Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session KA: Micro Fluids: General V |
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Chair: Daniel Maynes, Brigham Young University Room: Salt Palace Convention Center 150 A-C |
Tuesday, November 20, 2007 8:00AM - 8:13AM |
KA.00001: Propagation of particle-displacement waves in linear arrays of particles in Poiseuille flow between two parallel walls Matthew Baron, Jerzy Blawzdziewicz, Eligiusz Wajnryb A spherical particle subject to an incident Poiseuille flow in a narrow parallel-wall channel produces a disturbance flow corresponding to a two-dimensional harmonic-pressure dipole. Interaction of such far-field dipolar pressure-driven flows results in many interesting collective dynamical phenomena in regular particle arrays in a channel. In this talk we will focus on the propagation of collective particle-displacement waves in long linear arrays. We will describe the dispersions relation for transverse and longitudinal waves and discuss propagation of wave packets for different initial particle-displacement distributions. [Preview Abstract] |
Tuesday, November 20, 2007 8:13AM - 8:26AM |
KA.00002: Turbulent Flow through a Microchannel with Superhydrophobic Walls Kevin Jeffs, Daniel Maynes, Webb Brent A growing amount of research has focused on the reduction of drag in microfluidic transport. One approach is to fabricate micro-ribs and cavities in the channel wall that are then treated with a hydrophobic coating. Such treatment reduces the surface contact area between the flowing liquid and the solid wall, thus yielding walls with no-slip and nearly shear-free regions at the microscale. Most of the previous work has focused on the laminar flow where reductions in the frictional resistance as large as 87{\%} have been observed. Little research, however, has explored the potential drag reduction associated with turbulent flow through such microchannels. Results of an investigation of the turbulent fully developed flow in a parallel plate microchannel with microengineered surfaces will be discussed. A \textit{k-$\omega $ }turbulence modeling scheme is implemented for closure to the turbulent RANS equations. Results are presented for the friction factor as a function of the relevant governing dimensionless parameters. The Reynolds number was varied from 2,000 to 10,000 and compared to previously obtained laminar flow data. Results show, as with the laminar flow case, that as the shear-free region increases the friction factor decreases. The observed reduction, however, was found to be significantly greater in the turbulent flow cases than in the case of laminar flow. [Preview Abstract] |
Tuesday, November 20, 2007 8:26AM - 8:39AM |
KA.00003: Influence of the Vapor Cavity Depth on Liquid Flow through a Microchannel Exhibiting Superhydrophobic Walls Daniel Maynes, Kevin Jeffs, Brady Woolford, Brent Webb We report results of an analytical and experimental investigation of laminar flow in a parallel-plate microchannel with superhydrophobic walls. The walls are fabricated with hydrophobically coated micro-ribs and cavities that are oriented parallel to the flow direction and are modeled in an idealized fashion, with the shape of the liquid-vapor meniscus approximated as flat. An analytical model of the flow in the vapor cavity is employed and coupled with a numerical model of the liquid flow. The numerical predictions show that the effective slip length and the reduction in the classical friction factor-Reynolds number product increase with increasing relative cavity width and depth, and decreasing relative micro-rib/cavity module length. Comparisons are also made between the zero shear interface model and the liquid-vapor cavity coupled model. The results illustrate that the zero shear interface model under-predicts the overall flow resistance. Further, the deviation between the two models was found to be significantly larger for increasing values of both the relative rib/cavity module width and the cavity fraction. The trends in the frictional pressure drop predictions are in good agreement with experimental measurements made at similar conditions and a generalized expression for predicting the friction factor is proposed. [Preview Abstract] |
Tuesday, November 20, 2007 8:39AM - 8:52AM |
KA.00004: Numerical simulation of slug formation in micro-channels Sreedhar Manchu, Yves Dubief, William Louisos, Tim Harris, Darren Hitt We study the mixing of two immiscible fluids through a 90$^{\circ}$ junction of two micro-channels. The flow is simulated using both a particle-based method, dissipative particle dynamics (DPD), and a continuum Navier-Stokes approach with conservative level-set method. In both cases, immersed boundary methods simulate the walls. The Reynolds number is of the order of unity and the Weber number is varied for the purpose of the study. The measurement of the volume fraction of each fluid downstream of the junction exhibits a strongly periodic behavior. Under adequate conditions, micro-slugs that filled almost the entire width of the channel are formed. The simulation are compared with existing experimental data using water and octanol or aqueous glycerol. The flexibility of both codes is used to test optimization algorithms aiming at the control of the micro-slugs size and frequency. Preliminary results of this optimization study will be presented. [Preview Abstract] |
Tuesday, November 20, 2007 8:52AM - 9:05AM |
