Solomon Adera

High heat flux removal in advanced thermal management systems often relies on pool boiling, where surface microstructures strongly influence bubble dynamics and heat transfer. The geometry of these microstructures determines the critical heat flux (CHF) and the heat transfer coefficient (HTC). In this study, we experimentally investigate pool boiling of de-ionized water on silicon surfaces with re-entrant and vertical microcavities, re-entrant and vertical micropillars, and flat silicon surfaces. The CHF of re-entrant cavities is ≈ 142 W/cm 2 , representing ≈ 97% enhancement over vertical cavities (≈72 W/cm 2 ) and ≈ 43% over flat polished surfaces (≈99 W/cm 2 ). This improvement in CHF is attributed to the overhang geometry or hoodoo, which lowers the temperature locally at the cavity rim to sustain liquid replenishment, and accelerates bubble departure through the necked cross-section. In contrast, re-entrant pillars did not improve CHF, reaching ≈ 133 W/cm 2 versus ≈ 140 W/cm 2 for vertical pillars, as vapor trapping in re-entrant pillars promotes bubble coalescence and early transition to film boiling. Comparison of HTC values indicate that re-entrant features offer marginal benefits for cavities. However, HTC increased for micropillars because of enhanced bubble ebullition cycle. Furthermore, increasing the cap thickness from 0.5 µm to 2 µm enhanced the CHF of re-entrant microcavity surfaces from ≈ 136 W/cm 2 to ≈ 156 W/cm 2 . In contrast, the CHF of re-entrant micropillar surfaces remained unaffected when the cap thickness increased from 0.5 µm to 2 µm. Furthermore, our heat transfer measurements show that for both re-entrant cavity and re-entrant pillars, the wall superheat at the onset of nucleate boiling (ONB) decreased by ≈ 3-4 K, a result that points to the overall impact of re-entrant geometry on boiling. By elucidating the heat transfer mechanism, the new findings reported in this study provide useful insights that can guide the design of engineered surfaces for cooling next-generation power-dense electronics.

Boiling has been extensively studied over the past several decades due to its critical role in thermal-fluidic systems for power generation and electronics thermal management. By exploiting the large latent heat, boiling offers exceptional heat dissipation rate and remarkable wall temperature uniformity for power-dense applications. Typically, boiling performance is characterized by two key parameters. The first one is the critical heat flux (CHF), which is the maximum heating power per unit area (aka heat flux) that can be removed from the heat transfer surface before burnout. The second parameter is the heat transfer coefficient (HTC), which is the measure of cooling efficiency often described by the heat flux per unit superheat (excess wall temperature above saturation). Conventional boiling enhancement strategies rely primarily on surface modifications that employ micro/nanostructuring and/or chemical functionalization, which are neither durable nor well suited to handling transient heating loads. In this work, we introduce robust ultrasonic acoustic waves as an externally imposed and dynamically tunable approach to actively manipulate bubble nucleation, growth, and departure during nucleate boiling for heat transfer gain. Experiments were conducted on smooth copper plates (heated area = 15 mm × 15 mm) using degassed pure ethanol (200 proof, purity > 99.9%). By systematically varying the acoustic pressure amplitude ( P inc = 0–460 kPa) and wave incidence angle ( α = 0°–90°), we identified an optimal forcing condition. The heat dissipation rate is maximum when P inc = 46 kPa at α = 45°, consistent with our COMSOL simulation. Under optimal forcing, the CHF increased from ≈ 45 W/cm 2 to ≈ 72 W/cm 2 . We attribute the ≈ 60% enhancement in CHF to expedited bubble departure as observed by ≈ 36% reduction in departure diameter and ≈ 100% increase in departure frequency. A physics-based framework that is grounded on the dimensionless Bond number, which compares buoyancy and capillary forces, and the acoustic-capillary pressure ratio, which quantifies acoustic forcing relative to interfacial stability, shows that moderate forcing enhances bubble growth and departure while excessive forcing initiates interfacial destabilization that leads to bubble coalescence. The experiments further reveal a non-monotonic effect of forcing amplitude on boiling wherein weak forcing provides limited benefits whereas excessive forcing impacts CHF unfavorably. Collectively, these results establish boiling enhancement through acoustically assisted bubble departure as a robust and tunable strategy for on-demand transient thermal management of power-dense electronic devices.

