that the streaming non-fluorescent particles may also get trapped in the reservoir due to the influences from the accumulated fluorescent particles, which can significantly lower the separation purity. These influences decrease with the enhanced electrokinetic flow (by increasing the applied DC electric field) and the lowered AC field frequency. Since it takes place inside the reservoir and no in-channel mechanical or electrical parts are needed, the demonstrated rDEP particle sorter can be conveniently integrated with other components into lab-on-a-chip devices for diverse particle handling. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A6.00007: The Effect of Electrophoresis and Electroosmosis on Colloid Dynamics at a Micro-Nano-Channel Junction Gilad Yossifon, Yoav Green Understanding the electrokinetic interaction between nano-colliodal particles and a nano-channel is of particular interest in the fast growing field of micro- and nano-fluidics. It is well established that both nanocolloids and nanochannels/nanopores play an important role in biomolecular detection. Combining these may open new routes for a more sensitive detection platform. Our design consists of a nano-slot bounded by two micro-chambers, wherein we introduce the dispersed nano-colloids. We drive the fluid and particles into the channel via an electric field using electrophoresis and electro-osmosis. We have derived an analytic expression for the colloid dynamics within the microchamber, away from the nanoslot entrance vicinity where dielectrophoresis effects are dominant. We account for the two opposing mechanisms - electrophoresis and electroosmosis. The latter accounts for the dependence of nanochannel electro-osmotic permeability on electric Debye layer overlap intensity. These theoretical results stand in good qualitative agreement with the experimental findings. [Preview Abstract] |
Sunday, November 18, 2012 9:31AM - 9:44AM |
A6.00008: Rapid annealing of polycrystalline domains with a hexatic-to-disorder transition in colloidal crystals near electrodes C.S. Dutcher, N.H. Talken, T.J. Woehl, W.D. Ristenpart Colloids are known to form planar, hexagonal closed packed (HCP) crystals near electrodes in response to electrohydrodynamic (EHD) flow. Previous work has established that the EHD velocity increases as the applied AC frequency decreases, suggesting that the driving force for crystallization should increase at lower frequencies. Here we report the existence of an order-to-disorder transition at sufficiently low frequencies, despite the increase in the attractive EHD driving force. At large frequencies ($\sim$1000 Hz), spherical micron-scale particles form HCP crystals; as the frequency is decreased below $\sim$250 Hz, however, the crystalline structure transitions to randomly close packed (RCP) crystals. The transition is reversible and second order with respect to frequency, and independent measurements of the EHD aggregation rate confirm that the EHD driving force is indeed higher at the lower frequencies. We present evidence that the transition is instead caused by increased particle diffusivity due to increased particle height over the electrode at lower frequencies, and we demonstrate that the HCP-RCP transition facilitates rapid annealing of polycrystalline domains. [Preview Abstract] |
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