Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session GC: Microfluidics: Mixing I |
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Chair: Nadine Aubry, NJIT Room: Hilton Chicago Grand Ballroom |
Monday, November 21, 2005 10:34AM - 10:47AM |
GC.00001: Direct velocity measurement and enhanced mixing in laminar flows over ultrahydrophobic surfaces Jia Ou, Jonathan Rothstein A series of experiment are presented studying the kinematics of water flowing over drag-reducing ultrahydrophobic surfaces. The surfaces are fabricated from silicon wafers using photolithography and are designed to incorporate patterns of microridges with precise spacing and alignment. These surfaces are reacted with an organosilane to achieve high hydrophobicity. Microridges with different widths, spacing and alignments are tested in a microchannel flow cell with rectangular cross-section. The velocity profile across the microchannel is measured with micro particle image velocimetry ($\mu $-PIV) capable of resolving the flow down to length scales well below the size of the surface features. A maximum slip velocity of $>$60{\%} of the average velocity in the flow is observed at the center of the air-water interface supported between these hydrophobic microridges, and the no-slip boundary condition is found at the hydrophobic microridges. The $\mu $-PIV measurements demonstrate that slip along the shear-free air-water interface supported between the hydrophobic micron-sized ridges is the primary mechanism responsible for the drag reduction. The experiment velocity and pressure drop measurement are compared with the prediction of numerical simulation and an analytical model. By aligning the hydrophobic microridges at an acute angle to the flow direction a secondary flow is produced which can significantly enhance mixing in this laminar flow. [Preview Abstract] |
Monday, November 21, 2005 10:47AM - 11:00AM |
GC.00002: Details of chaotic advection in pulsed micro-mixing Arnaud Goullet, Nadine Aubry Many microfluidic applications require the mixing of reagents, but efficient mixing in these laminar systems is often difficult. In this presentation, we consider further the method of pulsed flow mixing which takes advantage of time dependency rather than spatial complexity. In particular, using computational fluid dynamics (CFD) we analyze the dynamics of the flow in a channel comprising two inlets and one outlet, with a pulsing of 90 degrees out of phase between the two inlets. By performing extensive numerical simulations and following material lines, the details of the mixing mechanism at the confluence of the inlets are shown. [Preview Abstract] |
Monday, November 21, 2005 11:00AM - 11:13AM |
GC.00003: ``Electrokinetic'' mixing instability: The sharp interface limit Hua-Yi Hsu, Neelesh Patankar An instability between two miscible liquid regions, of identical mechanical properties but different electrical conductivities, stressed by an external electric field parallel or perpendicular to the interface is studied. The problem is of interest due to its applications to mixing in microchannels. It is modeled by considering a sharp interface. The transport of the electrical conductivity is governed by a convection-diffusion equation. A shallow channel geometry is considered. It is shown that any velocity perturbation at the interface leads to a varying electrical conductivity, in its vicinity, due to the electromechanical coupling in the jump condition for the electrical conductivity. This in turns leads to a bulk charge density that gives a body force in the fluid equations. The body force generates a cellular motion that results in the instability. The critical condition for the instability is given in terms of a non-dimensional parameter $P_{\Sigma}$, which is a product of the Peclet number and another non-dimensional parameter that depends on the conductivity ratio of the two liquids. The results compare favorably with the experimental data and the diffuse interface based model by Santiago and co-workers. [Preview Abstract] |
Monday, November 21, 2005 11:13AM - 11:26AM |
GC.00004: Analysis of the reaction advection diffusion system for DNA hybridization in a microchannel Thomas John, Igor Mezic We consider the hybridization of short strand DNA in microchannels and model it as a reaction advection diffusion system in an extended space consisting of spatial dimensions as well as orientation. The model can be used to optimize the hybridization rate. We demonstrate with a shear superposition micromixer that it is possible to achieve chaotic mixing in space while simultaneously aligning the strands, thus increasing the rate of creation of double strand DNA. [Preview Abstract] |
Monday, November 21, 2005 11:26AM - 11:39AM |
GC.00005: Homogenization of advection diffusion equations with microstructure Igor Mezic, Thomas John We consider the flow of suspended rod-like microstructure in a thin long channel and perform homogenization over the cross section to obtain effective equations. We obtain correction terms to the effective diffusivity as obtained by Taylor's analysis when the diffusion of the microstructure is not isotropic. [Preview Abstract] |
Monday, November 21, 2005 11:39AM - 11:52AM |
GC.00006: Characterization of mixing in an electroosmotically stirred continuous micro mixer Ali Beskok, Ho Jun Kim We present theoretical and numerical studies of mixing in a straight micro channel with zeta potential patterned surfaces. A steady pressure driven flow is maintained in the channel in addition to a time dependent electroosmotic flow, generated by a stream-wise AC electric field. The zeta potential patterns are placed critically in the channel to achieve spatially asymmetric time-dependent flow patterns that lead to chaotic stirring. Fixing the geometry, we performed parametric studies of passive particle motion that led to generation of Poincare sections and characterization of chaotic strength by finite time Lyapunov exponents. The parametric studies were performed as a function of the Womersley number (normalized AC frequency) and the ratio of Poiseuille flow and electroosmotic velocities. After determining the non-dimensional parameters that led to high chaotic strength, we performed spectral element simulations of species transport and mixing at high Peclet numbers, and characterized mixing efficiency using the Mixing Index inverse. Mixing lengths proportional to the natural logarithm of the Peclet number are reported. Using the optimum non-dimensional parameters and the typical magnitudes involved in electroosmotic flows, we were able to determine the physical dimensions and operation conditions for a prototype micro-mixer. [Preview Abstract] |
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