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Flow boiling in microchannel heat sinks is capable of providing the high-heat-flux dissipation required for thermal management of next-generation wide bandgap power electronics at low pumping power and uniform surface temperatures. One of the primary issues preventing implementation of these technologies is the presence of flow boiling instabilities, which may reduce the heat transfer performance. However, the effect of individual instabilities, such as the parallel channel instability or pressure drop oscillations, on the overall heat transfer coefficient and critical heat flux in microchannel heat sinks has not been fully quantified. The primary cause of these dynamic flow boiling instabilities is the interaction between the inertia of a two-phase mixture in a heated channel and sources of compressibility located upstream of the inlet. In order to isolate the effect of pressure drop oscillations on flow boiling heat transfer performance, experiments are performed in a single-square microchannel cut into a copper heat sink, with a controlled level of upstream compressibility. The impact of pressure drop oscillations on the heat transfer coefficient and critical heat flux is characterized through analysis of both time-averaged steady-state data as well as high-frequency pressure signals synchronized with high-speed visualization. The dielectric working fluid HFE-7100 is used in all experiments with a saturation temperature of 60 °C at the channel outlet pressure. The occurrence and effect of pressure drop oscillations in 20-mm-long microchannels of three different channel widths (0.5, 0.75, and 1 mm) are related to mass flux, the degree of two-phase flow confinement, and the severity of pressure drop oscillations.

Flow boiling provides an effective means of heat removal but can suffer from thermal and hydrodynamic transients that compromise heat transfer performance and trigger device failure. In this study, the transient flow boiling characteristics in two thermally isolated, hydrodynamically coupled parallel microchannels are investigated experimentally. High-speed flow visualization is synchronized to high-frequency heat flux, wall temperature, pressure drop, and mass flux measurements to provide time-resolved characterization. Two constant and two transient heating conditions are presented. For a constant heat flux of 63 kW/m2 into each channel, boiling occurs continuously in both channels and the parallel channel instability is observed to occur at 15 Hz. Time-periodic oscillations in the pressure drop and average mass flux are observed, but corresponding oscillations in the wall temperatures are virtually nonexistent at this condition. At a slightly lower constant heat flux of 60 kW/m2, boiling remains continuous in one of the channels, but the other channel experiences time-periodic flow regime oscillations between single-phase and two-phase flow. At this condition, extreme time-periodic wall temperature oscillations are observed in both channels with a long period (~7 s) due to oscillations in the severity of the flow maldistribution. For the transient heating conditions, square-wave heating profiles oscillating between different heat flux levels are applied to the channels. Because of their relatively high frequency, the heating transients are attenuated by the microchannel walls, resulting in effectively constant heating conditions and flow boiling characteristics like that of the aforementioned constant heating conditions. This study illustrates the susceptibility of parallel two-phase heat sinks to flow maldistribution, particularly when undergoing transient flow regime oscillations.

An easy-to-use representation of vapor chambers is developed in terms of effective anisotropic properties. This approach enables accurate simulation of the vapor chamber represented as a solid conduction block by assigning appropriate values to its effective density, specific heat, in-plane thermal conductivity, and through-plane thermal conductance. These effective properties are formulated such that the vapor chamber operation in terms of steady-state and transient thermal responses matches a full, physical simulation of phase change and energy transport in the vapor core; they are intrinsic properties that can be applied independently of the boundary conditions and heat input.

We demonstrate the use of an evaporating, sessile droplet on a nonwetting substrate as a miniature micromixing device to conduct sample–dye reactions for absorbance-based colorimetry. The nonwetting substrate supports buoyancy-induced mixing inside the droplet for rapid completion of the measurement. The Bradford assay is used as a proof of concept, where a protein-containing sample is reacted with a reagent dye to measure the protein concentration. Viability of absorbance measurement through the droplet is first established using droplets in which the reactants are mixed prior to their deposition onto the substrate. In a second set of experiments involving in situ mixing, the reagent is directly added to a sessile droplet of the protein-containing sample, allowing the reactants to mix while the absorbance is being measured. Interplay between buoyancy-induced mixing, protein–reagent reaction, and protein adsorption onto the substrate leads to a complex temporal absorbance measurement signal. Videos corresponding to the signal data show that each of these mechanisms dominates during different phases of droplet evolution, causing a signal pattern containing peaks and valleys having a strong monotonic trend with the protein concentration. Overall, the second absorbance peak at which the reaction nears completion is the most sensitive to sample concentration. Heating of the substrate is demonstrated to dramatically speed up the mixing process. These protein concentration measurements, obtained with a simpler system and low reactant volumes, demonstrate that this droplet micromixing concept is a viable alternative to microtiter plates for colorimetric applications.

As the size, weight, and performance requirements of electronic devices grow increasingly demanding, their packaging has become more compact. As a result of thinning or removing the intermediate heat spreading layers, nonuniform heat generation from the chip-scale and component-level variations may be imposed directly on the attached microchannel heat sink. Despite the important heat transfer performance implications, the effect of uneven heating on the flow distribution in parallel microchannels undergoing boiling has been largely unexplored. In this study, a two-phase flow distribution model is used to investigate the impact of uneven heating on the flow distribution behavior of parallel microchannels undergoing boiling. Under lateral uneven heating (i.e., the channels are each heated to different levels, but the power input is uniform along the length of any given channel), it is found that the flow is significantly more maldistributed compared to the even heating condition. Specifically, the range of total flow rates over which the flow is maldistributed is broader and the maximum severity of flow maldistribution is higher. These trends are assessed as a function of the total input power, degree of uneven heating, and the extent of thermal connectedness between the channels. The model predictions are validated against experiments for a representative case of thermally isolated and coupled channels subjected to even heating and extreme lateral uneven heating conditions and show excellent agreement.

High-heat-flux removal is critical for the next-generation electronic devices to reliably operate within their temperature limits. A large portion of the thermal resistance in a traditional chip package is caused by thermal resistances at interfaces between the device, heat spreaders, and the heat sink; embedding the heat sink directly into the heat-generating device can eliminate these interface resistances and drastically reduce the overall thermal resistance. Microfluidic cooling within the embedded heat sink improves the heat dissipation, with two-phase operation offering the potential for dissipation of very high heat fluxes while maintaining moderate chip temperatures. To enable multichip stacking and other heterogeneous packaging approaches, it is important to densely integrate all fluid flow paths into the device; volumetric heat dissipation emerges as a performance metric in this new heat sinking paradigm. In this paper, a compact hierarchical manifold microchannel design is presented that utilizes an integrated multilevel manifold distributor to feed coolant to an array of microchannel heat sinks. The flow features in the manifold layers and microchannels are fabricated in silicon wafers using deep reactive-ion etching. The heat source is simulated via Joule heating using thin-film platinum heaters. The on-chip spatial temperature measurements are made using four-wire resistance temperature detectors. The individual manifold layers and the microchannel-bearing wafers are diced and bonded into a sealed stack via thermocompression bonding using gold layers at the mating surfaces. Thermal and hydrodynamic testing is performed by pumping the dielectric fluid HFE-7100 through the device at a known flow rate, temperature, and pressure at different levels of chip heat input. A volumetric heat density of up to 2870 W/cm3 is dissipated at a chip temperature less than 112 °C and microchannel pressure drop less than 27 kPa. The overall pressure drop is governed by flowing through the manifold, rather than the microchannels, in this compact heat sink that occupies envelope of 5 mm $\times 5$ mm $\times2.3$ mm including all the functional flow features.

