Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session L11: Microscale and Nanoscale Flows: Devices and Applications |
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Chair: Bin Liu, University of California, Merced Room: 155 A |
Monday, November 25, 2024 8:00AM - 8:13AM |
L11.00001: Evaporation-limited capillary flow in a heated microchannel. NABAJIT DEKA, Susmita Dash Capillary flow in porous media is significant in several applications including medical diagnosis, environmental monitoring and food safety analysis. Porous hydrophilic wicks such as textiles, paper, metallic weaves, and textured substrates are used in thermal management and desalination systems, leveraging their capillary action to efficiently manage liquid flow and heat transfer. Here, we study wicking and evaporation characteristics of a volatile liquid to maximize the rate of evaporation in a microchannel. Coupled equations of mass and energy conservation are solved in a microchannel to evaluate the equilibrium wicking length at different applied heat flux. We investigate the role of geometric parameters and heat flux on the wicking length. Wicking length combined with the width of the channel forms the evaporating surface area, increase in which increases the evaporation rate. At low heat flux, the wicking length is observed to decrease monotonically with an increase of the channel width. At high heat flux, an optimum width is obtained for which the equilibrium wicking length is the highest. Correspondingly, a set of geometric parameters, comprising of the width and depth of the channel is obtained for a given heat flux, which can serve as design criteria for evaporators to achieve high evaporation rate in phase change applications. |
Monday, November 25, 2024 8:13AM - 8:26AM |
L11.00002: An Experimental Study of Aluminum Pulsating Heat Pipes for Enhanced Cooling: Flow Characterization and the Effect of Aspect Ratio Arpan Ghimire Bohara, Mahedi Hassan, ZEESHAN AHMAD KHAN, Yaofa Li Managing high heat fluxes in electronic devices has become increasingly critical. In 2020, data centers in the USA used 100 billion liters of water annually for cooling, comparable to the water usage of Philadelphia. Future high-power electronic devices are expected to generate heat fluxes up to 5 kW/cm², exacerbating resource strain and complicating thermal management, particularly for NASA's essential systems such as planetary orbitary system, satellites etc. Conventional single-phase systems are inadequate for such high heat fluxes, necessitating two-phase thermal solutions like Pulsating Heat Pipes (PHPs), which have demonstrated superior heat extraction capabilities over traditional systems. This study explores aluminum flat PHPs with varying channel widths and turn numbers, while maintaining constant channel depth and total cross-sectional area with the purpose of optimizing current PHP designs. Experiments were conducted with filling ratios between 40% and 65% using pure ethanol. Instantaneous flow patterns were visualized with a high-speed camera and temperatures were captured at various locations using T-type thermocouples. Oscillatory flows and cooling improvement were observed for all configurations under study. The preliminary results indicate that the optimal aspect ratio is around 1.19. Our findings provide insights into the efficiency of aluminum flat PHPs for advanced electronic cooling, supporting effective thermal management in terrestrial and space applications. |
Monday, November 25, 2024 8:26AM - 8:39AM |
L11.00003: Membrane-Assisted Thermal Inkjet (MA-TIJ) Droplet Ejection for TIJ-Incompatible Inks Brandon Hayes, Heiko Dieter Kabutz, Kaushik Jayaram, Robert MacCurdy Thermal inkjet (TIJ) drop-on-demand (DOD) technology ejects a droplet of ink by rapidly heating a micro-resistor to cause explosive boiling. Throughout the last four decades, material limitations have been a long-standing issue in TIJ technology. Existing TIJ-compatible inks almost exclusively rely on solvent-based or aqueous-based inks that can nucleate a strong vapor bubble and withstand high temperatures, limiting material choices. We report a new TIJ-based architecture, ``membrane-assisted thermal inkjet (MA-TIJ),'' for drop ejection of previously TIJ-incompatible inks. We characterized jetting using fluid-structure interaction modeling as well as high-speed imaging and showed the ability to jet dimethyl sulfoxide and mineral oil using our MA-TIJ architecture, previously TIJ-incompatible inks, opening up the range of available materials in TIJ technology. |
Monday, November 25, 2024 8:39AM - 8:52AM |
L11.00004: Microfluidic cross-slot trapping to study single sperm under straining flow Javane Javaherchian, Farin Yazdan Parast, Moira K. O’Bryan, Reza Nosrati, Farzan Akbaridoust, Ivan Marusic Approximately one in six couples worldwide experiences infertility, with male factors solely contributing to 30% of these cases. A significant yet poorly understood phenomenon involves how sperm cells navigate the complex pathways of the female reproductive tract, encountering diverse fluid flow conditions. Thus, gaining a deeper understanding of the individual characteristics of sperm is crucial for advancing infertility management. However, due to their constant motion, analysing sperm presents a significant challenge. One common practice involves tethering individual sperm heads to slides, despite the risk of cell damage. In this study, we enhanced the microfluidic cross-slot trap design to immobilize individual sperm without physical tethering, ensuring long residence time with high stability in a minimally invasive environment. This platform provides uniform extensional flow and strain rate, making it a promising system for investigating the behaviour of single sperm in response to fluid flow. We subjected trapped sperm to different strain rates to examine the effect of strain rate on sperm flagellar beating patterns. Consequently, this advanced microfluidic device offers an effective platform for analysing the behaviours of individual sperm. |
