Dropwise condensation, widely recognized as a highly efficient heat transfer mechanism, is yet to be implemented in industrial applications. Recent advances in semi-analytical modeling of condensation have led to predictions of an optimal contact angle for enhancement of this phase change heat transfer process. Here, we present the first experimental study supporting these predictions. Using self-assembled monolayers of thiols on gold-coated smooth surfaces, we systematically investigated contact angles in the range 84 degrees to 115 degrees in a pressureand temperature-controlled environmental chamber in the absence of non-condensable gases. Our experimental results reveal that the optimal contact angle for condensation falls between 96 degrees and 105 degrees . Interestingly, while our results support predictions regarding the existence of an optimal contact angle, the specific values and their impact differ from previous reports. By experimentally demonstrating higher condensation heat transfer rates at intermediate contact angles, this study unequivocally shows that high hydrophobicity is not necessarily a desired property for a condenser surface. The insights gained from this work open new avenues for improving dropwise condensation in various industrial processes such as the steam cycle and liquid separation.

Thin-film evaporation in micropillar wicking structures is a promising passive cooling strategy for high heat flux electronics. This study numerically investigates thin-film evaporation in well-defined silicon micropillar wicks, where water is transported passively via capillary wicking from the reservoir to the evaporator. A coupled force balance and conservation law framework is employed to determine the meniscus shape, capillary pressure, fluid velocity in the micropillar wicks, and associated heat transfer characteristics. The dry-out heat flux, defined as the maximum heat flux the evaporator can dissipate when the smallest contact angle equals the receding contact angle, is evaluated for different wick designs. For uniform wicks with fixed micropillar geometry, the maximum dry-out heat flux is ≈84 W/cm2. To enhance thermo-fluidic performance, variable wicks are designed with sparse micropillars near the water reservoir and dense micropillars near the evaporator center. By dividing the wick into multiple sections with optimized diameters, the dry-out heat flux reaches ≈147 W/cm2, a 75% improvement over uniform wicks. Further optimization of the variable wicks using a genetic algorithm (GA) increases the dry-out heat flux to ≈165 W/cm2, a 96% enhancement compared to uniform wicks. Unlike uniform wicks, where dry-out starts at the evaporator center, optimized variable wicks experience dry-out at an intermediate location due to increased capillary pressure near the center. These findings provide useful insights into the design and optimization of wicking structures for thin-film evaporation in advanced passive cooling of electronic devices. Copyright © 2026 by ASME.

Boiling can effectively remove concentrated heat from electronic devices due to the large latent heat of phase change. The heat removal rate greatly depends on the bubble-surface interaction. In this study, we investigated the bubble dynamics and boiling heat transfer performance of hemi-solid hemi-liquid surfaces that are created by impregnating micro-nanostructured surfaces with oil. These surfaces exhibit record-low contact angle hysteresis (1–2°), providing bubbles with high mobility. Pool boiling of water on these surfaces, along with rigid surfaces with different wettability, was tested. High-speed images show that the average bubble departure diameter on oil-impregnated surfaces was ≈60% larger and the average bubble residence time was ≈70% longer compared to their counterparts without lubricant. We attribute this result to the wetting ridge, which increases the downward forces on departing bubbles and creates a physical barrier for bubble coalescence. The wetting ridge naturally forms when bubbles nucleate on oil-impregnated surfaces due to the unbalanced vertical component of interfacial forces. To provide insight into boiling heat transfer, we experimentally measured the critical heat flux (CHF), the maximum heat flux achievable before the boiling crisis. The CHF on oil-impregnated surfaces was ≈28-36 W/cm², comparable to that of the counterpart surface without oil (≈27 W/cm²). We attribute this result to oil depletion, which rendered the textured oil-impregnated surfaces superhydrophobic, as confirmed by surface analysis after boiling. In addition to experimentally measuring boiling heat transfer performance, this work provides new insight into bubble growth and departure mechanisms on textured oil-impregnated hemi-solid hemi-liquid surfaces. © 2025 The Authors