Controlling the shape and contact area that an impacting droplet makes with a solid substrate has significant implications in numerous industrial processes, including inkjet printing and spray cooling. Here, we report a unique approach that offers an extraordinary ability to precisely control and manipulate the contact shape of a droplet impinging on nonwetting well-structured silicon micropillars. Our experiments show that the wetted Wenzel-type contact area can take on various polygonal shapes, including square, rectangle, hexagon, octagon, and dodecagon, depending on the pillar density (diameter-to-spacing ratio), arrangement (inline versus staggered), and/or the droplet contact angle. Experiments show that inline pillars give rise to a square, rectangle, or octagon shape while staggered pillars give rise to a hexagon, dodecagon, or extended hexagon shape. Rooted in the fundamentals of contact line physics, we develope a closed form unified analytical model that accurately captures the steady-state and transient wetting morphology of the impinging droplet. Furthermore, we show that the model is applicable for analyzing entrapped bubble retraction mechanism during high-velocity droplet impact. Lastly, the outcomes of this study demonstrate the similarity of the shape of the wetted area induced by droplet impact on nonwetting surfaces with that obtained via sessile droplet evaporation on wetting surfaces. The shape selection strategy reported in this study has promising applications in facile microfabrication of lab-on-a-chip devices, polymer-based printed electronics, biomicroarrays, and droplet-based electronics thermal management.

Abstract 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.

• First experimental validation of optimal contact angle for dropwise condensation, confirming semi-analytical model predictions with experimental data. • Systematic investigation across contact angles from 84° to 115° using self-assembled thiol monolayers on gold-coated smooth surfaces. • Identified an optimal contact angle range (96°–105°) for enhancing condensation heat transfer rate, showing that intermediate, not highly hydrophobic, surfaces yield superior heat transfer rate in dropwise condenation. 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° to 115° in a pressure- and 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° and 105° . 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.

Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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 non-invasive, 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.

Abstract 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.

Abstract As an essential heat and mass transfer process, boiling is widely utilized in advanced cooling strategies, including immersion cooling for electronics. Surface microstructures can enhance boiling heat transfer by affecting bubble nucleation, growth, and departure from the heated surface. Past studies have shown that re-entrant surface structures can improve the boiling heat transfer coefficient. In this study, we investigate the effect of silicon dioxide capping layer thickness on the pool boiling performance of silicon surfaces with well-defined re-entrant microcavity structures. Re-entrant microcavities with oxide cap thicknesses of 2 μm, 1 μm, and 0.5 μm were fabricated on silicon surfaces using photolithography and plasma etching. Silicon surfaces with vertical microcavities and flat silicon surfaces were also tested for comparison. Pool boiling experiments were conducted in saturated de-ionized water using platinum thin-film heaters on the backside of silicon samples. The results show that the reentrant microcavities triggered nucleate boiling at a lower superheat than the vertical microcavities. The re-entrant surfaces with a 2 μm thick cap achieved the highest critical heat flux (CHF) of ≈ 156 W/cm2, corresponding to a ≈ 117 % increase over vertical cavities and a 58 % increase over flat surfaces. The re-entrant surfaces with 1 μm and 0.5 μm caps reached a CHF of ≈ 142 W/cm2 and ≈ 136 W/cm2, respectively. Increasing the capping layer thickness from 0.5 μm to 2 μm resulted in a ≈ 15% increase in CHF. These results demonstrate that the re-entrant microgeometry strongly enhances boiling heat transfer. Furthermore, increasing the capping layer thickness improves the CHF for re-entrant microcavity surfaces. This study provides insights into microscale surface modification for enhancing boiling in electronics thermal management.