Monday, November 25, 2024 8:52AM - 9:05AM |
L11.00005: Dynamical flow characterization of single-phase flow in a converging-diverging channels RAKESH KUMAR, RAJARAM LAKKARAJU, ARNAB ATTA We present a three-dimensional numerical model to investigate the dynamics of single-phase flow in a parallel branched microchannel, accounting for variable geometric dimensions of constrictions. The study aims to explore the complexities of fluid flow in microdevices featuring networks of branches and narrow passages. The results reveal non-linear fluctuations in velocity, pressure, acceleration, and shear stress along the flow direction, with a strong dependence on the angles of convergence or divergence at the constrictions. A modified Reynolds number is introduced as the primary parameter governing flow transitions, providing a novel approach to understanding the impact of geometric characteristics in microchannels with converging/diverging constrictions. Our findings show a significant increase in inertial forces, a phenomenon not typically observed in simple microchannels. Additionally, we analyze entropy generation and efficiency concerning the Reynolds number. The results suggest that microdevices with larger converging-diverging angles and smaller width ratios are more favorable, offering reduced pumping power and improved energy efficiency. These insights are critical for guiding design modifications aimed at enhancing the efficiency of micropumps or microvalves. |
Monday, November 25, 2024 9:05AM - 9:18AM |
L11.00006: Propeller can't propel at Intermediate Reynolds Numbers - numerical simulation Siyu Li, rong fu, Yang Ding, Haoxiang Luo Microrobots hold significant potential in biomedical applications, such as targeted drug delivery and minimally invasive surgery. Advances in microrobots have spurred interest in their propulsion mechanisms, particularly for propellers operating at low Reynolds numbers (Re). While extensive research has explored propeller performance at high Re in aviation and maritime contexts, the behavior of propellers at intermediate Re remains underexplored. In this study, we use a simplified model comprising a propeller and a disk to investigate why a toy submarine's propeller, when rotated forward, causes the submarine to move backward within the Re range of 5 to 130. Through 3D numerical simulations, we demonstrate that at high Re, the jet behind the propeller is well-defined. As Re decreases, the jet flow transitions to a configuration with an increasing angle to the axis and significant inward flow behind the propeller. This shift signifies a growing dominance of centrifugal forces. Additionally, a negative pressure region around the propeller expands towards the disk with decreasing Re. At lower Re, this negative pressure creates a substantial backward force on the disk, counteracting the forward thrust from the propeller and resulting in an overall backward force. |
Monday, November 25, 2024 9:18AM - 9:31AM |
L11.00007: Rapid multiplex patterning through conformally-mapped microfluidic flows Bin Liu, Jeremias Mitchell Garrett Gonzalez, Ajay Gopinathan The simultaneous manipulation of many microscale particles is challenging but essential to fundamental biophysical studies. Such a task is typically realized through multiple traps, or local direct fields incorporated with a feedback mechanism, leading to potential perturbations on particles and delayed responses associated with the feedback-control loop. Here, we achieve this goal by utilizing the uniformity of flow fields in micro-manipulation, realizable through a 3D microfluidic channel obeying symmetry principles. We show that any imperfection in symmetry can be amended by deforming a testing flow pattern to match the desired one. We demonstrate that this restored symmetry can be extended to any manipulation pattern through conformal mapping, which is differentiable along the entire path. These conformally-mapped flows can thus be used to entrain multiple microparticles into smooth and rapid patterns, with their accuracies as high as submicron scales, without any need of real-time feedback. We also extend such robust flow controls to the manipulation of Brownian particles and living microorganisms. |
Monday, November 25, 2024 9:31AM - 9:44AM |