A technique for reconstruction of liquid-gas interfaces based on high-speed stereo-imaging is applied to the liquid-vapor interfaces formed above a heated surface during pool boiling. Template matching is used for determining the correspondence of local features of the liquid-vapor interfaces between the two camera views. A sampling grid is overlaid on the reference image, and windows centered at each sampled pixel are compared with windows centered along the epipolar line in the target image to obtain a correlation signal. The three-dimensional coordinates of each matched pixel are determined via triangulation, which yields the physical world representation of the liquid-vapor interface. Liquid-vapor interface reconstruction is demonstrated during pool boiling for a range of heat fluxes. Textured mushroom-like vapor bubbles that are fed by multiple nucleation sites are formed close to the heated surface. Analysis of the temporal attributes of the interface distinguishes the transition with increasing heat flux from a mode in which vapor is released from the surface as a continuous plume to one dominated by the occurrence of intermittent vapor bursts. A characteristic morphology of the vapor mushroom formed during vapor burst events is identified. This liquid-vapor interface reconstruction technique is a time-resolved, flexible and non-invasive alternative to existing methods for phase-distribution mapping, and can be combined with other optical-based diagnostic tools, such as tomographic particle image velocimetry. Vapor flow morphology characterization during pool boiling at high heat fluxes can be used to inform vapor removal strategies that delay the occurrence of critical heat flux during pool boiling.

Vapor chambers with transformative evaporator wick designs capable of passively dissipating high heat fluxes over large areas, while maintaining low thermal resistances, can meet the thermal management needs of next-generation power semiconductor devices. Our prior work proposed a two-layer evaporator wick structure to enhance the performance of vapor chambers operating at high heat fluxes. The current study experimentally characterizes the capillary-fed boiling heat transfer behavior in such a two-layer evaporator wick, compared to a homogeneous (single-layer) wick, over a 1 cm2 evaporator area. The two-layer design comprises a thin base wick layer that is fed with liquid from a thick cap wick layer above using an array of vertical posts. The two-layer wick is fabricated using a sequence of sintering and laser-machining steps to form the base wick layer (200 µm), array of liquid-feeding posts, and cap wick layer (800 µm) using 90–106 µm copper particles. A test facility is constructed to replicate the conditions that exist at the evaporator of a vapor chamber; the novel facility design uses a physical restriction to prevent flooding of the wicks during testing. Two-layer wicks having 5 × 5 and 10 × 10 arrays of liquid feeding posts are characterized, along with a 200 µm-thick single-layer evaporator wick. The 10 × 10 array provides a >400% enhancement in the dryout heat flux compared to the single-layer wick. High-speed visualizations are used to identify the characteristic regimes of boiling operations for the wicks. The single-layer wick exhibits a partial dryout mode of operation, where a dry spot formed in the center of the heated evaporator area causes an increase in the thermal resistance with heat flux. In contrast, the distributed feeding provided by the two-layer wicks mitigates the development of this partial dryout regime and maintains a constant low resistance (∼0.1 K/W) during capillary-fed boiling until a complete dryout event occurs. This study demonstrates the significant enhancement in dryout heat flux offered by the liquid-feeding approach realized in the two-layer evaporator wicks characterized here.

Boiling is an effective heat transfer mechanism that is central to a variety of industrial processes including electronic systems, power plants, and nuclear reactors. Micro-/nano-structured surfaces have been demonstrated to significantly enhance the critical heat flux (CHF) during pool boiling, but there is no consensus on how to predict the structure-induced CHF enhancement. In this study, we develop an analytical model that takes into consideration key mechanisms that govern CHF during pool boiling on structured surfaces, namely, capillary wicking and evaporation of the liquid layer underneath the bubble. The model extends existing wicking-based CHF theories by introducing the competing evaporation mechanism. The model reveals a wicking-limited regime where CHF increases monotonically with the wicking flux, and an evaporation-limited regime where additional increases in the wicking flux do not significantly affect CHF. The model predictions are shown to agree with experimental CHF data from the literature for boiling of water on surfaces structured with square micropillar arrays. A parametric study is performed for such micropillared surfaces and has identified the optimal structure based on the competition between wicking and evaporation, capillary pressure and viscous resistance, and conduction and liquid-vapor interfacial resistance.

Vapor chambers can offer a passive heat spreading solution for thermal management in electronics applications ranging from mobile devices to high-power servers. The steady-state operation and performance of vapor chambers has been extensively explored. However, most electronic devices have inherently transient operational modes. For such applications, it is critical to understand the transient thermal response of vapor chamber heat spreaders and to benchmark their transient performance relative to the known behavior of metal heat spreaders. This study uses a low-cost, 3D, transient semi-analytical transport model to explore the transient thermal behavior of thin vapor chambers. We identify the three key mechanisms that govern the transient thermal response: (1) the total thermal capacity of the vapor chamber governs the rate of increase of the volume-averaged mean temperature; (2) the effective in-plane diffusivity governs the time required for the spatial temperature profile to initially develop; and (3) the effective in-plane conductance of the vapor core governs the range of the spatial temperature variation, and by extension, the steady-state performance. An experiment is conducted using a commercial vapor chamber sample to confirm the governing mechanisms revealed by the transport model; the model accurately predicts the experimental measurements. Lastly, the transient performance of a vapor chamber relative to a copper heat spreader of the same external dimensions is explored as a function of the heat spreader thickness and input power. The mechanisms governing the transient behavior of vapor chambers are used to explain the appearance of key performance thresholds beyond which performance is superior to the copper heat spreader. This work provides a foundation for understanding the benefits and limitations of vapor chambers relative to metal heat spreaders in transient operation and may inform the design of vapor chambers for improved transient performance.

A two-layer sintered porous evaporator wick for use in vapor chambers is shown to offer very high performance in passive high-heat-flux dissipation over large areas at a low thermal resistance. The two-layer wick has an upper cap layer dedicated to capillary liquid feeding of a thin base layer below that supports boiling. An array of vertical posts bridges these two layers for liquid feeding, while vents in the cap layer provide an unimpeded pathway for vapor removal from the base wick. The two-layer wick is fabricated using a combination of sintering and laser machining processes. The thermal resistance of the wicks during boiling is characterized in a saturated environment that replicates the capillary-fed working conditions of a vapor chamber evaporator. Thermal characterization tests are first performed using conventional single-layer evaporator wicks to analyze the effect of sintered particle size on capillary-fed boiling of water. Of the particle size ranges tested, wicks sintered from 180 to 212 μm-diameter particles provided the best combination of high dryout heat flux and a low boiling resistance. A two-layer evaporator wick comprising particles of this optimal size and a 15 × 15 array of liquid feeding posts yielded a maximum heat flux dissipation of 485 W/cm2 over a 1 cm2 heat input area while also maintaining a low thermal resistance of only ∼0.052 K/W. The thermal performance of the two-layer wick is compared against various hybrid and biporous evaporator wicks previously investigated in the literature. While previous wick designs are typically restricted to small areas and low power levels or high surface superheats when dissipating such heat fluxes, the unique area-scalability of the two-layer wick design allows it to achieve an unprecedented combination of high total power and low-thermal-resistance heat dissipation over larger areas than were previously possible.

