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Now showing items 1 - 7 of 7

  • Evolution of Microdroplet Morphology Confined on Asymmetric Micropillar Structures

    Ma, Binjian   Shan, Li   Dogruoz, Buis   Agonafer, Damena  

    The design of topological features to control the spreading of liquid has been widely investigated. Micropillar structures, for example, can retain stable droplets on the tip by inhibiting the contact line from advancing over a sharp solid edge. The pinning behavior of droplets on noncircular pillars, however, has received little attention. In this study, we analyze the retention of microdroplets with high and low surface tensions on axisymmetric and asymmetric porous micropillar structures. Circular, square, and triangular structures fabricated on silicon substrates are used to characterize the dynamic behavior of droplets before and after bursting. The critical pinning conditions are based on the visualization and pressure measurements of droplets. A theoretical model is developed based on a free energy analysis for predicting the change in pressure as the working fluid advances on the micropillar. For high surface tension liquids (e.g., water), the maximum pressure occurs when the contact line is pinned along the edge of the inner pore. For low surface tension liquids (e.g., Isopropanol and Novec 7500), the maximum pressure occurs when the contact line is pinned along the outer edge of the structure. The theoretical and experimental results demonstrate how a droplet pinned atop a triangular micropillar exhibits the smallest critical volume at the bursting moment. When using IPA solution (gamma =3D 23 mN/m) and Novec 7500 (gamma =3D 16 mN/m) as the working fluids, a change in the micropillar shape from circle to triangle, respectively, yields a 83% and 76% reduction in the critical burst volume. Meanwhile, the bursting pressure increases from 172 to 300 Pa and from 127 to 216 Pa for IPA and Novec 7500, respectively. These findings provide new insights to the rational design of surface micro/nanoengineered structures for tuning the surface wetting characteristics in scientific and engineering applications.
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  • Influence of Boundary Conditions on Sub-Millimeter Combustion RID B-5674-2012

    Prakash, Shaurya   Akberov, Roald   Agonafer, Damena   Armijo, Adrian D.   Shannon, Mark A.  

    Growing interest in small-scale, portable energy Systems Such as fuel cells has necessitated the development Of small-scale fuel processing or reforming systems. Many fuel reforming systems require reliable heat sources, as in some cases temperatures in excess of 600 degrees C may be required. Sub-millimeter combustors call provide such a heal source; however, a broader set of design rules are needed for constructing systematically engineered heat sources. In this article, experimental observations and computational fluid dynamics modeling results are presented For stable and steady confined flame structures within an alumina sub-millimeter combustor. Influence of inlet flow and thermal boundary conditions are evaluated through a parametric study. The inlet flow rates and relative gas composition. the thermal boundary conditions that include thermal conductivity of the walls, convection of heat to and from the walls, and radiation of heat energy through the walls all determine the position, structure, and temperature of the reacting fluid and combustor walls. The model shows the importance of radiative heat transfer in the formation of the steady-state flame structures within the microcombustor.
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  • Thermal management of die stacking architecture that includes memory and logic processor

    Dewan-Sandur, Bhavani P.   Kaisare, Abhijit   Agonafer, Dereje   Agonafer, Damena   Amon, Cristina   Pekin, Senol   Dishongh, Terry  

    The convergence of computing and communications dictates building up rather than out. As consumers demand more functions in their hand-held devices, the need for more memory in a limited space is increasing, and integrating various functions into the same package is becoming more crucial. Over the past few years, die stacking has emerged as a powerful tool for satisfying these challenging Integrated Circuit (IC) packaging requirements. Previously, present authors reported on the thermal challenges of various die stacking architectures that included memory (volatile and non-volatile) only. In this paper, the focus is on stacking memory and the logic processor on the same substrate. In present technologies, logic processor and memory packages are located side-by-side on the board or they are packaged separately and then stacked on top of each other (Package-on-package [PoP]). Mixing memory and logic processor in the same stack has advantage and challenges, but requires the integration ability of economies-of-scale. Geometries needed were generated by using Pro/Engineer((R)) Wildfire (TM) 2.0 as a Computer-Aided-Design (CAD) tool and were transferred to ANSYS((R)) Workbench (TM) 10.0, where meshed analysis was conducted. Package architectures evaluated were rotated stack, staggered stack utilizing redistributed pads, and stacking with spacers, while all other parameters were held constant. The values of these parameters were determined to give a junction temperature of 100 degrees C, which is an unacceptable value due to wafer level electromigration. A discussion is presented in what parameters need to be adjusted in order to meet the required thermal design specification. In that light, a list of solutions consisting of increasing the heat transfer co-efficient on top of the package, the use of underfill, improved thermal conductivity of the PCB, and the use of a copper heat spreader were evaluated. Results were evaluated in the light of market segment requirements.
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  • Experimental investigation of evaporation from asymmetric microdroplets confined on heated micropillar structures

