Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session LM: Microfluids: General VI: Magnetic and Electric Fields |
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Chair: Aloke Kumar, Oak Ridge National Laboratory Room: Long Beach Convention Center 202B |
Monday, November 22, 2010 3:35PM - 3:48PM |
LM.00001: Topology Optimization of Regenerators for Magnetic Refrigeration Fridolin Okkels, Grigorios Panagakos We show a free form geometrical optimization of a simple regenerator model and how it relates to improvements of magnetic refrigerator systems. Magnetic refrigeration systems utilize the magnetocaloric effect to separate the ambient temperature into hot and cold regions, through a thermodynamic cycle. In the specific model [1], a small-scale regenerator support the temperature difference, and we apply the method of topology optimization to improve the existing design. In order for the high-level implementation of topology optimization to work [2], the model has to be steady state, and therefore the refrigeration cycle has been reformulated, using harmonically varying fields, into an amplitude model. The amplitude model nicely reproduces the results from direct simulation of the thermodynamic cycle, and initial results from the topology optimization are presented. \\[4pt] [1] T. F. Petersen, ``Numerical modelling and analysis of a room temperature magnetic refrigeration system,'' PhD-thesis, DTU Ris{\o}, Denmark (2007)\\[0pt] [2] L.H. Olesen, F. Okkels, and H. Bruus, Int. J. Num. Meth. Eng. 65, 975 (2006) [Preview Abstract] |
Monday, November 22, 2010 3:48PM - 4:01PM |
LM.00002: Numerical Simulation on the Liquid Bridge Formation by the Applied Electric Pulse Jin Seok Hong, In Seok Kang In this work, liquid bridge (LB) formation by the applied electric field is analyzed numerically. Numerical simulation captures the temporal behavior of liquid surface during the LB formation between a top plate and a bottom nozzle. Numerical results show the three stages of LB formation; interface elevation, impact/fast spreading and slow spreading/stabilization. The effect of the applied voltage pulse is also studied in terms of minimal electrical energy for LB formation. Non-linear behavior such as bubble trapping at the impact of liquid to plate is also captured and explained qualitatively. Grounded and floating plate is considered. The wetting criterion for LB formation is suggested and explained in terms of capillary pressure. The linear decrease of the final contact radius with the top plate contact angle is shown from the numerical results. In addition, the effects of the liquid properties on the dynamics are briefly discussed. [Preview Abstract] |
Monday, November 22, 2010 4:01PM - 4:14PM |
LM.00003: Three-dimensional rotational dynamics of superparamagnetic microrods Marc Fermigier, Nais Coq, Sandrine Ngo, Olivia du roure, Denis Bartolo We investigate experimentally and theoretically the dynamics of paramagnetic microrods tethered to a solid wall and driven by a precessing magnetic field. We identify two distinct regimes : at low driving frequencies, the response of the rod is synchronous whatever the inclination of the field. Above a characteristic frequency, two qualitatively different behaviors are distinguished, depending on the inclination $\theta b$. For small field inclinations, the response of the filament remains synchronous at all frequencies. Conversely, when the field inclination exceeds a critical value $\sim55^\circ$, the response becomes asynchronous, and the tip of the rod follows a complex trajectory exhibiting three-dimensional back-and-forth patterns. A minimal model, neglecting the flexibility of the rod and the hydrodynamic interaction with the wall, captures the main features of both regimes. We thus show that the complex trajectory patterns are chiefly due to the geometrical nonlinearities in the magnetic dipolar coupling. The critical angle is itself set by a purely geometrical criterium, arising from the superparamagnetic nature of the rod. [Preview Abstract] |
Monday, November 22, 2010 4:14PM - 4:27PM |
LM.00004: Effectiveness of Magnetohydrodynamics in Microdevices for Fluid Flow Yogendra Panta, Wei Lin Magneto-hydrodynamics(MHD) offers an elegant means to control fluid flow in micro- and nano-devices without a need for any mechanical components with the device. In the presence of an external magnetic field in a microchannel filled with ionic sample solutions, the interaction between the electric currents and magnetic fields results Lorentz forces. Electric poential is supplied in the electrodes that patterned on the opposite walls of the channel. The Lorentz forces can be used to propel, stir, mix and/or manipulate fluid flow in the channel. Many works are reported about the MHD micro channel devices for various applications over the last thirty years, but there is still a need for better understanding of flow behavior in these microdevices. Also, there are insufficient studies of flow phenomenon under MHD in microtubules compared to rectangular cross sectioned microchannels. In this work, microtubules and rectangular microchannels are compared with 2D and 3D fluid flow simulation for testing their effectiveness. In presence and absence of external magnetic fields, an extensive parametric study was performed in order to find out the cross dependencies within the various experimental parameters. Numerical simulations were found in a good agreement with published data and esperimental results. [Preview Abstract] |
Monday, November 22, 2010 4:27PM - 4:40PM |
LM.00005: Interaction of two magnetic particles in a rotating magnetic field Tae Gon Kang, Martien Hulsen, Jaap den Toonder, Patrick Anderson, Han Meijer A three-dimensional direct simulation method was employed to solve flows with paramagnetic particles suspended in a non-magnetic fluid. The numerical scheme enables us to take into account both hydrodynamic and magnetic interactions between particles in a fully coupled manner, regardless of the shape of particles. As for the magnetic forces working on particles, the results obtained from our scheme are compared with those from the dipole-dipole interaction model. We confirm the critical angle separating the nature of magnetic interaction with the angle obtained by the point-dipole approximation. Dynamics of interacting two particles in a rotating field is investigated, demonstrating the capability of the method to tackle general problems. Chain dynamics is highly influenced by the Mason number, the ratio of viscous force to magnetic force. Below a critical Mason number, the chain of two particles rotates as a rigid body following the field, but with a phase lag. Above the critical Mason number, however, the chain rotates in overall sense but with an oscillatory motion on top of the rotation. It is also found that the magnetic susceptibility of particles is a factor with an influence on the chain dynamics. At one representative value of the susceptibility, we compared our numerical results with experimentally observed data. [Preview Abstract] |