Despite extensive study of boiling processes and their widespread use in industry, critical interactions between the fluid and surface during boiling remain poorly understood. Simplistic, static descriptions of the contact angle are still relied upon to describe the effects of surface wettability on dynamic interfacial processes that govern boiling. This work demonstrates the critical role of the dynamic wettability characteristics of a surface on bubble growth dynamics and boiling performance. In spite of their superior nucleation behavior, hydrophobic surfaces have received little attention for boiling applications due to their typically premature transition from efficient nucleate boiling to inefficient film boiling. Evaluation of hydrophobic surfaces with high contact angle hysteresis reveals that the heat transfer efficacy of these surfaces can be exploited in boiling, so long as the receding contact angle of the surface is sufficiently small to mitigate vapor spreading and thereby extend the nucleate boiling regime. A new paradigm of textured boiling surfaces – parahydrophobic surfaces that exhibit the “petal effect” and mimic the wetting behavior of a rose petal – are shown to have untapped potential in boiling applications resulting from highly hydrophobic behavior coupled with low receding contact angles.

The thermal resistance of a vapor chamber is primarily governed by conduction across the evaporator wick and the saturation temperature gradient in the vapor core. The relative contributions of these two predominant resistances can vary dramatically with vapor chamber operating conditions and geometry. In the limit of very thin form factors, the contribution from the vapor core thermal resistance dominates the overall thermal resistance of the vapor chamber; recent work has focused on working fluid selection to minimize overall thermal resistance in this limit. However, the wick thermal resistance becomes increasingly significant as its thickness increases to support higher heat inputs while avoiding the capillary limit. It therefore becomes critical to simultaneously consider the contributions of the wick and vapor core thermal resistances in the development of a generalized methodology for vapor chamber working fluid selection. The current work uses a simplified thermal-resistance-network-based vapor chamber model to explore selection of working fluids and wick structures that offer the minimum overall thermal resistance as a function of the vapor chamber thickness and heat input. An illustrative example of working fluid selection, for cases with and without the contribution of wick thermal resistance, is first used to demonstrate the potential significance of the wick thermal resistance on fluid choice. This influence of the wick on working fluid selection is further explained based on the wick properties (effective pore radius, permeability, and effective thermal conductivity). The ratio of effective pore radius to wick permeability is found to be the most critical wick parameter governing the overall vapor chamber resistance at thin form factors where minimizing the wick thickness is paramount; the wick conductivity becomes an equally important parameter only at thicker form factors. Based on this insight, a new approach for vapor chamber design is demonstrated, which allows simultaneous selection of the working fluid and wick that provides minimum overall thermal resistance for a given geometry and operating condition.

Two-phase flow through heated parallel channels is commonly encountered in thermal systems used for power generation, air conditioning, and electronics cooling. Flow boiling is susceptible to instabilities that can lead to maldistribution between the channels and thereby heat transfer performance reductions. In this study, the Ledinegg instability that occurs during flow boiling in two thermally isolated parallel microchannels is studied experimentally. A dielectric liquid (HFE-7100) is delivered to the parallel channels using a constant pressure source. Both channels are uniformly subjected to the same power, which is in increased in steps. Flow visualization is conducted simultaneously with pressure drop, mass flux, and wall temperature measurements to characterize the thermal-fluidic effects of the Ledinegg instability. When the flow in both channels is in the single-phase regime, they have equal wall temperatures due to evenly distributed mass flux delivered to each channel. Boiling incipience in one of the channels triggers the Ledinegg instability which induces a temperature difference between the two channels due to flow maldistribution. The temperature difference between the two channels grows with increasing power until boiling incipience occurs in the second channel. The wall temperatures of both channels then reduce significantly as the flow becomes more evenly distributed. The experimentally observed temperature excursion between the channels is reported here for the first time and provides an improved understanding of the thermal performance implications of the Ledinegg instability in thermally isolated parallel channels.

A hybrid two-layer evaporator wick is proposed for passive, high-heat-flux dissipation over large areas using a vapor chamber heat spreader. For such applications, the evaporator wick layer must be designed to simultaneously minimize the device temperature rise and minimize the flow resistance to a capillary feeding of the wick. This requires a strategy that exploits the benefits of a thin wick for reduced thermal resistance and a thick wick for liquid feeding. In the present design, a thick cap layer of wick material evenly routes liquid to a thin, low-thermal-resistance base layer through an array of vertical liquid-feeding posts. This two-layer structure decouples the functions of liquid resupply (cap layer) and capillary-fed boiling heat transfer (base layer), making the design scalable to heat input areas of ~1 cm2 for operation at 1 kW/cm2. A reduced-order model is developed to demonstrate the potential performance of a vapor chamber incorporating such a two-layer evaporator wick design. The model comprises simplified hydraulic and thermal resistance networks for predicting the capillary-limited maximum heat flux and the overall thermal resistance, respectively. The reduced-order model is validated against a higher fidelity numerical model and then used to analyze the performance of the vapor chamber with varying two-layer wick geometric feature sizes. The two-layer wick design is found to sustain liquid feeding at higher heat fluxes, without reaching the capillary limit, compared to single-layer evaporator wick designs.

Microchannel heat sinks allow the removal of dense heat loads from high-power electronic devices at modest chip temperature rises. Such heat sinks are produced primarily using conventional subtractive machining techniques or anisotropic chemical etching, which restricts the geometric features that can be produced. Owing to their layer-by-layer and direct-write approaches, additive manufacturing (AM) technologies enable more design-driven construction flexibility and offer improved geometric freedom. Various AM processes and materials are available, but their capability to produce features desirable for microchannel heat sinks has received a limited assessment. Following a survey of commercially mature AM techniques, direct metal laser sintering was used in this paper to produce both straight and manifold microchannel designs with hydraulic diameters of $500~\mu \text{m}$ in an aluminum alloy (AlSi10Mg). Thermal and hydraulic performances were characterized over a range of mass fluxes from 500 to 2000 kg/m2s using water as the working fluid. The straight microchannel design allows these experimental results to be directly compared against widely accepted correlations from the literature. The manifold design demonstrates a more complex geometry that offers a reduced pressure drop. A comparison of the measured and predicted performance confirms that the nominal geometry is reproduced accurately enough to predict pressure drop based on conventional hydrodynamic theory, albeit with roughness-induced early transition to turbulence; however, the material properties are not known with sufficient accuracy to allow for a priori thermal design. New design guidelines are needed to exploit the benefits of AM while avoiding undesired or unanticipated performance impacts.

Microchannel heat sinks are capable of removing dense heat loads from high-power electronic devices with low thermal resistance, but suffer from high pressure drops due to the small channel dimensions. Features that reduce the pressure drop, such as manifolds, increase fabrication complexity and are constrained by traditional subtractive manufacturing approaches. Additive manufacturing technologies offer improved design freedom and reduced geometric restrictions, expanding the types of features that can be produced and integrated into a heat sink. In this work, a novel permeable membrane microchannel (PMM) heat sink geometry is proposed and fabricated using direct metal laser sintering (DMLS) of an aluminum alloy (AlSi10Mg). In this PMM design, the cooling fluid is forced through thin, porous walls that act as both conducting fins and membranes that allow flow through their fine internal flow features for efficient heat exchange. The design leverages the ability of this fabrication process to incorporate complex, arbitrarily curved structures having internal porosity to enhance heat transfer and reduce pressure drop across the heat sink. The PMM heat sink geometry is benchmarked against a low-pressure-drop manifold microchannel (MMC) heat sink. A reduced-order model is used to explore the relative performance trends between the designs. Both heat sinks are experimentally characterized at flow rates of 50–500 mL/min using deionized water as the working fluid. At a constant pumping power of 0.018 W, the permeable membrane microchannel design offers both lower thermal resistance (17% reduction) and lower pressure drop (28% reduction) compared to the manifold microchannel heat sink.

