Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session LQ: Microfluidics: General |
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Chair: Carl Meinhart, University of California, Santa Barbara Room: 200E |
Monday, November 23, 2009 3:35PM - 3:48PM |
LQ.00001: Numerical Investigation of Liquid Flow through Micro-channels with Post Patterned Super-hydrophobic Walls A. Amin, D. Maynes, B.W. Webb We numerically investigate the effect of post patterned super-hydrophobic surfaces on the drag reduction for laminar liquid flow through micro-channels. Hydrophobic surfaces exhibiting micro-scale structures can significantly reduce the liquid-solid contact area resulting in reduced surface friction. The effects of cavity fraction (the ratio of cavity area to total surface area) and relative module width (ratio of post/cavity repeating length to channel hydraulic diameter) on the slip-length and on the Darcy friction factor-Reynolds number product, \textit{fRe}, were explored numerically for the post structured hydrophobic walls. The cavity fraction and relative module width vary from 0.5 to 0.9998 and from 0.01 to 1.5, respectively. In general, as both cavity fraction and relative module width increase \textit{fRe} decreases. The present results are compared with those for surfaces exhibiting rib/cavity patterns that are parallel and perpendicular to the flow direction. At high cavity fractions the post/cavity structuring produces larger slip-length and greater reduction in \textit{fRe} than either parallel or perpendicular rib/cavity structures. The results are also compared with scaling laws previously published in the literature. [Preview Abstract] |
Monday, November 23, 2009 3:48PM - 4:01PM |
LQ.00002: A universal scaling for viscous flows around microfabricated pillars Nimisha Srivastava, Carl Meinhart Complex geometries that involve an intricate network of channels and pillars are increasingly being used in microfluidic devices. Central to the successful operation of these devices is a fundamental theoretical framework that explains the effects and interplay of viscous, inertial and capillary forces in these geometries. One such geometry is a dense (1000 by 1000) array of micron-sized pillars. We will present a universal scaling (over four orders of magnitude) that predicts viscous, pressure driven flows in these pillars. We have developed a finite element model using COMSOL Multiphysics to simulate Stokes flow between pillars. Using curve fitting on flow through a wide range of height, diameter and gap (an order of magnitude), we were able to derive a unique model that will accurately predict flow rates in any given random array of pillars. We have found that the pressure driven viscous flow within pillars depends almost linearly with the height (h) of the pillars while it varies inversely with the square root of the diameter (d) of the pillars. The flow rate follows a 2.33 power of the gap between the pillars. In addition, we have, \textit{for the first time}, observed lubrication- like scaling in low Reynolds number (\textbf{\textit{Re}}$<$0.5) viscous flows around an array of microfabricated pillars. Our experiments and simulations have explored and validated the design space when \textbf{\textit{h}}/\textbf{\textit{g}} and \textbf{\textit{g}}/\textbf{\textit{d}} is between 1 and 10 --s as is the case in most microfluidic applications, which makes this finding imperative for future design of geometries involving pillars, tortuous channels and porous structures. [Preview Abstract] |
Monday, November 23, 2009 4:01PM - 4:14PM |
LQ.00003: A universal scaling for viscous flows around micro- and nano-fabricated pillars Carl Meinhart, Nimisha Srivastave, Changsong Ding, Noel MacDonald Complex geometries that involve an intricate network of channels and pillars are increasingly being used in microfluidic devices. Central to the successful operation of these devices is a fundamental theoretical framework that explains the effects and interplay of viscous, inertial and capillary forces in these geometries. One such geometry is a dense (1000 by 1000) array of micron-sized pillars. We will present a universal scaling (over four orders of magnitude) that predicts viscous, pressure driven flows in these pillars. We have developed a finite element model that simulates Stokes' flow between pillars. Building upon a universal scaling law for viscous losses in the pillars, we developed a model that accurately predicts flow rate through the pillars. We have found that pressure-driven viscous flow within the pillars depends nearly linearly with the height (h), inversely with the square root of the diameter (d), and a power law behavior with the gap between the pillars. In addition, we have observed lubrication- like scaling in low Reynolds number (Re $<$0.5) viscous flows around an array of microfabricated pillars. Numerical results compare well to experimental observation. [Preview Abstract] |
Monday, November 23, 2009 4:14PM - 4:27PM |
LQ.00004: Bistability in a simple fluid network due to viscosity contrast Brian Storey, John Geddes, David Gardner, Russell Carr We study the existence of multiple equilibrium states in a simple fluid network using Newtonian fluids and laminar flow. We demonstrate theoretically the presence of hysteresis and bistability, and we confirm these predictions in an experiment using two miscible fluids of different viscosity; sucrose solution and water. Possible applications include bloodflow, microfluidics, and other network flows governed by similar principles. [Preview Abstract] |
Monday, November 23, 2009 4:27PM - 4:40PM |
LQ.00005: Streaming Flow in Branching Micro/Mini Channels Donna Meyer, Zongqin Zhang, Chang Liu, Thomas Barek, Ahmed Fadl, Manfred Krafczyk A streaming-based study is presented in branching micro and minichannels of oscillating flows with no net mass flow. Zero-mean velocities result in distinct differences between forward and backward flow velocity profiles, causing near-wall particles in the fluid, and those near the channel center, to advance in opposite directions. Streaming velocities were found to be highly influenced by oscillating amplitude and frequency, as shown in numerical analyses and validated analytically. A kinematic viscosity of the fluid which is larger than the diffusivity of the particles were found to result in effective convective transport. Advantages of streaming flow-based phenomenon include enhanced mixing, pumpless fluid propulsion, multichannel fluid distribution, easy system integration with cost-effective operation. The distinguishing features of streaming flow lend themselves to numerous applications. [Preview Abstract] |