    Shan, Li   Li, Junhui   Ma, Binjian   Jiang, Xinyu   Dogruoz, Baris   Agonafer, Damena  

    Droplet evaporation is ubiquitous both in everyday life and numerous engineering, biomedical, and environmental applications. For example, the large amount of latent heat associated with the liquid-vapor phase change process makes droplet evaporation an ideal solution for cooling high-powered electronic devices. Over the past 100 years, many new theories and models of droplet evaporation have been proposed. Still much of the fundamental transport physics remains elusive, which makes it challenging to describe diverse evaporation behavior by a universal mechanism. In particular, evaporation from asymmetric droplets remains grossly unexplored due to their rarity in nature. However, recent advances in micro- and nanoengineering technology have made it possible to tune and maintain droplets in non-spherical geometries, where the interfacial transport rate becomes highly anisotropic along the circumferential direction. Such a characteristic represents a distinctive feature from evaporation of capped-spherical droplets and has not been comprehensively explained by existing theories. This paper exams the evaporation rate and heat transfer performance of continuously-fed water microdroplets confined on heated silicon micropillar structures with circular, square, and triangular cross sections. The evaporation experiments are performed at substrate temperatures ranging from 60 degrees C to 98 degrees C. The droplets exhibit a capped spherical shape on the circular micropillar but an asymmetric geometry on non-circular micropillars. Our experimental results show that increasing levels of droplet asymmetry significantly enhance evaporative transport. Specifically, for a 60 degrees C substrate, a triangular micropillar has 45% greater heat transfer coefficient than a circular one; at 98 degrees C, the increase reaches 71%. This enhancement is validated by multiphase numerical simulations which show agreement with experimental values. The enhanced heat transfer coefficient from asymmetric droplets is attributed to a reduced conduction resistance inside the droplet and smaller diffusion resistance at the droplet interface, both of which result from the non-uniform geometric features of the droplet along the circumferential direction. Finally, a thermal resistance network is developed to demonstrate the variation in the local droplet thickness and interfacial curvature along the circumferential direction for droplets confined on circular, square, and triangular micropillars. Asymmetric droplets yield smaller thicknesses near regions with high curvature, which results in a smaller conduction resistance and higher fraction of heat transport near the contact line region compared to a capped spherical droplet. Furthermore, with increasing droplet temperature, the thermal resistance associated with vapor diffusion is reduced significantly, leading to greater enhancement of evaporative heat transport. These findings not only provide insights in the evaporation behavior of asymmetric droplets, but also provide important guidance for relevant applications, such as the design of evaporative cooling systems.
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  • Investigation of the confinement effect on the evaporation behavior of a droplet pinned on a micropillar structure

    Li, Junhui   Shan, Li   Ma, Binjian   Jiang, Xinyu   Solomon, Abel   Iyengar, Madhusudan   Padilla, Jorge   Agonafer, Damena  

    Evaporation of sessile droplet suffers from reduced evaporation rate due to the confinement of vapor diffusion imposed by the bottom substrate. However, it is possible to change the evaporation behavior of a droplet by suspending it from the bottom substrate, in particular, supporting the droplet on a micropillar. This is expected to enable diffusion transport in the downward direction that will subsequently enhance evaporative transport. In this study, we investigate the diffusion confinement effect imposed by the bottom substrate and the side wall of the micropillar through numerical simulations and experimental investigation. The approximate solutions for total evaporation rate and local evaporative flux were subsequently derived from the total evaporation rate predicted by the simulation results. The simulation results, agreeing within 5% with the experimental measurements, show that increasing the micropillar height enhances the total evaporation rate from the suspended hemispherical droplet. This enhancement is due to a dramatic improvement of the local evaporation rate near the contact line region as micropillar heights increase. The micropillar heights examined for maximum evaporation rates were observed under substrate temperatures from 60-98 degrees C. The increasing pillar height leads to smaller vapor diffusion resistance but greater conduction resistance. (C) 2019 Elsevier Inc. All rights reserved.
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  • Investigation of the evaporation heat transfer mechanism of a non-axisymmetric droplet confined on a heated micropillar structure