This paper examines the ability of the axially dispersed plug-flow model to accurately predict breakthrough in adsorbent beds confined by rigid walls. The axially dispersed plug-flow model is used ...

Continuously increasing energy demand and growing concern about energy resources has attracted much research in the field of clean and sustainable energy sources. In this context, zero-emission fuels are required for energy production to reduce the usage of fossil fuel resources. Here, we present the synthesis of Pd-Ag-decorated reduced graphene oxide (rGO) nanostructures using a green chemical approach with stevia extract for hydrogen production and antibacterial studies under light irradiation. Moreover, bimetallic nanostructures are potentially lime lighted due to their synergetic effect in both scientific and technical aspects. Structural characteristics such as crystal structure and morphological features of the synthesized nanostructures were analyzed using X-ray diffraction and transmission electron microscopy. Analysis of elemental composition and oxidation states was carried out by X-ray photoelectron spectroscopy. Optical characteristics of the biosynthesized nanostructures were obtained by UV-Vis absorption spectroscopy, and Fourier transform infrared spectroscopy was used to investigate possible functional groups that act as reducing and capping agents. The antimicrobial activity of the biosynthesized Pd-Ag-decorated rGO nanostructures was excellent, inactivating 96% of Escherichia coli cells during experiments over 150 min under visible light irradiation. Hence, these biosynthesized Pd-Ag-decorated rGO nanostructures can be utilized for alternative nanomaterial-based drug development in the future.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a worldwide viral pandemic leading to global efforts to produce and distribute effective vaccines that prevent coronavirus virus disease 2019 (COVID-19) (1). The Spike protein and, more specifically, its receptor-binding domain (RBD) on the virus surface are responsible for binding to human angiotensin-converting enzyme 2 (hACE2) on the host cell. Hence, because of their central role in viral entry, the Spike and the RBD are established immunogens in SARS-CoV-2 vaccines (2, 3). mRNA-based and viral-vectored vaccines encoding the Spike protein have gained regulatory approvals and are being massively deployed presently. Despite their excellent protective efficacy against SARS-CoV-2, mRNA vaccines have well-known limited thermostability, and thus, global distribution is complicated by the requirement of ultracold freezing storage temperatures (4). As COVID-19 vaccines are needed worldwide to immunize sufficient populations to induce global herd immunity, including in low- and middle-income countries, there is more demand for vaccine doses than at any other time in history (5). A potent vaccine that is effective with a lower amount of antigen per dose would increase manufacturing and distribution capacity, facilitating the supply of global needs during the pandemic., This research was supported by the NIH (R43AI165089), the European Union’s Horizon 2020 research and innovation program under a Marie Skłodowska-Curie grant (E.R.), the CIFAR Azrieli Global Scholar program (J.P.-J.), the Ontario Early Researcher Awards program (J.P.-J.), and the Canada Research Chairs program (J.P.-J.). Cryo-EM images were collected at the Facility for Electron Microscopy Research (FEMR) at McGill University. FEMR is supported by the Canadian Foundation for Innovation, the Quebec government, and McGill University.

Surface wettability is known to have a major influence on the ebullition characteristics of a bubble growing from a solid surface. Yet, simplistic static characterization of the wetting behavior is still relied upon to indicate performance characteristics during boiling. In this study, a theoretical framework is developed for the wetting and dewetting processes occurring during bubble growth based upon the dynamic contact angles. This framework is incorporated into adiabatic volume-of-fluid simulations to capture the influence of the surface wettability on contact line and contact angle dynamics during bubble growth and departure. The simulations span a large range of dynamic wetting behaviors and fluid properties. The receding contact angle is shown to govern the early stages of bubble growth as the contact line recedes outward from the bubble center and is the dominant wetting characteristic that determines the maximum contact diameter and departure size. The advancing contact angle dictates the departure morphology as the contact line retracts inward and has a secondary role in determining the departure size. Following, improved reduced-order models are developed that establish fluid-property-independent correlations for the maximum contact diameter and departure diameter as a function of the dynamic contact angles. The results call for the need to redefine wettability classifications based on dynamic contact angles rather than static contact angle in the context of boiling. Hygrophilicity and hygrophobicity are redefined in this context, and an additional classification, ambiphilicity, is introduced for boiling surfaces exhibiting low receding contact angles and high advancing contact angles.

Evaporation from porous structured surfaces is encountered in a variety of applications including electronics cooling, desalination, and solar energy generation. Of major interest in the design of thermal systems for such applications is a prediction of the heat and mass transfer rates during evaporation from these surfaces. The present study develops a figure of merit (FOM) that characterizes the efficacy of evaporative heat transfer from microstructured surfaces. Geometric quantities such as the contact line length per unit area, porosity, and contact angle that are independent of details of the surface structure are utilized to develop the FOM, allowing for flexibility in its application to a variety of structured surfaces. This metric is calibrated against an evaporative heat transfer model and further benchmarked with evaporation heat transfer data from the literature. The FOM successfully captures the variation in evaporation heat transfer coefficient across different structures as well as the optimum dimensions for a given structure, and therefore can serve as a tool to survey available structures and also optimize their dimensions for heat and mass transfer enhancement.

A hierarchical manifold microchannel heat sink array is fabricated and experimentally characterized for uniform heat flux dissipation over a footprint area of 5 mm × 5 mm. A 3 × 3 array of heat sinks is fabricated into the silicon substrate containing the heaters for direct intrachip cooling, eliminating the thermal resistances typically associated with the attachment of a separate heat sink. The heat sinks are fed in parallel using a hierarchical manifold distributor that delivers flow to each of the heat sinks. Each heat sink contains a bank of high-aspect-ratio microchannels; five different channel geometries with nominal widths of 15 μm and 33 μm and nominal depths between 150 μm and 470 μm are tested. The thermal and hydraulic performance of each heat sink array geometry is evaluated using HFE-7100 as the working fluid, for mass fluxes ranging from 600 kg/m2 s to 2100 kg/m2 s at a constant inlet temperature of 59 °C. To simulate heat generation from electronics devices, a uniform background heat flux is generated with thin-film serpentine heaters fabricated on the silicon substrate opposite the channels; temperature sensors placed across the substrate provide spatially resolved surface temperature measurements. Experiments are also conducted with simultaneous background and hotspot heat generation; the hotspot heat flux is produced by a discrete 200 μm × 200 μm hotspot heater. Heat fluxes up to 1020 W/cm2 are dissipated under uniform heating conditions at chip temperatures less than 69 °C above the fluid inlet and at pressure drops less than 120 kPa. Heat sinks with wider channels yield higher wetted-area heat transfer coefficients, but not necessarily the lowest thermal resistance; for a fixed channel depth, samples with narrower channels have increased total wetted areas owing to the smaller fin pitches. During simultaneous background and hotspot heating conditions, background heat fluxes up to 900 W/cm2 and hotspot fluxes up to 2700 W/cm2 are dissipated. The hotspot temperature increases linearly with hotspot heat flux; at hotspot heat fluxes of 2700 W/cm2, the hotspot experiences a temperature rise of 16 °C above the average chip temperature.

