Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session G11: Microscale Flows: Porous Media and Thermal Transport |
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Chair: James Palko, University of Michigan Room: 3007 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G11.00001: Optimum design for effective water transport through a double-layered porous hydrogel inspired by plant leaves Hyejeong Kim, Hyeonjeong Kim, Hyungkyu Huh, Hyung Ju Hwang, Sang Joon Lee Plant leaves are generally known to have optimized morphological structure in response to environmental changes for efficient water usage. However, the advantageous features of plant leaves are not fully utilized in engineering fields yet, since the optimum design in internal structure of plant leaves is unclear. In this study, the tissue organization of the hydraulic pathways inside plant leaves was investigated. Water transport through double-layered porous hydrogel models analogous to mesophyll cells was experimentally observed. In addition, computational experiment and theoretical analysis were applied to the model systems to find the optimal design for efficient water transport. As a result, the models with lower porosity or with pores distributed widely in the structure exhibit efficient mass transport. Our theoretical prediction supports that structural features of plant leaves guarantee sufficient water supply as survival strategy. This study may provide a new framework for investigating the biophysical principles governing the morphological optimization of plant leaves and for designing microfluidic devices to enhance mass transport ability. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G11.00002: Flow-induced fiber deformation in a confined microchannel: \emph{in situ} mechanical testing of gels Camille Duprat, Helene Berthet, Jason Wexler, Olivia du Roure, Anke Lindner Photopolymerized hydrogels are a functional template for micro-particle fabrication, microflowsensors and microbiology experiments. The control and knowledge of their mechanical properties are paramount to many applications. We have designed a novel robust method to determine these properties. We measure the deformation of a gel beam of precisely controlled shape, under a controlled flow forcing, which provides a direct measurement of the Young's modulus of the gel upon its fabrication. We then use this method to determine the mechanical properties of the commonly used poly(ethylene glycol) diacrylate (PEGDA) under various experimental conditions. The mechanical properties of the gel can be highly tuned, yielding two orders of magnitude in the Young's modulus. We provide a simple control parameter, the UV exposure time, to have a great control over the network properties, and rationalize these observations by studying the kinetics of the polymerization reaction. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G11.00003: Characterization of Fluid Flow in Paper-Based Microfluidic Systems Noosheen Walji, Brendan MacDonald Paper-based microfluidic devices have been presented as a viable low-cost alternative with the versatility to accommodate many applications in disease diagnosis and environmental monitoring. Current microfluidic designs focus on the use of silicone and PDMS structures, and several models have been developed to describe these systems; however, the design process for paper-based devices is hindered by a lack of prediction capability. In this work we simplify the complex underlying physics of the capillary-driven flow mechanism in a porous medium and generate a practical numerical model capable of predicting the flow behaviour. We present our key insights regarding the properties that dictate the behaviour of fluid wicking in paper-based microfluidic devices. We compare the results from our model to experiments and discuss the application of our model to design of paper-based microfluidic devices for arsenic detection in drinking water in Bangladesh. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G11.00004: Flow control for a paper-based microfluidic device by adjusting permeability of paper Ilhoon Jang, Gangjune Kim, Simon Song The paper-based microfluidics has attracted intensive attention as a prospective substitute for conventional microfluidic substrates used for a point-of-care diagnostics due to its superior advantages such as the cost effectiveness and production simplicity. Generally, a paper-based microfluidic device utilizes capillary force to drive a flow. Recent studies on flow control in such a device aimed at obtaining accurate and quantitative results by varying a channel geometry like width and length. According to the Darcy's law describing a flow in a porous media like paper, a flow rate can be adjusted the permeability of paper. In this study, we investigate a flow control method by adjusting the permeability of paper. We utilize the wax printing for the adjustment and the fabrication of paper channels. A rectangular wax pattern was printed on one inlet channel of a Y-channel geometry. By varying the brightness of the wax pattern, a relationship between the flow rate and permeability changes due to the wax was investigated. As a result, we obtained an effective permeability contour with respect to the wax pattern length and brightness. In addition, we developed a paper-based micromixer of which the mixing ratio was controlled precisely by adjusting the permeability. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G11.00005: Nanofluidic control by nanoporous materials using electrocapillary effects Yahui Xue, Huiling Duan, Juergen Markmann, Patrick Huber, Joerg Weissmueller Electrocapillary techniques exhibit great advantages in nonmechanical electrofluidic manipulation, e.g., flow actuation in micro-/nano- channels. One issue of interest is the spontaneous imbibition of fluids in bodies with a nanoscale pores size. Contrary to previous studies we here use a metallic nanoporous body. This allows us to control the electrode potential at the solid-fluid interface. Nanoporous gold (NPG) with uniform pore- and ligament size of 45 nm was fabricated by dealloying an Ag75Au25 alloy. Spontaneous imbibition of aqueous electrolytes obeys the Lucas-Washburn law. Interestingly, the estimated tortuosity has the low value of 3.2 (3 is expected for an isotropic sponge). Electrocapillary effects were then used to manipulate the imbibition dynamics. As a result of the enhanced wetting by the electrocapillary effects, we observed an acceleration of the imbibition by 30{\%}. When air as the pore fluid is replaced with cyclohexane, we show for aqueous electrolyte imbibition in nanoporous gold that the fluid flow can be reversibly switched on and off through electric potential control of the solid--liquid interfacial tension. Our findings demonstrate that the high electric conductivity along with the pathways for fluid/ionic transport render nanoporous gold a versatile, accurately controllable electrocapillary pump and flow sensor for minute amounts of liquids with exceptionally low operating voltages. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G11.00006: Effect of Axial Fluid Conduction on Thermal Transport in Superhydrophobic Microchannels Adam Cowley, Daniel Maynes, Julie Crockett Convective heat transfer in a rib/cavity structured superhydrophobic microchannel is explored numerically. The cavities are assumed to be in the Cassie state (not wet) and the liquid meniscus is modeled as flat. The ribs are oriented perpendicular to the flow direction and are smaller than the channel hydraulic diameter. A constant heat flux condition is prescribed at the top of the ribs while the gas/liquid interface is approximated as adiabatic. The varied parameters include Peclet number, relative cavity size, and relative channel-wall spacing. The influence of fluid axial conduction is explored and it is found that axial conduction plays a significant role in superhydrophobic microchannels. Aggregate results are presented in the form of an average Nusselt number and the ratio of the temperature jump length to the hydrodynamic slip length. These results are compared to two previous studies: one where axial conduction was neglected and another where diffusion is assumed to be dominant. Overall, the results show that heat transfer is decreased for a superhydrophobic channel when compared to a classical smooth channel and that axial conduction exerts influence over a much larger range of parameters than prevails for classical no-slip channels. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G11.00007: Fluorescence Thermometry Characterization of Microchannel Cooling Performance with Sidewall Heating Tae Jin Kim, Carlos Hidrovo Microchannel cooling of complex circuitry in microelectronics and LOC systems is an area of continued research that is constantly evolving. As such, it is important to properly evaluate the heat removal efficiency of the microchannels in near wall heating configurations. In this talk we evaluate the cooling efficiency of microchannels with microheaters embedded on the sidewalls. The microchannels are fabricated using soft lithography and the embedded joule heaters are created by filling molten low melting temperature alloys in two satellite microchannels and solidifying them. In order to assess the thermal transport rate, fluorescent images of the fluid mixed with two temperature-sensitive dyes were captured and pre-conditioned with an image-distortion correction algorithm. By taking their ratiometric value versus temperature measurements, results show that the heat removal efficiency initially increases as a function of Re and then plateaus at about 50 percent once Re is greater than 20. This behavior is the result of a decreasing advective resistance with increasing flow rate, where the ratio of the substrate-environment resistance to the wall-fluid convective resistance determines the ultimate performance of the cooling microchannel. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G11.00008: Multiscale simulation of time-dependent thermal transpiration in large-scale systems Duncan A. Lockerby, Alexander Patronis, Matthew K. Borg, Jason M. Reese We describe the development of an efficient hybrid continuum-molecular approach for simulating non-isothermal, low-speed, internal rarefied gas flows, both in time and space. This is applied to transient flow in macro-scale Knudsen compressors, which is governed by both rarefied gas and continuum fluid dynamics. The method is an extension of the hybrid approach presented by Patronis et al. (2013) [J. Comp. Phys., 255, pp 558-571], which is based on the framework originally proposed by Borg et al. (2013) [J. Comp. Phys., 233, pp 400-413] for the simulation of micro/nano flows of high-aspect-ratio. The efficiency of the multiscale method allows the investigation of alternative Knudsen-compressor configurations to be undertaken. A comparison is made with published experimental data of the transient response (of pressure) in differentially heated reservoirs suddenly connected by a micro capillary. The multiscale simulation results compare very closely to the experimental data and are obtained at a fraction of the cost of a full kinetic or molecular solution. Recommendations for future development and opportunities are discussed. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G11.00009: Controlling foam drainage in a 2D microchamber using thermocapillary stress Vincent Miralles, Isabelle Cantat, Marie-Caroline Jullien We investigate the drainage of a 2D microfoam in a vertical Hele-Shaw cell, and show that the Marangoni stress at the air-water interface generated by a constant temperature gradient applied \textit{in situ} can be tuned to control the drainage. The temperature gradient is applied in such a way that thermocapillarity and gravity have an antagonist effect. We characterize the drainage over time by measuring the liquid volume fraction in the cell and find that thermocapillarity can overcome the effect of gravity, effectively draining the foam towards the top of the cell, or exactly compensate it, maintaining the liquid fraction at its initial value over at least 60 s. We quantify these results by solving the mass balance in the cell, and provide insight on the interplay between gravity, thermocapillarity and capillary pressure governing the drainage dynamics. Finally we use this model system to provide insight in the drainage dynamics for a more complex interfacial rheology, using insoluble surfactants inducing a solutocapillary effect. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G11.00010: Volume-Of-Fluid Simulation for Predicting Two-Phase Cooling in a Microchannel Catherine Gorle, Pritish Parida, Farzad Houshmand, Mehdi Asheghi, Kenneth Goodson Two-phase flow in microfluidic geometries has applications of increasing interest for next generation electronic and optoelectronic systems, telecommunications devices, and vehicle electronics. While there has been progress on comprehensive simulation of two-phase flows in compact geometries, validation of the results in different flow regimes should be considered to determine the predictive capabilities. In the present study we use the volume-of-fluid method to model the flow through a single micro channel with cross section 100 x 100 $\mu$m and length 10mm. The channel inlet mass flux and the heat flux at the lower wall result in a subcooled boiling regime in the first 2.5mm of the channel and a saturated flow regime further downstream. A conservation equation for the vapor volume fraction, and a single set of momentum and energy equations with volume-averaged fluid properties are solved. A reduced-physics phase change model represents the evaporation of the liquid and the corresponding heat loss, and the surface tension is accounted for by a source term in the momentum equation. The phase change model used requires the definition of a time relaxation parameter, which can significantly affect the solution since it determines the rate of evaporation. The results are compared to experimental data available from literature, focusing on the capability of the reduced-physics phase change model to predict the correct flow pattern, temperature profile and pressure drop. [Preview Abstract] |
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