Understanding drop friction on textured surfaces has implications in microfluidics and lab-on-a-chip devices. In this work, we investigated the drop friction on lubricant-coated pillars by systematically varying pillar height and density. First, we measured the friction force on a moving drop using a cantilever force sensor that has ±0.1 μN sensitivity. This measurement shows that drop friction on tall dense pillars is comparable to drop friction on short pillars, a significant result that suggests the presence of a Landau-Levich-Derjaguin (LLD) film underneath the moving drop. Second, we validated the force measurement by estimating the lubricant layer thickness by using white-light interferometry. Third, we visualized the lubricant film underneath the moving drop using reflection interference contrast microscopy. The three independent diagnostic tools and measurement techniques complement each other and reaffirm that drops oleoplane on tall dense pillars, while they graze over the pillar tops in tall sparse pillars. The critical density that forces this transition to drop friction is ≈50%. Furthermore, the experimental results show that friction on microholes and micropillars is comparable when the solid fraction is the same. The results reported in this study contradict past studies that reported the absence of an oil layer on tall pillars. Besides improving current understanding, the insights gained from this work provide design guidelines for turning drop friction on-off on demand for microfluidics applications.

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.

Cytoplasmic abundant heat-soluble (CAHS) proteins, a stress-responsive intrinsically disordered protein from tardigrades, have been discovered to form gel-like networks providing structural support during dehydration, thus enabling anhydrobiosis. However, the mechanism by which CAHS proteins protect the dehydrating cellular membrane remains enigmatic. Using giant unilamellar vesicles (GUVs) as a model membrane system, here we show that encapsulated CAHS12 undergoes a reversible structural transformation that reinforces membrane integrity and preserves encapsulated components, mimicking natural anhydrobiosis. CAHS12-containing GUVs demonstrated stability for weeks and mechanical robustness under dehydration, elevated temperature, and osmotic stresses. Molecular simulations suggest that CAHS12 forms a filamentous network within the vesicle lumen that mitigates membrane collapse and preserves compartmental architecture. Synthetic cells with cell-free transcription-translation capabilities withstand desiccation and recover biochemical activities, akin to the tun state of the tardigrade. This discovery opens up synthetic cell applications in bioengineering, cold-chain-independent biomanufacturing, and adaptive biointerfaces.

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.

Boiling is recognized as one of the promising thermal management strategies for various industries including electronics cooling. This study introduces a novel approach to enhance boiling heat transfer rate by using engineered nucleation sites on silicon micropillar wicking structures. We investigated the impact of hierarchically stacking nucleation sites on well-defined silicon micropillars. Heat transfer measurements were conducted using ethanol as the working fluid. Preliminary results show that stacking nucleation sites vertically on silicon micropillars significantly reduces surface superheat during nucleate boiling. Our data show a near-zero superheat at the onset of nucleate boiling (ONB), a significantly smaller kickback temperature compared to conventional pillar array structures. This low surface superheat persists across a wide range of heat fluxes, significantly improving the heat transfer coefficient (HTC) of the boiling heat transfer process. Compared to micropillar array structures, our vertically stacked nucleation sites on silicon pillars enhanced the HTC by over 400%. This study represents the first investigation of synergistically engineered microcavities with micropillars for boiling heat transfer augmentation. The results reported in this study are beyond promising and forge new avenues for advanced thermal management solutions for electronics cooling.

Dropwise condensation is renowned for its superior heat transfer efficiency compared to the more common filmwise regime. Despite extensive research over the years, a comprehensive understanding of this phenomenon remains elusive, limiting its industrial application. In this study, we experimentally investigate analytical predictions suggesting the existence of an optimal contact angle and challenge the common assumption that increasing hydrophobicity will invariably enhance heat transfer performance. By creating self-assembled monolayers of thiols on gold-coated surfaces, we systematically examined contact angles ranging from 84° to 115°. Experiments conducted in a temperature and pressure controlled climatic chamber provided strong evidence for an optimal contact angle between 96° and 105°. These findings not only support analytical models but also represent a paradigm shift in the design and optimization of condenser surface coatings. The insights gained from this research hold significant implications for industries such as power generation, chemical processing, liquid separation, desalination, and thermal management, while also expanding our understanding of this intriguing process.

