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.