Fixed-bed adsorption is widely used in industrial gas separation and is the primary method for atmosphere revitalization in space. This paper analyzes the uncertainty of a one-dimensional, fixed-bed adsorption model due to uncertainty in several model inputs, namely, the linear-driving-force (LDF) mass transfer coefficient, axial dispersion, heat transfer coefficients, and adsorbent properties. The input parameter uncertainties are determined from a comprehensive survey of experimental data in the literature. The model is first calibrated against experimental data from intra-bed centerline concentration measurements to find the LDF coefficient. We then use this LDF coefficient to extract axial dispersion coefficients from mixed, downstream concentration measurements for both a small-diameter bed (dominated by wall-channeling) and a large-diameter bed (dominated by pellet-driven dispersion). The predicted effluent concentration and temperature profiles are most strongly affected by uncertainty in LDF coefficient, adsorbent density, and void fraction. The uncertainty analysis further reveals that ignoring the effect of wall-channeling on apparent axial dispersion can cause significant error in the predicted breakthrough times of small-diameter beds.

Evaporating flows in parallel channels occurring in a variety of industrial heat exchange processes can encounter non-uniform flow distribution between channels as a result of two-phase flow instabilities. Such flow maldistribution can have a negative impact on the performance, robustness and predictability of these systems. Two-phase flow modeling can assist in understanding the mechanistic behavior of this flow maldistribution, as well as determine parametric trends and identify safe operating conditions. The work described in this paper expands on prior two-phase flow distribution modeling efforts by including and assessing the effect of thermal conduction in the walls surrounding the parallel channels. This thermal conduction has a critical dampening effect on wall temperature gradients. In particular when a channel is significantly starved of flow rate and risks dryout, channel-to-channel thermal coupling can redistribute the heat load from the flow-starved channel to neighboring channels. The model is used to simulate the two-phase flow distribution in a system of two parallel channels driven by a constant flow rate pump. A comparison between thermally isolated and coupled channels indicates that thermally coupled channels are significantly less susceptible to maldistribution. Furthermore, a parametric study reveals that flow maldistribution is only possible in thermally coupled systems beyond a certain critical heat flux threshold. This threshold heat flux increases as the lateral wall conductance is increased, converging to a constant value in the limit of very high lateral conductance.

Dynamic flow boiling instabilities are studied experimentally in a single, 500 µm-diameter glass microchannel subjected to a uniform heat flux. Fluid flow is driven through the microchannel in an open-loop test facility by maintaining a constant pressure difference between a pressurized upstream reservoir and a reservoir at the exit that is open to the ambient; the working fluid is HFE-7100. This hydrodynamic boundary condition resembles that of an individual channel in a parallel-channel heat sink where the channel mass flux can vary in time. Simultaneous high-frequency measurement of reservoir, inlet, and outlet pressures, pressure drop, mass flux, inlet and outlet fluid temperatures, and wall temperature is synchronized to high-speed flow visualizations enabling transient characterization of the thermal-fluidic behavior. Part 1 of this study investigates the rapid-bubble-growth instability at the onset of boiling; the effect of flow inertia and inlet liquid subcooling is assessed. The mechanisms underlying the rapid-bubble-growth instability, namely, a large liquid superheat and a large pressure spike, are quantified; this instability is shown to cause flow reversal and can result in large temperature spikes. Low flow inertia exacerbates the rapid-bubble-growth instability by starving the heated channel of liquid replenishment for longer durations and results in severe temperature increases. In the case of high flow inertia or high inlet liquid subcooling, flow reversal is still observed at the onset of boiling, but results in a minimal wall temperature rise because liquid quickly replenishes the heated channel. A companion paper (Part 2) investigates the effect of flow inertia, inlet liquid subcooling, as well as heat flux on the thermal-fluidic oscillations during time-periodic flow boiling that follows the initial incipience at the onset of boiling considered here.

Dynamic instabilities during flow boiling in a uniformly heated microchannel are investigated. The focus of this Part 2 of the study is on the effect of operating conditions on the instability type and the resulting time-periodic hydrodynamic and thermal oscillations, which have been established after the initial boiling incipience event. Part 1 of this study investigated the rapid-bubble-growth instability at the onset of boiling in the same experimental facility. Fluid is driven through the single 500 μm-diameter glass microchannel by maintaining a constant pressure difference between a pressurized upstream reservoir and a reservoir downstream that is open to the ambient, so as to resemble the hydrodynamic boundary conditions of an individual channel in a parallel-channel heat sink. Simultaneous high-frequency measurement of pressure drop, mass flux, and wall temperature is synchronized to high-speed flow visualizations enabling transient characterization of the thermal-fluidic behavior. The effect of flow inertia, inlet liquid subcooling, and heat flux on the hydrodynamic and thermal oscillations and time-averaged performance is assessed. Two predominant dynamic instabilities are observed: a time-periodic series of rapid-bubble-growth instabilities, and the pressure drop instability. A spectral analysis of the time-periodic data is performed to determine the characteristic oscillation frequencies. The heat flux, ratio of flow inertia to upstream compressibility, and degree of inlet liquid subcooling significantly affect the thermal-fluidic characteristics. High inlet liquid subcoolings and low heat fluxes result in time-periodic transitions between single-phase flow and flow boiling that cause large-amplitude wall temperature oscillations due to a time-periodic series of rapid-bubble-growth instabilities. Low inlet liquid subcoolings result in small-amplitude thermal-fluidic oscillations and the pressure drop instability. Low flow inertia exacerbates the pressure drop instability and results in large-amplitude thermal-fluidic oscillations whereas high flow inertia reduces their severity.

Two-phase passive heat transport devices such as vapor chambers, loop heat pipes, and capillary pumped loops utilize porous evaporators for phase change and to drive fluid transport. Nucleate boiling can occur within such capillary-fed porous evaporators, especially under high-heat-flux operation, as has been visually observed in various experimental studies in the literature. However, prior modeling efforts have typically only considered single-phase flow of liquid through a completely saturated porous medium for characterizing the dryout limit and thermal performance. The present work offers a new semi-empirical model for prediction of thermal resistance and dryout during boiling in capillary-fed evaporators. Thermal conduction across the solid and volumetric evaporation within the pores are solved to obtain the temperature distribution in the porous structure. Capillary-driven lateral liquid flow from the outer periphery of the evaporator to its center, with vapor flow across the thickness, is considered to obtain the local liquid and vapor pressures. The capillary pressure and the relative permeabilities (fraction of single-phase permeabilities) for two-phase flow in the porous medium are modeled as a function of the local liquid saturation. The heat flux at which the liquid saturation at the center of the evaporator becomes zero is defined as the dryout limit of the evaporator. Experiments are conducted on sintered copper particle evaporators of different particle sizes and heater areas to collect data for model calibration. To demonstrate the wider applicability of the model for other types of porous evaporators, the model is further calibrated against a variety of dryout limit and thermal resistance data collected from the literature. The model is shown to predict the experimentally observed trends in the dryout limit with mean particle/pore size, heater size, and evaporator thicknesses.