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 ≈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 (≈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.

This work investigates drop impact dynamics on state-of-the-art lubricant-infused micro/nanotextured surfaces. The results of this study show the presence of an optimal lubricant layer thickness (\ensuremath{\approx}3-5 𝜇m) that maximizes drop breakup and splashing. Moreover, our experiments show that drop splashing can be suppressed by increasing lubricant viscosity. Lastly, the density mismatch between the drop and the lubricant oil has also been shown to amplify the breakup of the radially expanding liquid rim into tiny droplets. The insights gained from this work provide new avenues to suppress and/or amplify drop breakup during high-velocity impact.

Abstract Boiling is an essential heat and mass transfer process that is employed in numerous applications ranging from power generation and seawater desalination to thermal management of electronic components. Nucleate boiling, where vapor bubbles nucleate, grow, and depart from active nucleation sites, provides high heat transfer coefficient due to the large latent heat of phase change and the enhanced mixing of liquid near the heated surface caused by continuous bubble departure. Strategies for enhancing the nucleate boiling heat transfer include increasing active nucleation site density with micro/nano-sized roughness, improving vapor/gas entrainment in cavities, and increasing the apparent contact angle of the working fluid. Millimeter- or submillimeter-scale re-entrant grooves fabricated on metallic surfaces have been shown to enhance the heat transfer coefficient during nucleate boiling. Here, we experimentally measure the heat transfer coefficient of surfaces with microscale re-entrant cavities during pool boiling of de-ionized water. Boiling experiments were conducted on silicon surfaces with different surface structures, including vertical/cylindrical and re-entrant microcavities. The results reveal that, compared to the vertical microcavity surfaces, the reentrant microcavity surface reaches a maximum of 50% higher heat transfer coefficient under the same heat flux. Importantly, we observed a sudden temperature drop and heat flux rise at the onset of nucleate boiling for re-entrant microcavity surfaces. The heat transfer enhancement is attributed to the stronger vapor entrapment ability (that is, resistance to wetting by the working fluid) of the re-entrant microcavities. The results of this study provide design guidelines for re-entrant cavities for enhancing boiling heat transfer rate.

Two-dimensional (2D) superlattices, formed by stacking sublattices of 2D materials, have emerged as a powerful platform for tailoring and enhancing material properties beyond their intrinsic characteristics. However, conventional synthesis methods are limited to pristine 2D material sublattices, posing a significant practical challenge when it comes to stacking chemically modified sublattices. Here we report a chemical synthesis method that overcomes this challenge by creating a unique 2D graphene superlattice, stacking graphene sublattices with monodisperse, nanometer-sized, square-shaped pores and strategically doped elements at the pore edges. The resulting graphene superlattice exhibits remarkable correlations between quantum phases at both the electron and phonon levels, leading to diverse functionalities, such as electromagnetic shielding, energy harvesting, optoelectronics, and thermoelectrics. Overall, our findings not only provide chemical design principles for synthesizing and understanding functional 2D superlattices but also expand their enhanced functionality and extensive application potential compared to their pristine counterparts.

Recently, there is much interest in droplet condensation on soft or liquid or liquidlike substrates. Droplets can deform soft and liquid interfaces resulting in a wealth of phenomena not observed on hard, solid surfaces (e.g., increased nucleation, interdroplet attraction). Here, we describe a unique collective motion of condensate water droplets that emerges spontaneously when a solid substrate is covered with a thin oil film. Droplets move first in a serpentine, self-avoiding fashion before transitioning to circular motions. We show that this self-propulsion (with speeds in the 0.1-1 mm s^{-1} range) is fueled by the interfacial energy release upon merging with newly condensed but much smaller droplets. The resultant collective motion spans multiple length scales from submillimeter to several centimeters, with potentially important heat-transfer and water-harvesting applications.

