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 R30: Microfluids: General VII |
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Chair: Paulo Arratia, University of Pennsylvania Room: Ballroom IV |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R30.00001: Tracking the Growth Rate of Nanopillar Formations Caused by Large Thermocapillary Forces Euan McLeod, Sandra Troian Viscous nanofilms are known to deform spontaneously into periodic arrays of nanopillars when exposed to a strong transverse thermal gradient. Comparison of the characteristic pillar spacing in experiment with predictions of linear instability analysis suggests that thermocapillary forces, and not acoustic phonon pressure or electrostatic image charge, are likely the dominant destabilizing mechanism [1]. Examination of the dynamical shapes and growth rates of emerging peaks provides an even more stringent test of the physical mechanism underlying the deformation process. Here we report measurements based on white light interferometry in which the reflected intensity from individual color channels is used to monitor the growth and shape of 3D formations in molten polymer nanofilms. Numerous experiments were conducted to isolate the influence of various operating parameters. These measurements exhibit an extended regime characterized by exponential growth which persists well beyond small amplitude deformations. The corresponding growth rates agree well with predictions of linear stability theory based on thermocapillary flow; however, the inferred film viscosities are systematically larger than bulk values. \\[4pt] [1] E. McLeod, Y. Liu, and S. M. Troian, Phys. Rev. Lett. 106, 175501(2011) [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R30.00002: Dynamics of flexible molecules in thinning fluid filaments Paulo E. Arratia, Gabriel Juarez Newtonian liquids that contain small amounts ($\sim $ppm) of flexible polymers can exhibit viscoelastic behavior in extensional flows. In this talk, we report the results of experiments on the thinning and breakup of polymeric fluids in a simple microfluidic device. We aim to understand the stretching dynamics of flexible polymers by direct visualization of fluorescent DNA molecules, a model polymer. A Boger fluid, composed of 100 ppm polyacrylamide and 85{\%} w/w glycerol, is seeded with stained lambda---DNA molecules ( $<$ 10{\%} v/v) imaged by high speed epifluorescence microscopy. We observe that the strong flow in the thinning fluid threads provide sufficient forces to stretch the DNA molecules away from their equilibrium coiled state. The distribution of stretch lengths, however, is very heterogeneous due to molecular individualism and initial conditions. Once the molecules are stretched to their full length and aligned with the flow, they translate along the fluid thread as rigid rods until the point of pinch off. After pinch off, both the fluid and molecules return to a relaxed state. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R30.00003: Optimum Transport in Flat Heat Pipes Yu-Wei Liu, Marin Sigurdson, Payam Bozorgi, Noel MacDonald, Carl Meinhart In this study we investigate wetting behavior of the wick structure and the maximum theoretical heat transfer rate of a 40 cm titanium flat heat pipe. Large scale flat heat pipes are designed for high performance electronics cooling. Wick designs in flat heat pipes are typically limited by viscous drag and capillary pressure, and do not transport fluids as sufficient rates to meet practical cooling requirements. An analytical model is used to describe flow through wick structure with array of pillars. The capillary pressure and viscous drag are obtained by surface energy calculation and numerical simulations, respectively. To verify the model, we conducted wetting tests on the wick samples with different pillar dimensions. The model agrees qualitatively with the experiments, but under predicts the viscous drag. We extend the model to calculate the pressure drop for liquid and vapor flows, which predicts the upper limit for heat transport in flat heat pipes. Pillar parameters are optimized for maximum heat transfer rate, which approaches several hundred of Watts. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R30.00004: Validation of CFD models for microscale nanoprecipitation reactor using $\mu$-PIV and confocal $\mu$-LIF Yanxiang Shi, Michael G. Olsen, Rodney O. Fox Over the past a few decades, computational fluid dynamics (CFD) models have become more and more important in the process of reactor design in chemical engineering. Compared to experimental methods, they can provide comprehensive information on the flow field as well as other fields, such as concentration. However, they also need to be validated against experimental data to ensure the accuracy. In this work, the micro-scale particle image velocimetry ($\mu$-PIV) is employed in conjunction with the confocal-base micro-scale laser induced fluorescence ($\mu$-LIF) to specifically validate CFD models for use in microscale nanoprecipitation reactor. The former is for the velocity field measurement and the latter gives us the mixture fraction information. Both RANS and LES are used to simulate the field flow. For RANS, a DQMOM-IEM micromixing model is used to predict the mixture fraction field while only a scalar transport equation is solved in the LES simulations. Comparisons between simulation results and experimental data show that RANS might not be the right tool for such reactors. LES, on the other hand, gives reasonably satisfactory predictions. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R30.00005: Thermophoresis of a temperature responsive polymer Jennifer Kreft Pearce, Klinton Kilgore, Audrey Hammack, Jacob Ford Thermophoresis, the migration of a species due to a temperature gradient, has been shown to be a possible mechanism for manipulating molecules in microfluidic devices. The mechanism governing thermophoresis is complex making its dependence on different physical factors hard to predict. We experimentally investigate thermophoresis of a polymer which exhibits inverse temperature dependence of its solubility in water. For sufficiently high average temperatures, two forms of the molecule are present. We measure the Soret coefficient of both and find that one has positive S$_{T}$ and the other negative. We investigate the cause of this sign change using a Lattice Boltzmann based simulation. We find that the conformation of the polymer can influence its migration in a temperature gradient. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R30.00006: Microscale heat transfer enhancement using spinodal decomposition Pietro Poesio, Dafne Molin, Nicolas G. Hadjiconstantinou, Gian Paolo Beretta In many cases, miniaturization is limited by our ability to quickly remove heat; current state-of-the-art cooling approaches have significant limitations, particularly for high heat flux applications. Recent studies have shown that phase separation of a binary liquid-liquid mixture quenched to a temperature below the spinodal curve can be used to enhance heat transfer in small-scale devices. In particular, it has been shown that the self propulsion of single droplets formed during the intermediate stage of spinodal decomposition can produce considerable agitation and, as a result, enhanced heat transport. Spinodal phase separation dynamics can be described by the coupled Cahn-Hilliard/Navier-Stokes equations; unfortunately, simulation of these equations at the device scale is computationally costly due to the mulltiscale nature of spinodal decomposition, which requires resolution of the phase interface between the two fluids which is of atomistic size. In this talk we discuss possible approaches for reducing this computational cost by calculating the resulting transport from synthetic fluctuating fields that simulate the effect of spinodal decomposition but are generated stochastically without solving the Cahn-Hilliard equation at close-to-atomistic resolution. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R30.00007: Improved theory on AC electrothermal flows Sophie Loire, Paul Kauffmann, Igor Mezic We compare simulations from new theory to experimental measurements on AC eletrothermal flows (ACET) for micromixing application on 96 microwell (10 $\mu$L) plate for high conductivity physiological solutions. This application leads to certain design constraints (electrode sizes, voltage range, conductivity). Beneath each microwell filled with saline solution ($\sigma$=0.02 mS/cm, to 16 mS/cm.), a sinusoidal voltage (0 to 40Vpp, 1MHz) is applied between 3 interdigitated gold electrodes 35 $\mu m$ thick, separated by a 150$\mu m$ gap. Due to this design, the ACET flows, measured by $\mu PIV$, doesn't follow the present theory. Similarly to natural convection, a bifurcation like behaviour is observed : the flows appear only above a critical voltage. The velocities scale as $V^p$ with $p\geq4$ with $p$ increasing with conductivities. We analyse the validity conditions of the weak temperature gradient approximations. Accordingly we propose a thermal-electrical strong coupling model, which is traditionally neglected. We also study the competition between ACET and natural convection appearing in this configuration. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R30.00008: Controlling the trajectories of bubble trains at a microfluidic junction Pravien Parthiban, Saif Khan The increasing number of applications facilitated by digital microfluidic flows has resulted in a sustained interest in not only understanding the diverse, interesting and often complex dynamics associated with such flows in microchannel networks but also in developing facile strategies to control them. We find that there are readily accessible flow speeds wherein resistance to flow in microchannels decreases with an increase in the number of confined bubbles present, and exploit this intriguing phenomenon to sort all bubble of a train exclusively into one of the arms of a nominally symmetric microfluidic loop. We also demonstrate how the arm into which the train filters into can be chosen by applying a \textit{temporary} external stimulus by means of an additional flow of the continuous liquid into one the arms of the loop. Furthermore, we show how by tuning the magnitude and period of this temporary stimulus we can switch controllably, the traffic of bubbles between both arms of the loop even when the loop is \textit{asymmetric}. The results of this work should aid in developing viable methods to regulate traffic of digital flows in microfluidic networks. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R30.00009: Measurement of charge of a droplet induced by contact electrification Dongwhi Choi, Horim Lee, Do Jin Im, Kwan Hyoung Kang The contact electrification is the charge transfer between two surfaces by contact and separation. We have developed the experimental method to measure the amount of charge of a droplet induced by contact electrification. In the method, the uniform electric field is applied to a droplet suspended in dielectric oil. The horizontal movement of a droplet is determined by the balance between electric and drag force. The drag force exerted on a droplet has been calculated by Hadamard-Rybczinski solution. The effects of a droplet size, electrolyte concentration of an aqueous droplet on the amount of charge have been examined. [Preview Abstract] |
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