An experimental investigation is conducted to examine the effects of operating conditions and channel size on the liquid film thickness of vapor bubbles in adiabatic air-water flows within the slug flow regime. Acrylic test sections are fabricated to contain a single microchannel of square cross-section with hydraulic diameters of 510 μm and 1020 μm. High-speed visualizations are used to map the flow regimes in these channels in order to determine the range of liquid and gas flow rates for which slug-regime flows are sustained. Subsequently, a tomographic optical imaging technique is employed to quantitatively reconstruct the liquid–gas interface of vapor bubbles at selected operating conditions. This technique relies on visualization of fluorescent particles seeded into the liquid phase in order to identify the phase boundaries within thin sections of the flow. Using the reconstructions, the thickness of the liquid film in the corner of the square channel cross-section is extracted. This film thickness is found to decrease with increasing capillary number, and a simple expression is proposed for calculation of the film thickness; predictions from this model match the measurements with a mean absolute error (MAE) of 21.6%; 85.7% of all predicted data points fall within an error band of ±30%. Additionally, film thickness data from the literature are compared to model predictions and a comparable MAE of 21.8% is found with 77.8% of data points falling within an error band of ±30%.

Structured porous materials show great potential as extended surfaces in heat-exchange applications that also require design for load-bearing capability. In particular, lattice-frame materials (LFM) are known for their superior strength-to-weight ratio; this work presents a comprehensive experimental and numerical study of fluid flow and heat transfer in porous LFMs. Flow through a periodic unit cell of the material is simulated to characterize the forced-convection performance under hydraulically and thermally fully developed conditions. The performance of LFMs with a tetrahedral ligament configuration is characterized as a function of Reynolds number in the laminar regime (150 Re

The current pandemic of COVID-19 caused by SARS-CoV-2 (severe acute respiratory syndrome coronavirus-2) has raised significant public health concerns. Rapid and accurate testing of SARS-CoV-2 is urgently needed for early detection and control of the disease spread. Here, we present an RT-LAMP coupled glass nanopore digital counting method for rapid detection of SARS-CoV-2. We validated and compared two one-pot RT-LAMP assays targeting nucleocapsid (N) and envelop (E) genes. The nucleocapsid assay was adopted due to its quick time to positive and better copy number sensitivity. For qualitative positive/negative classification of a testing sample, we used the glass nanopore to digitally count the RT-LAMP amplicons and benchmarked the event rate with a threshold. Due to its intrinsic single molecule sensitivity, nanopore sensors could capture the amplification dynamics more rapidly (quick time to positive). We validated our RT-LAMP coupled glass nanopore digital counting method for SARS-CoV-2 detection by using both spiked saliva samples and COVID-19 clinical nasopharyngeal swab samples. The results obtained showed excellent agreement with the gold standard RT-PCR assay. With its integration capability, the electronic nanopore digital counting platform has significant potential to provide a rapid, sensitive, and specific point-of-care assay for SARS-CoV-2.

Flow boiling in a network of heated parallel channels is prone to instabilities that can cause uneven flow distribution, thereby degrading the heat transfer performance of the system and limiting predictability. This study experimentally investigates flow maldistribution between two parallel microchannels that arises due to the Ledinegg instability. The channels are heated uniformly and are thermally isolated from each other, such that both channels are subjected to the same input power regardless of the flow distribution. The channels are hydrodynamically connected in parallel and deionized water is delivered at a constant total flow rate shared by both channels. Direct measurements of the flow rate, wall temperature, and pressure drop in individual channels are performed simultaneously with flow visualization. At low power levels, when both channels remain in the single-phase liquid regime, the flow is evenly distributed between the channels and they attain the same wall temperature. As the power is increased, boiling incipience in one of the channels triggers the Ledinegg instability, which causes the flow to become maldistributed and induces a temperature difference between the channels. The severity of flow maldistribution, as well as the temperature difference between the channels, grows with increasing power. In the most extreme condition measured in this study, 96.5% of the total flow rate is directed to the channel operating in the single-phase liquid regime, while the boiling channel is starved and receives just 3.5% of the flow. The quantitative account of the flow maldistribution and temperature non-uniformity presented here provides a mechanistic understanding of the effects of Ledinegg instability-induced flow maldistribution on the heat transfer characteristics of thermally isolated parallel microchannels.

Two-phase flow boiling is susceptible to the Ledinegg instability, which can result in non-uniform flow distribution between parallel channels and thereby adversely impact the heat transfer performance. This study experimentally assesses the effect of thermal coupling between parallel microchannels on the flow maldistribution caused by the Ledinegg instability and compares the results to our prior theoretical predictions. A system with two parallel microchannels is investigated using water as the working fluid. The channels are hydrodynamically connected via common inlet/outlet plenums and supplied with a constant total flow rate. The channels are uniformly subjected to the same input power (which is increased in steps). Two separate configurations are evaluated to assess drastically different levels of thermal coupling between the channels, namely thermally isolated and thermally coupled channels. Synchronized measurements of the flow rate in each individual channel, wall temperature, and pressure drop are performed along with flow visualization to compare the thermal-hydraulic characteristics of these two configurations. Thermal coupling is shown to reduce the wall temperature difference between the channels and dampen flow maldistribution. Specifically, the range of input power over which flow maldistribution occurs is noticeably smaller and the maximum severity of flow maldistribution is reduced in thermally coupled channels. The data provide a quantitative account of the effect of lateral thermal coupling in moderating flow maldistribution, which is corroborated by comparison to the predictions from our two-phase flow distribution model. This combined experimental and theoretical evidence demonstrates that, under extreme conditions when one channel is significantly starved of flow rate and risks dryout, channel-to-channel thermal coupling can redistribute the heat load from the flow-starved channel to the channel with excess flow. Due to such a possibility of heat redistribution, the coupled channels are significantly less prone to flow maldistribution compared to thermally isolated channels.

A methodology is proposed for predictive modeling of the liquid-gas interface shape and saturated flow boiling heat transfer coefficient in two-phase microchannel flows within the annular regime. The mechanistic model accounts for the effects of surface tension and interface curvature, gravity, and shear stress in determining the liquid film shape; one-dimensional conduction is assumed to occur across the variable-thickness film to calculate wall heat transfer coefficients locally along the channel length. Model performance is benchmarked against 251 experimentally measured heat transfer coefficient values taken from the literature for annular-regime flow boiling in microchannels of a rectangular cross-section. These data are successfully predicted with a mean absolute error of 21.7%, and 72.1% of the points lie within an error band of ±30%. The match to data is poorest at lower vapor qualities corresponding to the onset of the annular regime, for which the heat transfer coefficient is underpredicted; an experimental investigation is performed to better understand the disparity under these operating conditions. Liquid film shapes are measured during adiabatic annular flow through microchannels of square cross-section for a range of channel hydraulic diameters (160 µm, 510 µm, 1020 µm) and operating conditions, so as to control the void fraction and Weber number of the flow. Using air and water as the working gas and liquid, respectively, trends in film behavior are identified and compared against model predictions. The experimental findings reveal the non-negligible impact of capillary pumping on the interface morphology at the onset of the annular regime.