Droplets on nanotextured oil-impregnated surfaces have high mobility due to record-low contact angle hysteresis (∼1-3°), attributed to the absence of solid-liquid contact. Past studies have utilized the ultralow droplet adhesion on these surfaces to improve condensation, reduce hydrodynamic drag, and inhibit biofouling. Despite their promising utility, oil-impregnated surfaces are not fully embraced by industry because of the concern for lubricant depletion, the source of which has not been adequately studied. Here, we use planar laser-induced fluorescence (PLIF) to not only visualize the oil layer encapsulating the droplet (aka wrapping layer) but also measure its thickness since the wrapping layer contributes to lubricant depletion. Our PLIF visualization and experiments show that (a) due to the imbalance of interfacial forces at the three-phase contact line, silicone oil forms a wrapping layer on the outer surface of water droplets, (b) the thickness of the wrapping layer is nonuniform both in space and time, and (c) the time-average thickness of the wrapping layer is ∼50 ± 10 nm, a result that compares favorably with our scaling analysis (∼50 nm), which balances the curvature-induced capillary force with the intermolecular van der Waals forces. Our experiments show that, unlike silicone oil, mineral oil does not form a wrapping layer, an observation that can be exploited to mitigate oil depletion of nanotextured oil-impregnated surfaces. Besides advancing our mechanistic understanding of the wrapping oil layer dynamics, the insights gained from this work can be used to quantify the lubricant depletion rate by pendant droplets in dropwise condensation and water harvesting.

Abstract Recently, there is much interest in droplet condensation on soft or liquid/liquid-like substrates. Droplets can deform soft and liquid interfaces resulting in a wealth of phenomena not observed on hard, solid surfaces (e.g., increased nucleation, inter-droplet attraction). Here, we describe a unique complex collective motion of condensate water droplets that emerges spontaneously when a solid substrate is covered with a thin oil film. Droplets move first in a serpentine, self-avoiding fashion before transitioning to circular motions. We show that this self-propulsion (with speeds in the 0.1–1 mm s−1 range) is fuelled by the interfacial energy release upon merging with newly condensed but much smaller droplets. The resultant collective motion spans multiple length scales from submillimetre to several centimetres, with potentially important heat-transfer and water-harvesting applications.

Recently, there is much interest in droplet condensation on soft or liquid/liquid-like substrates. Droplets can deform soft and liquid interfaces resulting in a wealth of phenomena not observed on hard, solid surfaces (e.g., increased nucleation, inter-droplet attraction). Here, we describe a unique complex collective motion of condensate water droplets that emerges spontaneously when a solid substrate is covered with a thin oil film. Droplets move first in a serpentine, self-avoiding fashion before transitioning to circular motions. We show that this self-propulsion (with speeds in the 0.1-1 mm/s range) is fuelled by the interfacial energy release upon merging with newly condensed but much smaller droplets. The resultant collective motion spans multiple length scales from submillimetre to several centimetres, with potentially important heat-transfer and water-harvesting applications.

Abstract Boiling is an important heat and mass transfer process that has tremendous benefits in thermal management applications where timely and efficient heat removal is critical for operation. Boiling is inherently limited by the critical heat flux (CHF), which is the maximum heat flux that can be dissipated from the heated surface via liquid-to-vapor phase change. Heat fluxes that exceed CHF cause surface dry-out that initiates thermal runaway and eventual device failure or burnout. Past studies have used micro/nanoengineered surfaces to improve wicking and liquid replenishment mechanism to delay CHF and/or increase the CHF limit. In this work, we investigate the CHF limit of micro-nanotextured oil-impregnated surfaces in pool boiling. Pool boiling experiments in water were conducted for copper plates with different surface morphologies, including Krytox oil impregnation. The bubble nucleation, growth, and departure behaviors were captured using a high-speed camera at 1000 frames per second during nucleate boiling. Due to the presence of an annular wetting ridge, which forms near the base of the growing bubble, the bubble growth and coalescence mechanism on oil-impregnated surfaces was distinct from that on un-impregnated micro-nanostructured hydrophobized (i.e., superhydrophobic) surfaces. Our experimental results show that (a) the bubble departure diameter on the oil-impregnated surfaces was 1.6-times larger than that on the superhydrophobic surfaces and (b) the CHF on oil-impregnated surfaces was similar to that on the un-impregnated superhydrophobic surfaces (≈20–30 W/cm2), a result that we attribute to oil depletion. Results show that oil impregnation does not improve the CHF limit in boiling.