Monday, November 23, 2009 4:40PM - 4:53PM |
LQ.00006: Microfluidics on a wire Tristan Gilet, Denis Terwagne, Nicolas Vandewalle In this talk, we discuss the behavior of droplets sliding on fibers. An on/off transition is observed when a droplet comes around an intersection between several fibers: large droplets cross the junction while small droplets remain blocked. We show that simple fiber networks may perform advantageously most operations of digital microfluidics, such as multiplexing: intersections could be the basic component of new fiber-based microfluidic devices. [Preview Abstract] |
Monday, November 23, 2009 4:53PM - 5:06PM |
LQ.00007: Systems-level analysis of AC microfluidics and the problem of trapped gas bubbles S{\O}ren Vedel, Laurits H{\O}jgaard Olesen, Henrik Bruus Using pulsatile pressure and flow rate, we extend the equivalent circuit (EC) approach for systems-level analysis of microfluidic systems to also include dynamic, transient effects such as inertia and compliance. The dynamic time scales of microfluidics are typically on the order of millisecond, or equivalently frequencies in the low kHz regime. A novel pressure source has been developed and successfully tested for the experimental generation of flow under these conditions for two microfluidic setups. Good agreement was found between the experimental observations and the results of corresponding systems-level EC model [1]. Trapped air bubbles in the microfluidic system severely influences its performance, while also leading to erroneous predictions from the systems-level analysis. We present theoretical analysis of the physics of bubble adhesion to the system walls, leading to insights to their removal. \\ \noindent{}[1] S. Vedel, L.H. Olesen, H. Bruus, Lab Chip (submitted 2009), http://arxiv.org/abs/0907.2679 [Preview Abstract] |
Monday, November 23, 2009 5:06PM - 5:19PM |
LQ.00008: Convective Heat-Transfer Characteristics of Laminar Flow Through Smooth- and Rough-Wall Microchannels V.K. Natrajan, K.T. Christensen The convective heat-transfer behavior of laminar flow through smooth- and rough-wall microchannels is investigated by performing \textit{non-intrusive} measurements of fluid temperature using a microscale adaptation of two-color laser-induced fluorescent thermometry for flow through a heated copper microchannel testbed of hydraulic diameter $D_{h}=600$\,$\mu$m. These measurements, in concert with pressure-drop measurements, are performed for a smooth-wall case and two different rough-wall cases with roughness that is reminiscent of the surface irregularities one might encounter due to imperfect fabrication methods. Pressure-drop measurements reveal the onset of transition above $\mathrm{Re_{cr}}=1800$ for the smooth-wall case and deviation from laminar behavior at progressively lower Re with increasing surface roughness. The local Nusselt number (Nu) for smooth-wall flow over the range $200\leq \mathrm {Re} \leq \mathrm{Re_{cr}}$ agree well with macroscale predictions in both the thermally-developing and -developed regimes. With increasing roughness, while an enhancement in local Nu is noted in the thermally-developing regime, these differences do not exist upon attainment of a thermally- developed state. Examination of temperature profiles across the microchannel suggest that the thermal boundary layer may be regenerated by roughness, resulting in a delay in the attainment of thermally-developed flow. [Preview Abstract] |
Monday, November 23, 2009 5:19PM - 5:32PM |
LQ.00009: Optimization of Microchannel Characteristics for Enhanced Heat Transfer in the Laminar Regime R. Saksena, K.T. Christensen The present effort explores optimization of microchannel characteristics for enhanced convective heat transfer in the laminar regime with specific application to the development of complex microfluidic networks for self-cooling material systems. Of particular interest is optimization of the layout of the microscale flow passages in a manner that both maximizes heat transfer while simultaneously minimizing any additional pressure-drop penalty compared to straight microchannels. This optimization is achieved using a multi-objective genetic algorithm in conjunction with numerical simulations in the flow regime relevant to self- cooling applications ($\mathrm{Re}<100$) that span a parameter space of both wavelength and amplitude for sinuous microchannels. Experimental validation of the identified optimal configurations is performed in a controlled heat-transfer environment using microchannels written with a unique robocasting fugitive-ink-deposition printer at UIUC. [Preview Abstract] |
Monday, November 23, 2009 5:32PM - 5:45PM |
LQ.00010: Hysteresis and wall-effects in low Reynolds number propulsion by driven elastic filaments Sarah Clark, Prabhakar Ranganathan, James Friend There is currently intense interest in developing micron-sized robots for uses such as minimally invasive surgery. Although progress has been made in miniaturizing the motor, the hydrodynamic behavior of associated propellers is far from being fully understood. An example is an elastic filament driven by a torque at one end where the shape assumed by the filament is strongly coupled to the hydrodynamics forces. Investigation of these dynamics has only recently commenced, for instance Manghi et al. [PRL 96, 068101 (2006)] uncovered an intriguing shape transition in an elastic filament spun in a bulk fluid. Since such transitions can be expected to have a crucial bearing on the operation of microbot swimmers we examine this behavior in detail with simulations. We also study the effect of planar no-slip walls on the propulsion characteristics. The slender filament is represented as a bead-spring chain and inter-bead hydrodynamic-interactions are described using the appropriate Greens functions. We study the origin of the shape transition and hysteresis in detail and show the relationship to sedimenting filaments. We show that the presence of a boundary either perpendicular or parallel to the axis of the applied torque has a significant effect on the overall motion. We also point out the possible detrimental consequences of these effects on operation of microbots in the vicinity of conduit walls. [Preview Abstract] |
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