Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session M27: Microfluids: General VI |
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Chair: Chuan-Hua Chen, Duke University Room: Ballroom I |
Tuesday, November 22, 2011 8:00AM - 8:13AM |
M27.00001: Streaming potential revisited Ehud Yariv, Ory Schnitzer, Itzchak Frankel Streaming-potential phenomena refer to the generation of bulk electric fields by imposed relative motion between a charged solid and the Debye layer adjacent to it. Realistic scenarios are adequately described by the thin-Debye-layer limit $\delta\to 0$ ($\delta$ denoting the dimensionless Debye thickness), which has been addressed by Cox (1997). Cox's analysis has established that the perturbation to the flow, neglected in the earlier investigations, gives rise to an $O(\delta^4)$ force that dominates that contributed by Maxwell stresses. Cox's theory is founded upon the assumption of $O(1)$ Hartmann and P\'eclet numbers. We demonstrate that the product of these numbers is actually $O(\delta^{-2})$ and accordingly revisit the generic problem of streaming-potential. Electric-current matching between the Debye layer and the bulk provides an inhomogeneous Neumann condition governing the electric field in the latter. This field, in turn, results in a velocity perturbation animated by a Smoluchowski-type slip condition. Owing to dominant convection, the present analysis yields an asymptotic structure considerably simpler than that of Cox (1997): the electro-viscous effect now already appears at $O(\delta^2)$ and is contributed by both Maxwell and viscous stresses. The present paradigm is illustrated for the prototypic problem of a sphere sedimenting in an unbounded fluid, with the resulting drag correction differing from that calculated by Cox (1997). [Preview Abstract] |
Tuesday, November 22, 2011 8:13AM - 8:26AM |
M27.00002: Electrokinetic lift Ory Schnitzer, Ehud Yariv, Itzchak Frankel Electrolyte flow relative to a charged surface induces a bulk electric field (the ``streaming potential'' phenomena). This field, and the flow perturbation it animates, generate both electrical and hydrodynamic ``electro-viscous'' forces whose magnitude has been a matter of ongoing controversy. Recently we have revisited this problem, predicting $O(\delta^2)$ scaling (as opposed to earlier prediction of $\delta^4$ and $\delta^6$), $\delta \ll 1$ being the dimensionless Debye width. These electro-viscous forces can explain the anomalous repulsion of polystyrene microspheres from an adjacent wall in the presence of an imposed shear flow, observed by Prieve and co-workers. Owing to the symmetry properties of the linear Stokes equations, such repulsion is inadmissible in the absence of inertial effects. This particle--wall interaction is analyzed using our revised scheme. The undisturbed flow consists of three components: the `driving' shear mechanism and the `induced' particle translation and rotation. We consider a small dimensionless particle--wall gap $\epsilon$. At leading-order, both the lift and additional drag are contributed by the inner gap region. The lift force is $O(\delta^2\epsilon^{-3})$ while the additional drag is $O(\delta^2\epsilon^{-2})$. The streaming-potential mechanism underlying these forces arises from the `induced' rather than the driving component. [Preview Abstract] |
Tuesday, November 22, 2011 8:26AM - 8:39AM |
M27.00003: From diffusive motion to sub-diffusive motion with local aggregation: the effect of surface contamination in dipolophoresis Jae Sung Park, David Saintillan We investigate the effects of surface contamination in suspensions of ideally polarizable spheres in an applied electric field using large-scale particle simulations. In the case of clean particles, suspensions are known to undergo dipolophoresis (DIP), or combination of dielectrophoresis (DEP) and induced- charge electrophoresis (ICEP), in which ICEP dominates the dynamics over DEP. Surface contamination is modeled as a thin dielectric layer coated uniformly around the particle surface, which can be captured by a means of a correction factor multiplying the induced zeta potential or slip velocity driving ICEP. Because the correction factor is always less than unity, ICEP is gradually suppressed as surface contamination becomes significant, resulting in a transition in the suspensions from diffusive motions to sub-diffusion and local aggregation. Large- scale simulation results are presented on the suspension microstructure, as well as on particle velocities, which are strongly reduced for contaminated particles. We demonstrate a clear transition from diffusion to sub-diffusion by calculating particle mean-square displacements, and we explain it as a consequence of the non-integrable local waiting time distributions that arise in the sub-diffusive regime due to the trapping of the particles inside chains and clusters. [Preview Abstract] |
Tuesday, November 22, 2011 8:39AM - 8:52AM |
M27.00004: Directly resolving particles in an electric field: local charge, force, torque, and applications Qianlong Liu Prosperetti's seminal Physalis method for fluid flows with suspended particles is extended to electric fields to directly resolve finite-sized particles and to investigate accurately the mutual fluid-particle, particle-particle, and particle-boundary interactions. The method can be used for uncharged/charged dielectrics, uncharged/charged conductors, conductors with specified voltage, and general weak and strong discontinuous interface conditions. These interface conditions can be in terms of field variable, its gradients, and surface integration which has not been addesed by other numerical methods. In addition, for the first time, we rigorously derive the force and torque on the finite-sized particles resulting from the interactions between harmonics. The method, for the first time, directly resolves the particles with accurate local charge distribution, force, and torque on the particles, making many applications in engineering, mechanics, physics, chemistry, and biology possible, such as heterogeneous materials, microfluidics, electrophotography, electric double layer capacitors, and microstructures of nanodispersions. The efficiency of the method is demonstrated with up to one hundred thousand 3D particles, which suggests that the method can be used for many important engineering applications of broad interest. [Preview Abstract] |