The design of structured surfaces for increasing the heat flux dissipated during boiling and evaporation processes via enhanced liquid rewetting requires prediction of the liquid meniscus shape on these surfaces. In this study, a general continuum model is developed to predict the three-dimensional meniscus shape of liquid films on micro/nanostructured surfaces based on a minimization of the system free energy that includes solid-liquid van der Waals interaction energy, surface energy, and gravitational potential. The continuum model is validated at the nanoscale against molecular dynamics simulations of water films on gold surfaces with pyramidal indentations, and against experimental measurements of water films on silicon V-groove channels at the microscale. The validated model is used to investigate the effect of film thickness and surface structure depth on the meniscus shape. The meniscus is shown to become more conformal with the surface structure as the film thickness decreases and the structure depth increases. Assuming small interface slope and small variation in film thickness, the continuum model can be linearized to obtain an explicit expression for the meniscus shape. The error of this linearized model is quantitatively assessed and shown to increase with increasing structure depth and decreasing structure pitch. The model developed can be used for accurate prediction of three-dimensional meniscus shape on structured surfaces with micro/nano-scale features, which is necessary for determining the liquid delivery rate and heat flux dissipated during thin-film evaporation. The linearized model is useful for rapid prediction of meniscus shape when the structure depth is smaller than or comparable to the liquid film thickness.

Coalescence-induced condensate droplet jumping from superhydrophobic surfaces can be exploited in condensation heat-transfer enhancement, imparting self-cleaning behavior to surfaces, anti-icing coatings, and other industrial uses. An intriguing application would exploit this phenomenon to achieve thermal rectification using a sealed vapor chamber with opposing superhydrophilic and superhydrophobic surfaces. During forward operation, continuous evaporation occurs from the heated superhydrophilic evaporator surface; self-propelled jumping returns the condensate droplets from the cooler superhydrophobic surface to the evaporator, allowing passive recirculation of the working fluid without a reliance on gravity or capillary wicking for fluid return. In reverse operation, when the superhydrophobic surface is heated, the absence of a mechanism for fluid return from the cooler superhydrophilic side to the evaporator restricts heat transport across the (low conductivity) vapor gap. The effectiveness of this jumping-droplet thermal diode can be enhanced by maximizing the distance between the opposing superhydrophobic and superhydrophilic surfaces. It is, therefore, important to investigate the height to which the condensate droplets jump from the superhydrophobic surfaces such that replenishment of liquid to the superhydrophilic surface is maintained. In this study, we systematically investigate the height to which the condensate droplets jump from the hierarchical superhydrophobic surfaces having truncated microcones coated with nanostructures. The condensate droplet jumping height increases with a decrease in microcone pitch. A general theoretical model that accounts for surface adhesion, line tension, and initial wetting states of multiple coalescing droplets of different radii is used to predict and explain the experimentally observed trend of droplet jumping height with surface roughness. The insights provided regarding the effect of surface topography on droplet jumping height are critical to the design of high-performance thermal diode devices.

Two-phase heat exchangers are used in a variety of industrial processes in which the boiling fluid flows through a network of parallel channels. In some situations, the fluid may not be uniformly distributed through all the channels, causing a degradation in the thermal performance of the system. A methodology for modeling two-phase flow distributions in parallel-channel systems is developed. The methodology combines a pressure-drop model for individual parallel channels with a pump curve into a system flow network. Due to the non-monotonicity of the pressure drop as a function of flow rate for boiling channels, many steady-state solutions exist for the system flow equations. A new numerical approach is proposed to analyze the stability of these solutions, based on a generalized eigenvalue problem. The method is specifically designed for analyzing systems with hundreds of identical parallel channels. The method is first applied to analyze the flow distribution and stability behavior in two-channel and five-channel systems. The asymptotic behavior of the flow stability is then analyzed for increasing numbers of channels, and it is shown that the stability behavior of a system with a constant flow-rate pump curve simplifies to the stability behavior for a constant pressure-drop pump curve. A parametric study is conducted to assess the influence of inlet temperature, heat flux, and flow rate on the stability of the uniform flow distribution solution as well as on the severity of flow maldistribution. Below some critical inlet subcooling, uniform flow distribution is always stable and maldistribution does not occur, regardless of heat flux and flow rate. Above this critical inlet subcooling, there is a range of operating parameters for which uniform flow distribution is unstable. With increasing inlet subcooling, this range broadens and the severity of the associated maldistribution increases.

The behavior of a vapor chamber is strongly coupled to the thermophysical properties of the working fluid within. It is well known that these properties limit the maximum power (heat load) at which a vapor chamber can operate, due to incidence of the capillary limit. At this limit, the available capillary pressure generated within the wick structure balances the total pressure drop incurred along the path of fluid flow within the wick. A common figure of merit prioritizes working fluids that maximize this capillary-limited operating power. The current work explores working fluid selection for ultra-thin vapor chambers based on a thermal performance objective, rather than for maximized power dissipation capability. A working fluid is sought in this case that provides the minimal thermal resistance while ensuring a capillary limit is not reached at the target operating power. A resistance-network-based model is used to develop a simple analytical relationship for the vapor chamber thermal resistance as a function of the working fluid properties, operating power, and geometry. At small thicknesses, the thermal resistance of vapor chambers becomes governed by the saturation temperature gradient in the vapor core, which is dependent on the thermophysical properties of the working fluid. To satisfy the performance objective, it is shown that the choice of working fluid cannot be based on a single figure of merit containing only fluid properties. Instead, the functional relationship for thermal resistance must be analyzed taking into account all operating and geometric parameters, in addition to the thermophysical fluid properties. Such an approach for choosing the working fluid is developed and demonstrated.

This study uses high-speed imaging to characterize microchannel slug flow boiling using a novel experimental test facility that generates an archetypal flow regime suitable for high-fidelity characterization of key hydrodynamic and heat transfer parameters. Vapor and liquid phases of the fluorinated dielectric fluid HFE-7100 are independently injected into a T-junction to create a saturated two-phase slug flow, thereby eliminating the flow instabilities and flow-regime transitions that would otherwise result from stochastic generation of vapor bubbles by nucleation from a superheated channel wall. Slug flow boiling is characterized in a heated, 500 μm-diameter borosilicate glass microchannel. A thin layer of optically transparent and electrically conductive indium tin oxide coated on the outside surface of the microchannel provides a uniform heat flux via Joule heating. High-speed flow visualization images are analyzed to quantify the uniformity of the vapor bubbles and liquid slugs generated, as well as the growth of vapor bubbles under heat fluxes ranging from 30 W/m2 to 5160 W/m2. A method is demonstrated for measuring liquid film thickness from the visualizations using a ray-tracing procedure to correct for optical distortions. Characterization of the slug flow boiling regime that is generated demonstrates the unique ability of the facility to precisely control and quantify hydrodynamic and heat transfer characteristics. The experimental approach demonstrated in this study provides a unique platform for the investigation of microchannel slug flow boiling transport under controlled, stable conditions suitable for model validation.