KA.00005: Drop deformation between parallel plates Patrick Anderson, Pieter Janssen Studying the nature of flow in confined geometries has become increasingly important due to downsizing of equipment. Examples include microfluidic devices as lab-on-a-chip and flow through porous media. Here, we focus on the flow of a single drop in a matrix fluid confined between two parallel walls, where the distance between the walls is in the order of the drop diameter. To model this system a three-dimensional boundary integral method is used with the inclusion of the two parallel walls in the free-space kernels of the boundary integral method. The deformation of a drop in shear flow as function of the capillary number and the distance between the walls is studied. The drop shapes found in the presence of the walls substantially differ from the typical ellipsoidal shaped drops found in unbounded flows. Overall deformation, expressed in the Taylor deformation parameter, increases when reducing the distance between the walls. Furthermore, the angle of the major drop axis with the velocity direction also decreases. A detailed analysis decribing the dynamics of breakup of drops in confined geometries is discussed. [Preview Abstract] |
Tuesday, November 20, 2007 9:05AM - 9:18AM |
KA.00006: Use of Nano-patterning to Manipulate Particle-Wall Interactions for Micron Scale Objects in Shear flow Ranojoy Duffadar, Jeffrey Davis, Maria Santore Computational models have been developed to understand the adhesion dynamics of micron- and submicron-scale objects in low Reynolds number flows over planar surfaces with randomly distributed electrostatic heterogeneity at $O$(10nm) length scales. These surfaces are net-repulsive but present spatially varying colloidal potentials. In addition to hydrodynamics and the heterogeneous colloidal field, the model includes surface roughness and contact and frictional forces between the particles and the nano-textured surface to investigate particle motion both in flowing solution and upon wall contact. The predicted particle trajectories, velocities, adhesion thresholds, and deposition rates agree quantitatively with experimental results. Adhesion regime diagrams are constructed to quantify the regions in parameter space in which no contact, skipping, rolling, and arrest are observed. These diagrams and characteristic adhesion signatures are reminiscent of pattern recognition and adhesion in biological systems, such as leukocyte rolling. This dynamic adhesion behavior relies on local fluctuations in the electrostatic heterogeneity and is not observed for systems with ordered heterogeneity at the same length scales. [Preview Abstract] |
Tuesday, November 20, 2007 9:18AM - 9:31AM |
KA.00007: Slip behavior of the confined polymer melt near periodically roughened surface: comparison between molecular dynamics and continuum simulations Anoosheh Niavarani, Nikolai Priezjev Molecular dynamics (MD) simulations are used to investigate the behavior of the slip length in the Couette flow of a polymer melt. For atomically smooth surfaces and weak wall-fluid interactions, the shear rate dependence of the slip length is a non-monotonic function, with a distinct local minimum. For corrugation wavelengths larger than the radius of gyration of polymers, the decay of the slip length with corrugation amplitude obtained from MD simulations agrees well with the continuum predictions for the following cases: (1) Stokes solution with constant local slip length, (2) Stokes solution with local shear-rate-dependent slip length, and (3) Navier- Stokes solution with local rate-dependent slip length. If the corrugation wavelength is less than or on the order of the radius of gyration, the continuum predictions (the Stokes solution) overestimate the values of the slip length extracted from MD simulations. The analysis of the conformational properties of the polymer melt indicates that polymer chains tend to stretch in the direction of shear at the peaks of the sinusoidal wave and align themselves along the bottom of the grooves. The MD simulations show also that the slip length at large corrugation amplitudes and small wavelengths correlates well with the radius of gyration inside the grooves. [Preview Abstract] |
Tuesday, November 20, 2007 9:31AM - 9:44AM |
KA.00008: DEP and EWOD Forcing for Application in Digital Microfluidics Patrick Young, Kamran Mohseni Accurate descriptions of actuation forces and resultant droplet velocities must be available when designing a microfluidic device makinguse of discretized flows. Currently, the most promising methods of droplet actuation in microfluidic devices are electrowetting on dielectric (EWOD) for conductive droplets and dielectrophoresis (DEP) for electrically insulating droplets, where in both cases droplets are transported by sweeping an applied voltage along a microchannel. Numerical modeling of the droplet dynamics for EWOD and DEP configurations has been done using approximations of the electrostatic effect, but incorporation of the electrostatic force density into a direct simulation of the fluid mechanics is desired. This talk focuses on the relationship between EWOD and DEP. The equations that govern the forcing of both mechanisms are presented in detail, including a resolution of the seemingly contradictory model of forcing in DEP; that being the Korteweg-Helmholtz and Kelvin polarization formulations. Numerical results are presented that compare the net force and force distribution in EWOD and DEP. The effect of electrode size and patterning on the total imposed force on the droplet is presented. [Preview Abstract] |
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