By enabling an atomically smooth and chemically homogeneous interface, state-of-the-art lubricant-infused surfaces minimize contact line pinning, which directly translates to remarkable droplet mobility and ultralow drop friction. A unique feature of these surfaces is the formation of a wrapping layer-a nanometric lubricant film that encapsulates droplets. However, the mechanism that governs the formation of the wrapping oil layer and its thickness remains poorly understood to date. In this study, we develop and experimentally validate a theoretical modeling framework for the wrapping layer thickness by balancing two competing forces: curvature-induced Laplace pressure and van der Waals interaction-induced disjoining pressure. Using planar laser-induced fluorescence microscopy, we directly visualized and measured the wrapping layer thickness across a range of droplet radii, lubricant viscosities, and lubricant thicknesses used to impregnate the underlying textured substrate. Our results show that the wrapping layer thickness, which is insensitive to lubricant viscosity and initial thickness, scales with the droplet radius to the 1/3rd power. After lending credence to our analytical approach by validating model predictions with experiment, we estimated the volume of the wrapping layer using a simple, yet important, scaling argument. Moreover, we estimated the wetting ridge volume by capturing the steady-state shape of the oil meniscus that forms near the droplet base. Our analysis and theretical treatment show that the volume of oil in the wrapping layer is four orders of magnitude smaller than that of the wetting ridge, a result that points to the annular wetting ridge as the major source of lubricant depletion by moving droplets. The insights gained from this work improve the current understanding of wrapping layer dynamics and its impact on lubricant depletion.

Understanding droplet-surface interactions has broad implications in microfluidics and lab-on-a-chip devices. In contrast to droplets on conventional textured air-filled superhydrophobic surfaces, water droplets on state-of-the-art lubricant-infused surfaces are accompanied by an axisymmetric annular wetting ridge, the source and nature of which are not clearly established to date. Generally, the imbalance of interfacial forces at the contact line is believed to play a pivotal role in accumulating the lubricant oil near the droplet base to form the axisymmetric wetting ridge. In this study, we experimentally characterize and model the wetting ridge that plays a crucial role in droplet mobility. We developed a geometry-based analytical model of the steady-state wetting ridge shape that is validated by using experiments and numerical simulations. Our wetting ridge model shows that at steady state (1) the radius of the wetting ridge is approximate to 30% higher than the droplet radius, (2) the wetting ridge rises halfway to the droplet radius, (3) the volume of the wetting ridge is half (approximate to 50%) of the droplet volume, and (4) the wetting ridge shape does not depend on the oil viscosity used for impregnation. The insights gained from this work improve our state-of-the-art mechanistic understanding of the wetting ridge dynamics.

Boiling is a critical heat transfer mechanism in power generation, chemical processing, desalination, and electronics thermal management. Its performance is quantified by two key metrics: critical heat flux (CHF), the maximum heat removal rate before transition to film boiling, and heat transfer coefficient (HTC), the efficiency of heat transport per unit area per unit wall superheat. Traditional strategies to enhance CHF and HTC—such as surface micro/nanostructuring—often require intricate surface design and complex fabrication procedures and are not amenable in practical applications. This study investigates externally applied acoustic fields as a noninvasive, dynamically tunable method to control bubble departure during boiling. Acoustic actuation promotes vapor removal through capillary waves, streaming, and radiation pressure, enhancing bubble detachment and suppressing coalescence. Experiments conducted at three voltage levels reveal that moderate acoustic forcing (1 V) increases bubble departure frequency, reduces bubble size, and enhances microlayer rewetting, leading to a 42% increase in CHF and higher HTC across all superheats. In contrast, excessive forcing (6 V) disrupts microlayer stability, reducing HTC. High-speed imaging and heat transfer measurements corroborate these findings, establishing acoustic forcing as a tunable mechanism for optimizing boiling heat transfer, providing advantages over passive surface treatments in high-power-density applications. Copyright © 2025 by ASME

The rising power density in modern power electronic devices, especially those using wide-bandgap (WBG) semiconductors like gallium nitride (GaN) and silicon carbide (SiC), has led to significant thermal management challenges. Localized heat fluxes can exceed 1 kW/cm2, surpassing the capabilities of traditional cooling methods such as indirect liquid or air cooling. In this study, we evaluate direct immersion cooling using dielectric fluid as a scalable solution for managing extreme heat loads in SiC devices. Unlike conventional methods, immersion cooling provides direct access to the heat source, eliminating the need for heat spreaders and thermal interface layers. We experimentally assess the performance of immersion cooling with 3M FC-3283 fluid, employing a four-wire Kelvin setup to measure surface and fluid temperatures, and validate our results using ANSYS Icepak simulations. The system achieved a heat transfer coefficient (HTC) of up to 820 W/m2·K, a 65× improvement over natural convection air cooling (12.5 W/m2·K). Immersion cooling enabled power dissipation up to 21 W, far exceeding the 1.4 W limit of air cooling, while maintaining safe junction temperatures. Simulations showed agreement with experiments, with device temperatures remaining below the 150 °C threshold even at 70 W. These findings establish immersion cooling as a high-performance, scalable solution for next-generation power electronics and offer insights into optimizing fluid selection, PCB design, and packaging for SiC devices. Copyright © 2025 by ASME