    Shan, Li   Ma, Binjian   Li, Junhui   Dogruoz, Baris   Agonafer, Damena  

    In scaling computer performance, closely integrating heterogeneous components in a single system-inpackage assembly provides faster signal communication and a tighter footprint - but also generates more heat: state-of-the-art microelectronic devices can produce more than 1 kW cm(-2). The next generation high-powered electronics will need two-phase liquid cooling, such as droplet evaporation, which utilizes the latent heat of vaporization to remove excessive heat. Compared to traditional single-phase cooling techniques, two-phase cooling offers both high efficiency and an exceptionally high heat dissipation rate. Although droplet evaporation has been explored for over a hundred years, many fundamental transport mechanisms are still not well understood. For example, most droplet evaporation studies focused only on spherical droplets, which possess a uniform curvature, kappa (for a low Bond Number), along the meniscus interface. Evaporation from a non-spherical droplet, due to the change in perimeter-to-area ratio and the meniscus curvature, exhibits very different interfacial mass transport features from a spherical droplet. In particular, a higher perimeter-to-solid-liquid-area ratio will yield a larger fraction of thin film region and therefore a smaller thermal resistance in an evaporating droplet, while a high local curvature will facilitate a stronger local vapor diffusion rate. In this study, we develop a numerical model to investigate the evaporation behavior of asymmetrical microdroplets suspended on heated porous micropillar structures. We explore the equilibrium profiles and mass transport characteristics of droplets with circular, triangular, and square contact shapes, using the Volume of Fluid (VOF) method, and we use a simplified Schrage model [1] to study the evaporative mass transport at the liquid-vapor interface. The results show that microdroplets evaporating on a triangular substrate possess 12.8% smaller effective film thickness compared to that on a circular substrate, due to a longer length of the contact line. During the evaporative heat transfer process, the triangular-based droplets also exhibit the smallest temperature difference between the droplets' solid-liquid interface and ambient temperature, which leads to a higher heat transfer coefficient (21% larger than a spherical droplet at a supplied heat flux of 500 W/cm(2)). When the supplied heat increases to a higher value (e.g., 1000 W/cm(2)), the shape effect becomes less significant where the diffusion resistance is dominated by the liquid-vapor interfacial temperature instead of the meniscus curvature. (C) 2019 Elsevier Ltd. All rights reserved.
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  • Aerogel for microsystems thermal insulation: System design and process development

    Smith, Brian   Romero, David   Agonafer, Damena   Gu, Jason   Amon, Cristina H.  

    Extreme miniaturization in the microelectronics component market along with the emergence of system-on-chip applications has driven interest in correspondingly small-scale thermal management designs requiring novel material systems. This paper concentrates on aerogel, which is an amorphous, nanoporous dielectric oxide fabricated through a sol-gel process. Its extremely high porosity leads to very low thermal conductivity and dielectric constants. Significant research has been devoted to its electrical properties; however, there are several emerging applications that can leverage the thermal characteristics as well. Two promising applications are investigated in this paper: a monolithically integrated infrared sensor that requires thermal isolation between sensor and silicon substrate, and an ultra-miniature crystal oscillator device which demands thermal insulation of the crystal for low-power operation. This paper identifies the potential benefits of aerogel in these applications through system modeling, demonstrates aerogel's compatibility with standard low-cost microfabrication techniques, and presents results of thermal testing of aerogel films compared with other microelectronics insulators and available data in the literature. The goal is to explore system thermal design using aerogel while demonstrating its feasibility through experimentation. The combination of numerical simulations, Bayesian surrogate modeling, and process development helps to refine candidate aerogel applications and allow the designer to explore thermal designs which have not previously been possible in large-scale microelectronics system production.
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