Tuesday, November 22, 2011 8:52AM - 9:05AM |
M27.00005: Multiscale study of nanoparticle-wall interactions in electroosmotic flow A.T. Conlisk, Harvey Zambrano, Zhizi Peng In electroosmotic transport (EOT), particle mobility results not only from the dragging exerted by the electrolyte, but also from the force exerted by the External Electric Field (EEF), and from the interactions with the walls and with the solvent. The objective of this work is to develop a unified theory of the motion of colloidal particles near walls and compare with the experiments of Kazoe and Yoda for EOT. In the present study a novel continuum approach is developed to study the particle interactions with polystyrene beads. Moreover, we conduct Non-equilibrium Molecular Dynamics Simulations (NEMDS) of a nanoparticle as it moves near a solid-liquid interface subjected to an EEF. We investigate the response of the particle to changes in the surface electrostatics and the electrolyte concentration. Therefore, we perform NEMDS of a silica particle immersed in an electrolyte. The electrolyte solution is mounted on a silica substrate and the particle is constrained to move parallel to the surface so that we can extract the forces acting between the particle and the wall. We vary the electrolyte concentration, the particle size and the surface electrostatics. [Preview Abstract] |
Tuesday, November 22, 2011 9:05AM - 9:18AM |
M27.00006: Role of Background Electrolyte in Electrokinetic Locomotion by Reaction-Induced Charge Auto-Electrophoresis Jeffrey L. Moran, Jonathan D. Posner Bimetallic particles propel themselves through aqueous solutions by harvesting chemical energy from hydrogen peroxide fuel and converting it to fluid motion through reaction-induced charge auto-electrophoresis. We present a scaling analysis and computational simulations that describe the physics underlying the locomotion of these particles. The model shows that the motion results from electrical body forces in the surrounding fluid, which are generated by a coupling of an asymmetric dipolar charge density distribution and the electric field it generates. The simulations and scaling analysis make the predictions, in agreement with experiments, that the speed of the autonomous motion depends linearly on fuel concentration and particle surface charge and inversely on solution conductivity. [Preview Abstract] |
Tuesday, November 22, 2011 9:18AM - 9:31AM |
M27.00007: Electrokinetically induced flocculation of Enteroaggregative \textit{Escherichia coli} Aloke Kumar, Ninell Mortensen, Mansueta Harris, Partha Mukherjee, Scott Retterer, Mitchel Doktycz Enteroaggregative \textit{Escherichia coli} (EAEC) is a diarrheal microbe, whose aggregative dynamics is involved in its pathogenic behavior. We investigated EAEC's electrokinetic response in miniaturized and microfluidic devices. We found a novel response of the microbe under low magnitude, uniform and oscillating electric fields. In this electrokinetically induced response, microbial adhesion to a glass substrate decreases significantly, leading to a loss of EAEC's biofilm forming abilities. Some earlier studies had indicated that that microbial adhesion and detachment at surfaces can be prompted only by charge-transfer processes at the electrode and not applied electrical potentials - such an inference is not corroborated by our work. Instead, we found that electric fields promote the formation of large mesoscopic microbial aggregations (flocs) in the solution. The presence of frequency dependent relaxation phenomena is explored and the observed results are extended to other microbes. [Preview Abstract] |
Tuesday, November 22, 2011 9:31AM - 9:44AM |
M27.00008: Magnetoviscosity and thread-like agglomerations in ferrofluids Philip Yecko, A. Cali, W.-K. Lee, S. Nunez, J. Prescod, R. Smith, A.D. Trubatch, M. Vieira We report on experiments and simulations performed on small non-magnetic glass balls falling under gravity through a magnetized ferrofluid. The applied magnetic field is oriented horizontally, normal to the fall, and is uniform but its magnitude can be adjusted over a wide range. Using the Advanced Photon Source x-ray beamline at Argonne, we were able to achieve sufficient spatial and temporal resolution to track the dynamics of these $500 \mu$m diameter spheres simultaneously with an array of magnetic particle {\em macro-chains} -- thread-like agglomerations each several mm long and $2-10 \mu$m thick. The enhanced drag induced by the macro-chains is enormous: up to four times larger than for unmagnetized fluid, a value greater than is predicted by the prevailing magneto-viscosity model. We provide direct visualization of a possible mechanism by which macro-chains impede the transverse motion of spheres. Numerical simulations can reproduce the observed drag, without modeling it physically, by implementing a simple magnetization dependent anisotropic viscosity. [Preview Abstract] |
Tuesday, November 22, 2011 9:44AM - 9:57AM |
M27.00009: Non-Clogging Resistive Pulse Sensing with Electrohydrodynamic Cone-Jet Bridges Chuan-Hua Chen, Yuejun Zhao, David Bober A new paradigm of resistive pulse sensing (Coulter counting) is developed using a liquid bridge in lieu of a solid pore as the sensing aperture, where the flexible liquid aperture circumvents the clogging issue of conventional Coulter counters. The electrohydrodynamic bridge is formed between two opposing Taylor cones and stabilized by radial polarization stresses. Passage of a colloidal particle through the upstream conical apex triggers a current oscillation at the resonant frequency of the cone-jet bridge. The relative current change is indicative of the particle- to-jet diameter ratio. [Preview Abstract] |
Tuesday, November 22, 2011 9:57AM - 10:10AM |
M27.00010: ABSTRACT HAS BEEN MOVED TO M29.00009 |
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