L11.00008: Acoustic streaming flow driven about an array of sharp-edged obstacles Michael Gary Olsen, Md. Abdul Karim Miah, Jaime J Juarez Acoustic streaming is a process that is used as a flow control mechanism for mixing, sorting, and enhanced transport phenomena. In this work, we present experimental results examining the superposition of acoustic streaming and bulk flow in a microchannel that incorporates an array of sharp-edge obstacles with fixed porosity of 90%. In the absence of bulk flow, we perform experiments over a parameter space consisting of obstacle morphology (circle, square, triangle, cross) and input voltage (4 V – 12 V) with a fixed frequency of 5.8 kHz. Microscopic particle image velocimetry (microPIV) measurements yield a velocity range from 37 μm/s to 674 μm/s. In all shapes, an overall clockwise rotation was found at the right side of the PZT and anticlockwise rotation at the left side of PZT. Although the peak acoustic streaming velocities are different for each shape, we find that the velocity scales nearly quadratically as a function of applied voltage (Uo∼V2), which is consistent with scaling analyses of acoustic streaming in microfluidic systems. A bulk flow of ~185 μm/s is imposed on the microchannel at the same time as a 10 V, 5.8 kHz signal. We find that the resulting flow field can be reconstructed by adding the bulk flow field without streaming to the acoustic streaming flow field without bulk flow. This observation will lead to simplified models of oscillating microstreaming under conditions where the streaming component can be considered steady, because the pressure-driven flow and acoustic driven flow can be modelled separately and then simply superimposed, eliminating the need to model both phenomena simultaneously. Furthermore, this work may find applications in a variety of fields, including, but not limited to, microfluidic flow control, heat transfer, and sample mixing |
Monday, November 25, 2024 9:44AM - 9:57AM |
L11.00009: Fluid-induced Snapping of Elastic Shells Vitus Østergaard-Clausen, Lucas Rudzki, Hemanshul Garg, Pier Giuseppe Ledda, Matteo Pezzulla We study the fluid-induced snapping of spherical shells in cylindrical channels at low Reynolds numbers. Experiments demonstrate that above a critical flow rate, spherical shells undergo a snapping instability, drastically changing the internal geometry of the channel. By combining experiments, axisymmetric simulations, and theoretical analyses, we vary the geometrical and material parameters of our system to identify the instability threshold, expressed as a critical Cauchy number (i.e., the ratio between viscous and elastic forces). Our findings are summarized in a phase map, expressed in terms of the Cauchy number and a geometrical dimensionless parameter that describes the shell-channel system. This phase map shows a collapse of our experimental and numerical results into a single master curve. It highlights the good agreement between experiments and simulations and provides a design rule for channels with snapping valves. |
Monday, November 25, 2024 9:57AM - 10:10AM |
L11.00010: Thermofluidics Design Evolution of Biomimetic Micropillar Wick for Thin-Film Evaporative Cooling Anand S, Chander Shekhar Sharma Thin-film evaporative cooling in heat pipe and vapor chamber is a promising approach for passive cooling of microelectronics. This study reports the development of a capillary wick design, consisting of an array of wedged micropillars inspired by the peristome of Nepenthes alata. The sharp wedge corners lead to high meniscus curvature. The resulting large capillary pumping pressure, coupled with high permeability of the array, can achieve ~234% higher dryout heat flux compared to the cylindrical micropillar array. However, fabrication of these micropillars using optical lithography and DRIE results in large radius of curvature at the wedge corners, which significantly decreases its thermofluidic performance. We adopt several alternative fabrication approaches to improve the wedge corner sharpness, including the use of serif structures and e-beam lithography. We find that e-beam lithography achieves the requisite wedge geometry. However, upscaling the process to large footprint area is challenging. In view of these challenges, we propose a new design of wedged micropillars that compensates for finite corner radius, can be fabricated using optical lithography and DRIE, and can deliver dry-out heat flux performance equivalent to that of sharp wedge corner micropillars. |
Monday, November 25, 2024 10:10AM - 10:23AM |
L11.00011: Pulse-driven microfluidic pumps with varying depth flow channels for fine flow rate tuning Shuyu Zhang, Rafael V Davalos, Anne E Staples The delivery of liquid pharmaceuticals across the skin has been based on powered infusion pumps and syringes for over half a century. Both types of devices pose significant limitations for patients, including injection site pain, bulkiness, and embarrassment. To address some of these limitations, we have developed featherweight, wearable, arterial pulse-driven microfluidic infusion pump devices. Small changes in microfluidic channel design parameters like channel length and width have sometimes resulted in a change in the character of the correlation between the driving heart rate and blood pressure and the resulting device flow rate. To address this, we allowed the channel depth to vary along its length, hypothesizing that this would result in finer control over the precise flow rate produced by a given device. To test this hypothesis, we developed a pump prototype with a slanted channel with a depth varying from 40 to 80 μm that yielded a constant flow rate of 0.2 μL/s regardless of the driving heart rate or blood pressure. The results from this device provide evidence that varying-depth flow channel devices may expand the number of achievable flow rates in our devices, compared to constant depth devices, allowing for finer control over device flow rates. |
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