Monday, November 22, 2010 4:40PM - 4:53PM |
LM.00006: Electric-field-induced pattern formation in colloidal suspensions Jae Sung Park, David Saintillan We use numerical simulations to investigate the long-time dynamics and pattern formation in semi-dilute suspensions of colloidal spheres in a viscous electrolyte under a uniform electric field. Dielectrophoretic interactions between particles occur as a result of Maxwell stresses in the fluid, and the dynamics under these interactions are analyzed in the thin Debye layer limit. Simulations in large-scale suspensions in a thin gap are performed with periodic boundary conditions in the directions perpendicular to the electric field. Results show the rapid formation of finite chains in the field direction, followed by a slow coarsening process by which chains coalesce into hexagonal sheets and eventually rearrange to form mesoscale cellular structures, in agreement with recent experimental observations. The effects of suspension volume fraction, electrode spacing and field strength on this phase transition are described, and a simple explanation for the observed wave number selection is proposed based on the analysis of interactions between two identical finite-length chains. [Preview Abstract] |
Monday, November 22, 2010 4:53PM - 5:06PM |
LM.00007: Stretching Tethered Polymer by Traveling Wave Electric Fields Hsien-Hung Wei, Yeng-Chin Li, Ten-Chin Wen In this work, we theoretically explore the use of traveling wave electric fields in stretching a charged polymer chain whose one end is pinned at a surface. A simple elastic dumbbell model is employed to elicit the essences of the stretching. In a simple sinusoidal field, the chain merely stretches and contracts back and forth with a zero cycle-averaged extension, as expected. In a traveling electric field, on the contrary, the chain can be periodically pulled by subsequent strokes of the field without being fully contracted, and therefore can exhibit some extension during a cycle. And yet, the chain will not stretch at all if the field travels too fast. The detailed response would depend on the Deborah number, the ratio of the elastic force to the stretch force, and alpha, the ratio of the traveling field speed to the characteristic electrophretic velocity of the chain. We not only show how the stretching is characterized by these parameters, but also provide the criteria for realizing the stretching in terms of electrode dimensions, the chain size, and the strength and frequency of an applied traveling field. A possible application to molecular sensing is also discussed. [Preview Abstract] |
Monday, November 22, 2010 5:06PM - 5:19PM |
LM.00008: Electrothermal Flow in Microfluidic Reservoirs Xiangchun Xuan, Junjie Zhu, Sriram Sridharan, Guoqing Hu Electrokinetic flow is an efficient technique for manipulating liquids and samples in microfluidic devices. However, there exists inevitable Joule heating in electrokinetic flow due to the liquid's resistance to the electrical current. As such, both temperature rises and gradients are caused in the liquid, which has long been known to affect the electrokinetic fluid and sample transport within the fluid conduit such as a micro capillary or a microchannel. So far, however, no work has been done on Joule heating effects in microfluidic reservoirs that are the origins of all fluid and sample motions. In this talk we present an experimental and numerical study of electrokinetic fluid flow in microfluidic reservoirs. We demonstrate that fluid circulations can be induced by electrothermal effects inside the reservoir, which is potentially useful for trapping biomolecules or enhancing sample mixing. [Preview Abstract] |
Monday, November 22, 2010 5:19PM - 5:32PM |
LM.00009: Electrospinning of a viscous-capillary jet within dielectric liquid bath Guillaume Riboux An experimentally characterization of the whipping motion of an electrified micro-jet of glycerine immersed within a liquid bath is carried out. In particular, the determination of the evolution of the frequency, the wavelength and the amplitude of the whipping oscillations as a function of the dimensionless parameters: the capillary number, the electrical Bond number and a residence to electrical relaxation time ratio. The presence of whipping requires threshold values of the three parameters to be reached. The electrified cone radius strongly depend on the capillary and electrical Bond numbers. The whipping behaviour, which depends on the capillary number but only weakly on the electrical Bond number, presents three different regimes: periodic, quasi-periodic or chaotic. Results showed that the wavelength and the frequency of the jet whipping depend strongly of the electrical Bond number. The phase velocity of the whipping jet is constant and proportional to the visco-capillary velocity. The detected whipping envelope showed self-similar behavior after appropriate normalization and evolved downstream as a 3/2 power law of the normalized distance. [Preview Abstract] |
Monday, November 22, 2010 5:32PM - 5:45PM |
LM.00010: An efficient discretization of the Poisson-Boltzmann equation with applications to electrostatic force calculation Mohammad Mirzadeh, Todd Squires, Frederic Gibou We present a finite difference discretization of the non-linear Poisson-Boltzmann (PB) equation over complex geometries that has second order accurracy. The level-set method is adopted to represent the interface and Octree (in three dimensions) or Quadtree (in two dimensions) data stuructures are used to generate adaptive grids. Such an approach garanties that the finest grid resolution is located near the interface where EDL forms and creates very large electric field. Several numerical experiments are carried which indicate the second order accuracy both in the case of Dirichlet and Neumann boundary conditions in $L_2$ and $L_\infty$ norms. Finally, we use our method to study the electrostatic interaction of double layers between charged particles in an unbounded bulk electrolyte as well as in a channel where the channel width is of the order of Debye length. [Preview Abstract] |
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