Droplets residing on textured oil-impregnated surfaces form a wetting ridge due to the imbalance of interfacial forces at the contact line, leading to a wealth of phenomena not seen on traditional lotus-leaf-inspired non-wetting surfaces. Here, we show that the wetting ridge leads to long-range attraction between millimeter-sized droplets, which coalesce in three distinct stages: droplet attraction, lubricant draining, and droplet merging. Our experiments and model show that the magnitude of the velocity and acceleration at which droplets approach each other horizontally is the same as the vertical oil rise velocity and acceleration in the wetting ridge. Moreover, the droplet coalescence mechanism can be modeled using the classical mass-spring system. The insights gained from this work will inform future fundamental studies on remote droplet interaction on textured oil-impregnated surfaces for optimizing water harvesting and condensation heat transfer.

Dynamic hemiwicking behavior is observable in both nature and a wide range of industrial applications ranging from biomedical devices to thermal management. We present a semi-analytical modeling framework (without empirical fitting coefficients) to predict transient capillary-driven hemiwicking behavior of a liquid through a nano/microstructured surface, specifically a micropillar array. In our model framework, the liquid domain is discretized into micropillar unit cells to enable the time marching of the hemiwicking front. A simplified linear pressure drop is assumed along the hemiwicking length such that the local meniscus curvature, contact angle, and effective liquid height are determined at each time step in our transient model. This semi-analytical model is validated with experimental data from our own experiments and from published literature for different fluids. Our model predicts hemiwicking dynamics with <20% error over a broad range of micropillar geometries with height-to-pitch ratio ranging between ≈0.34 and 6.7 and diameter-to-pitch ratio in the range of ≈0.25–0.7 and without any fitting parameters. For lower diameter-to-pitch ratio data points related to sparse micropillar array arrangements, we suggest modifications to the semi-analytical model. This work sheds light on complex and dynamic solid–liquid–vapor interfacial interactions which could serve as a guide for the design of textured surfaces for wicking enhancement in multi-phase thermal and mass transport technologies and applications.

Liquid-liquid heat exchangers that operate in marine environments are susceptible to biofouling, which decreases the overall heat exchange between hot and cold liquids by increasing the conduction resistance. Recently, micro/nanostructured oil-impregnated surfaces have been shown to significantly reduce biofouling. However, their potential as a heat exchanger material has not been studied. Neither is it obvious since the oil used for impregnation increases the wall thickness and the associated conduction resistance. Here, by conducting extensive field and laboratory studies supported by theoretical modeling of heat transfer in oil-infused heat exchanger tubes, we report the synergistic benefits of micro/nanostructured oil-impregnated surfaces for reducing biofouling while maintaining good heat transfer. These benefits justify the use of lubricant-infused surfaces as heat exchanger materials, in particular in marine environments.

Harvesting largely ignored and wasted electromagnetic (EM) energy released by electronic devices and converting it into direct current (DC) electricity is an attractive strategy not only to reduce EM pollution but also address the ever-increasing energy crisis. Here we report the synthesis of nanoparticle-templated graphene with monodisperse and staggered circular nanopores enabling an EM-heat-DC conversion pathway. We experimentally and theoretically demonstrate that this staggered nanoporous structure alters graphene's electronic and phononic properties by synergistically manipulating its intralayer nanostructures and interlayer interactions. The staggered circular nanoporous graphene exhibits an anomalous combination of properties, which lead to an efficient absorption and conversion of EM waves into heat and in turn an output of DC electricity through the thermoelectric effect. Overall, our results advance the fundamental understanding of the structure-property relationships of ordered nanoporous graphene, providing an effective strategy to reduce EM pollution and generate electric energy.