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1. Multiscale Evaporation Rate Measurement Using Microlaser-Induced Fluorescence

   https://doi.org/10.1115/1.4046767  

   Suh, Youngjoon; Lin, Cheng-Hui; Gowda, Hamsa; Won, Yoonjin (September 2020, Journal of Electronic Packaging)

   Abstract As the heat generation at device footprint continuously increases in modern high-tech electronics, there is an urgent need to develop new cooling devices that balance the increasing power demands. To meet this need, cutting-edge cooling devices often utilize microscale structures that facilitate two-phase heat transfer. However, it has been difficult to understand how microstructures enhance evaporation performances through traditional experimental methods due to low spatial resolution. The previous methods can only provide coarse interpretations on how physical properties such as permeability, thermal conduction, and effective surface areas interact at the microscale to effectively dissipate heat. This motivates researchers to develop new methods to observe and analyze local evaporation phenomena at the microscale. Herein, we present techniques to characterize submicron to macroscale evaporative phenomena of microscale structures by using microlaser-induced fluorescence (μLIF). We corroborate the use of unsealed temperature-sensitive dyes by systematically investigating the effects of temperature, concentration, and liquid thickness on the fluorescence intensity. Considering these factors, we analyze the evaporative performances of microstructures using two approaches. The first approach characterizes the overall and local evaporation rates by measuring the solution drying time. The second approach employs an intensity-to-temperature calibration curve to convert temperature-sensitive fluorescence signals to surface temperatures, which calculates the submicron-level evaporation rates. Using these methods, we reveal that the local evaporation rate between microstructures is high but is balanced with a large capillary-feeding. This study will enable engineers to decompose the key thermofluidic parameters contributing to the evaporative performance of microscale structures. 

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2. Analysis of Drying Front Propagation and Coupled Heat and Mass Transfer During Evaporation From Additively Manufactured Porous Structures Under a Solar Flux

   https://doi.org/10.1115/1.4063766  

   Chakraborty, Partha Pratim; Derby, Melanie M (February 2024, ASME Journal of Heat and Mass Transfer)

   Abstract Drying front propagation and coupled heat and mass transfer analysis from porous media is critical for soil–water dynamics, electronics cooling, and evaporative drying. In this study, de-ionized water was evaporated from three 3D printed porous structures (with 0.41 mm, 0.41 mm, and 0.16 mm effective radii, respectively) created out of acrylonitrile butadiene styrene (ABS) plastic using stereolithography technology. The structures were immersed in water until all the pores were invaded and then placed on the top of a sensitive scale to record evaporative mass loss. A 1000 W/m2 heat flux was applied with a solar simulator to the top of each structure to accelerate evaporation. The evaporative mass losses were recorded at 15 min time intervals and plotted against time to compare evaporation rates from the three structures. The evaporation phenomena were captured with a high-speed camera from the side of the structures to observe the drying front propagation during evaporation, and a high-resolution thermal camera was used to capture images to visualize the thermal gradients during evaporation. The 3D-structure with the smallest effective pore radius (i.e., 0.16 mm) experienced the sharpest decrease in the mass loss as the water evaporated from 0.8 g to 0.1 g within 180 min. The designed pore structures influenced hydraulic linkages, and therefore, evaporation processes. A coupled heat-and-mass-transfer model modeled constant rate evaporation, and the falling rate period was modeled through the normalized evaporation rate. 

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3. Critical assessment of water enthalpy characterization through dark environment evaporation

   https://doi.org/10.1126/sciadv.adn6368  

   Caratenuto, Andrew; Zheng, Yi (September 2024, Science Advances)

   Comparative evaporation rate testing in a dark environment, commonly used to characterize a reduced vaporization enthalpy in interfacial solar evaporators, requires the assumption of equal energy input between cases. However, this assumption is not generally valid, leading to misleading characterization results. Interfacial evaporators yield larger evaporation rates in dark conditions due to enlarged liquid-vapor surface areas, resulting in increased evaporative cooling and larger environmental temperature differentials. Theoretical and experimental evidence is provided, which shows that these temperature differences invalidate the equal energy input assumption. The results indicate that differences in evaporation rates correspond to energy input variations, without requiring enthalpy to be reduced below theoretical values. These findings offer alternative explanations for previous claims of reduced vaporization enthalpy and contradict enthalpy-related conclusions drawn from differential scanning calorimetry. We conclude that postulating a reduced vaporization enthalpy using the dark environment method is inaccurate and that re-evaluation of vaporization enthalpy reduction is required. 

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4. Effects of Droplet Volumes on Acoustothermal Heating in 128° YX LiNbO ₃ Substrates

   https://doi.org/10.1109/SENSORS52175.2022.9967169  

   Das, Pradipta Kr.; Huang, Yuqi; Evans-Nguyen, Theresa; Bhethanabotla, Venkat R. (October 2022, 2022 IEEE Sensors)

   Surface acoustic wave (SAW) devices can generate significant heat due to acoustic damping when liquid droplets are placed on them, and this heating (acoustothermal heating) can be used for microscale heating purposes. However, SAW devices are often used in biosensing applications where significant acoustothermal temperature rise can damage the proteins or the biomolecules and destroy the sensor performances. In this paper, we have performed thermal camera-based experiments to study the heating phenomena and how they can be controlled by varying droplet sizes. We found that the temperature rise linearly increases with increasing SAW power whereas it decreases with increasing droplet volume. Hence, a larger liquid volume and lower SAW power can be used in biosensors to avoid significant heating. 

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5. Multi-Fidelity Design of Porous Microstructures for Thermofluidic Applications

   https://doi.org/10.1115/1.4064813  

   Eweis-Labolle, Jonathan Tammer; Zhao, Chuanning; Won, Yoonjin; Bostanabad, Ramin (October 2024, Journal of Mechanical Design)

   As modern electronic devices are increasingly miniaturized and integrated, their performance relies more heavily on effective thermal management. In this regard, two-phase cooling methods which capitalize on thin-film evaporation atop structured porous surfaces are emerging as potential solutions. In such porous structures, the optimum heat dissipation capacity relies on two competing objectives that depend on mass and heat transfer. Optimizing these objectives for effective thermal management is challenging due to the simulation costs and the high dimensionality of the design space which is often a voxelated microstructure representation that must also be manufacturable. We address these challenges by developing a data-driven framework for designing optimal porous microstructures for cooling applications. In our framework, we leverage spectral density functions to encode the design space via a handful of interpretable variables and, in turn, efficiently search it. We develop physics-based formulas to simulate the thermofluidic properties and assess the feasibility of candidate designs based on offline image-based analyses. To decrease the reliance on expensive simulations, we generate multi-fidelity data and build emulators to find Pareto-optimal designs. We apply our approach to a canonical problem on evaporator wick design and obtain fin-like topologies in the optimal microstructures which are also characteristics often observed in industrial applications. 

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