Electrode design is investigated for electrical capacitance tomography on a 1 mm-thick 3D planar domain. Arrays of square electrodes are located on both sides of the domain, which is filled with dielectric material. Anomalies of interest are voids in the material with low dielectric constant. Simulated tomography is conducted with various electrode patterns over a range of electrode sizes in order to compare performance. The high-aspect-ratio domain requires a large finite element mesh for simulating electric fields and for defining spatial sensitivity responses of electrode pairs. Compared to the canonical 2D pipe cross-section problem, this high-aspect-ratio system requires solving the reconstruction problem on a much larger mesh and is more severely underdetermined. An efficient methodology for characterizing the sensitivity responses of various electrode pairs is developed. An image reconstruction algorithm creates binary images by sequentially composing a list of occupied cells, conditioned with a cell-to-cell energy formulation. A parametric study of four candidate electrode patterns is conducted using electrode sizes between 0.8 mm and 1.6 mm, in which each potential design is used to perform tomographic reconstruction of 100 images. A new image-error metric is proposed, which is used to evaluate each electrode design. It is found that an electrode matrix pattern on each substrate, with the patterns offset, results in lower mean image error than other candidate patterns.

The maximum heat load that a heat pipe can sustain at steady state is governed by the balance between the capillary pressure provided by the wick and the flow resistance to liquid resupply at the evaporator. At heat loads beyond the capillary limit, the wick will dry out at the evaporator. However, the nature of the imposed heat load may be highly transient in various applications ranging from consumer electronic devices to server processors, depending on the end user needs and workloads. For such scenarios, it becomes critical to assess the operation of heat pipes in response to brief transient heat loads which could be higher than the notional capillary limit that governs dryout at steady state. In the current study, experiments are performed to demonstrate that, under transient heating conditions, a heat pipe can sustain heat loads higher than the steady-state capillary limit for brief periods of time without experiencing dryout. The time-to-dryout during which the heat pipe can sustain a higher heat load is characterized for a step input higher than the steady-state capillary limit. The time-to-dryout is found to decrease as the magnitude of the step heat input increases.

Freezing of water flowing through a small channel can be used as a nonintrusive flow control mechanism for microfluidic devices. However, such ice valves have longer response times compared to conventional microvalves. To control and reduce the response time, it is crucial to understand the factors that affect the flow freezing process inside the channel. This study investigates freezing in pressure-driven water flow through a glass channel of 500 μm inner diameter using measurements of external channel wall temperature and flow rate synchronized with high-speed visualization. The effect of flow rate on the freezing process is investigated in terms of the external wall temperature, the growth duration of different ice modes, and the channel closing time. Freezing initiates as a thin layer of ice dendrites that grows along the inner wall and partially blocks the channel, followed by the formation and inward growth of a solid annular ice layer that leads to complete flow blockage and ultimate channel closure. A simplified analytical model is developed to determine the factors that govern the annular ice growth, and hence the channel closing time. For a given channel, the model predicts that the annular ice growth is driven purely by conduction due to the temperature difference between the outer channel wall and the equilibrium ice-water interface. The flow rate affects the initial temperature difference, and thereby has an indirect effect on the annular ice growth. Higher flow rates require a lower wall temperature to initiate ice nucleation and result in faster annular ice growth (and shorter closing times) than at lower flow rates. This study provides new insights into the freezing process in small channels and identifies the key factors governing the channel closing time at these small length scales commonly encountered in microfluidic ice valve applications.

In article number 2005637, Jonathan F Lovell and co-workers show that the SARS-CoV-2 RBD surface protein becomes a potent immunogen when presented in nanoparticle format Using a vaccine adjuvant that spontaneously converts soluble recombinant antigens into stable particles, immunization studies in mice and rabbits shows that the particle-based RBD elicits strong immune responses and potent antibodies capable of neutralizing the virus

A single subcooled jet of water which undergoes boiling upon impingement on a discrete heat source is studied experimentally using time-resolved stereo particle image velocimetry (PIV). The impinging jet issues from a 3.75 mm diameter sharp-edged orifice in a confining orifice plate positioned 4 orifice diameters from the target surface. The behavior at jet Reynolds numbers of 5,000 and 15,000 is compared for a constant jet inlet subcooling of 10 °C. Fluorescent illumination allows for simultaneous imaging of both the flow tracers and the vapor bubbles in the flow. Flow structure, time-averaged velocities, and turbulence statistics are reported for the liquid regions within the confinement gap for a range of heat inputs at both Reynolds numbers, and the effect of the vapor generation on the flow is discussed. Vapor generation from boiling is found to modify the liquid velocities and turbulence fluctuations in the confinement gap. Flow in the confinement gap is dominated by vapor flow, and the vapor bubbles disrupt both the vertical impinging jet and horizontal wall jet flow. Moreover, vapor bubbles are a significant source of turbulence kinetic energy and dissipation, with the bubbly regions above the heated surface experiencing the most intense turbulence modification. Spectral analysis indicates that a Strouhal number of 0.023 is characteristic of the interaction between bubbles and turbulent liquid jets.

Tomographic particle image velocimetry (PIV) is used to study the effect of confinement gap height on the liquid flow characteristics in jet impingement boiling. This first application of tomographic PIV to flow boiling is significant given the complexity of confined two-phase jet impingement. A jet of subcooled water at a Reynolds number of 5,000 impinges onto a circular heat source undergoing boiling heat transfer at a constant heat input. Confinement gap heights of 8, 4, and 2 jet diameters are investigated. A visual hull method is used to reconstruct the time-varying regions of the vapor in the flow. The vapor motion is found to govern the liquid flow pattern and turbulence generation in the confinement gap. Time-averaged velocities and regions of turbulent kinetic energy in the liquid are highest for a confinement gap height of 8 jet diameters, with lower velocity magnitude and turbulence being observed for the smaller spacings. Coherent vortical structures identified with the λ 2 -criterion are found to occur most frequently near the moving vapor interface. The most intense regions of turbulent kinetic energy do not coincide with the location of coherent structures within the flow. Irrotational velocity fluctuations in the liquid phase caused by vapor bubble pinch-off are the primary cause of the high turbulent kinetic energy measured in these regions. At a gap height of H / d = 2 the vapor plume is constrained as it grows from the heat source, causing bulk flow oscillations in the downstream region of the confinement gap.

An instrumentation technique is developed using embedded capacitive sensors to measure the thickness and evenness of coverage of a thin layer of dielectric thermal interface material (TIM) between two substrates. The technique requires an array of sensors embedded into one substrate, with an electrically conductive opposing substrate. Local capacitance measurements are sensitive to both local bond layer thickness and local voiding. We propose a means for using an array of capacitance measurements to interpret both bond layer thickness and local voiding at every sensor location. An algorithm is developed which reveals both characteristics from a single set of capacitance measurements. Experiments are conducted with thermal grease layers of different bond layer thicknesses and void distributions using a prototype system constructed on printed circuit boards. The thickness and void distribution are successfully mapped across the bond layer using the algorithm developed. The technique offers a sensing approach for in situ instrumentation of layers of thermal grease in a thermal test vehicle.

The development of absorbent materials with high selectivity for oils and organic solvents is of great ecological importance for removing pollutants from contaminated water sources. We have developed a facile solution-immersion process for creating polydimethylsiloxane (PDMS)-functionalized sponges for oil–water separation. Sponge materials with densities ranging from 8 to 26 mg/cm3 were investigated as candidate skeletons. After functionalization, the lowest-density melamine sponge exhibits superior superhydrophobic and superoleophilic properties, absorption capacity, oil–water selectivity, and absorption recyclability. Via suction through such a functionalized sponge, we have experimentally demonstrated that various kinds of oils can be continuously separated from immiscible liquid mixtures without any water uptake. The widely available raw materials (melamine sponge and PDMS solution) and simple synthesis steps yield a cost-effective and scalable process for fabrication of absorbent materials that can ...