Droplet nucleation, growth, and departure during dropwise condensation is not fully understood to date. A model that fully captures this energy dense and often highly stochastic phase change process and the associated energy release is currently missing. In fact, detailed analytical and numerical heat and mass transfer models use numerous hard-to-prove assumptions that require a closer look as more diagnostic tools become available with advances in science. This work revisits and critically analyzes one of the central assumptions that is routinely used in modeling dropwise condensation. We introduce an innovative outside-the-box characterizing technique that utilizes elastic scattering transmission spectroscopy to directly quantify nucleation site density, a major development that will have significant implications in current dropwise condensation models. By applying anomalous diffraction theory and inverse scattering analysis, our novel technique provides useful insight into the probability of nucleation events within a temperature- and pressure-controlled environment that is free from non-condensable gases. The results reported in this study mark the first application of elastic scattering transmission spectroscopy in fluid mechanics and heat transfer, providing an innovative state-of-the-art analytical tool for analyzing thermo-fluidic transport processes at submicron length scale, particularly those processes that lie beyond the resolution limits of conventional imaging techniques. Copyright © 2025 by ASME

Condensation is an essential phase change process in nature and in industry. In nature it provides life-sustaining water to plants and animals. In industry, it is central to numerous engineering processes including power generation, seawater desalination, distillation, environmental control, and electronics thermal management. Current knowledge base in the heat transfer community shows that, expedited droplet departure (i.e., higher departure frequency) during condensation is beneficial for improving the heat transfer rate in dropwise condensation. However, the proof and physical justification for this widely adopted belief is missing in literature. In this work, we critically re-examine and test the validity of this belief and understanding by conducting well-controlled condensation experiments in a temperature and pressure controlled environmental chamber. For this study, condensation on surfaces with varying contact angles are conducted under nearly identical subcooling conditions. The results are analyzed by comparing trends in heat flux, droplet departure frequency, average droplet departure radius, and the maximum droplet radius before departure in between successive sweeping events to put the aforementioned ambiguity to rest. Our results reveal that while condensate droplet departure frequency increases monotonically with contact angle, it does not follow the non-monotonic heat flux trend reported in dropwise condensation. In contrast, the maximum droplet radius on the surface exhibits strong correlation with the experimentally measured heat flux trend, suggesting that maximum droplet size on the condenser surface serves as a more reliable proxy for dropwise condensation heat transfer rate. To explore the underlying mechanism, condensate droplet growth rate behavior is examined across a range of equilibrium contact angles. Our results show that condensate droplet growth rate also increases with increasing contact angle. These observations point to a fundamental trade-off: as contact angle increases, droplets grow faster but remain on the surface for shorter durations. This competing interplay between growth rate and surface retention (or residence time) points to the existence of an optimal contact angle for dropwise condensation. Our findings not only challenge a long-standing belief regarding the direct correlation between departure frequency and heat transfer rate but offer a fresh perspective and new framework for the design and optimization of condenser surfaces. We anticipate this work to sow the seed for follow-up studies that re-examine and re-evaluate key assumptions that are embedded in the widely used dropwise condensation models. Copyright © 2025 by ASME

Since the advent of integrated circuits several decades ago, effective heat dissipation continues to be the primary factor limiting the performance and reliability of modern high-power analog and digital electronics. Due to its high latent heat of phase change, boiling has been considered a viable thermal management solution for electronics. Traditionally, boiling is limited by the critical heat flux (CHF), which is the maximum heating power per unit area that can be dissipated before boiling crisis leading to catastrophic thermal runaway. Here, we use state-of-the-art additive manufacturing to fabricate a boiling surface (15 mm × 15 mm) with slanted copper micropillars (75 µm × 75 µm) at 45°, 60°, 75°, and 90° tilt angle to increase CHF. In our experiments using ethanol, the CHF of the 75 μm tall three-dimensional (3D) printed pillars was ≈65 W/cm2, ≈57 W/cm2, ≈61 W/cm2, and ≈70 W/cm2 for 45°, 60°, 75°, and 90° slant angles. These CHF values are 20-50% higher than the CHF on flat copper plates (≈45 W/cm2). Importantly, we observed early onset of nucleate boiling (ONB) for the 3D printed copper micropillars at ≈2 K superheat as opposed to ≈14 K for the flat copper plates. The early ONB led to high heat transfer coefficient of ≈453 kW/m2·K, ≈154 kW/m2·K, ≈167 kW/m2·K, and ≈418 kW/m2·K for the slanted micropillars at 45°, 60°, 75°, and 90° tilt angles, respectively. This is orders of magnitude higher that the heat transfer coefficient for flat copper plates (19 kW/m2·K). We anticipate improved boiling performance by optimizing the pillar dimensions, arrangement, and tilt angle. The results reported in this study demonstrate the potential of additively manufactured copper micropillars for thermal management applications in electronics cooling. Copyright © 2025 by ASME