Parsa Faghihi

Efficient condensate removal is critical for sustaining high heat‐transfer rates. While coalescence‐induced droplet jumping on superhydrophobic surfaces has been widely studied as a passive mode of water removal, its efficiency is limited by an intrinsically low energy conversion efficiency (<6%). Single‐droplet jumping, driven by internal Laplace pressure difference, enhances energy conversion efficiency and thus serves as a promising alternative. Here, superhydrophobic SU8 micropillar arrays with controlled geometries (dimensions in the range of 5‒20 ) are developed and used to conduct evaluations of design parameters that enhance number of single‐droplet jumping during condensation. Using environmental scanning electron microscopy, nucleation, growth, and jumping dynamics are investigated across pillars with effective aspect ratios, defined as measured pillar heights divided by edge spacings at tops of pillars; ranging from 1.0 to 3.4 and three geometries: square, triangular, and hexagonal. Our results reveal critical design criteria for enabling single‐droplet jumping during water vapor condensation: (i) sufficient confinement energy release; (ii) minimization of the nucleation‐to‐jump period; and (iii) maximizing number of jumping sites. This work demonstrates a passive method for fast and efficient transport and shedding of droplets having micrometer to millimeter length scales.

What ultimately limits heat removal during boiling, and can the boiling crisis be surpassed? The critical heat flux is traditionally viewed as a terminal threshold beyond which vapor blanketing suppresses heat transfer. However, in strongly subcooled water, a regime known as microbubble emission boiling (MEB) has been observed to sustain heat fluxes exceeding classical limits. Yet its physical origin has remained a subject of intense debate since the 1980s. Here, we show that the hydrodynamic instability underlying the boiling crisis represents a bifurcation point rather than an absolute thermal limit. Under sufficient subcooling, rapid condensation intercepts vapor coalescence, redirecting the postinstability evolution toward high-frequency, collapse-driven convection that continuously renews the surface, a state we identify as condensation-driven oscillatory boiling (COB). Near-wall temperature measurements, velocity fields from intrinsic tracers, and pressure spectra reveal a characteristic oscillatory timescale that quantitatively reproduces the sustained macroscopic heat flux exceeding 1.4 kW cm ⁻² . We derive a dimensionless kinetic criterion, Π < 1, that defines the boundary between film boiling and COB as a competition between condensation and coalescence timescales. Hysteresis tests establish this regime as a reproducibly stable branch of the post-CHF response. This mechanistic reinterpretation resolves the long-standing question of MEB’s origin, framing the boiling crisis as a stable hydrodynamic transition to a high-efficiency transport state and providing a foundation for predicting subcooled boiling at extreme heat fluxes.

Flow condensation is a cornerstone of refrigeration systems, heat pumps, and electronics cooling technologies, and its enhancement is critical for improving efficiency and reducing heat exchanger size. Numerous enhancement strategies have been proposed for flow condensation, including micro- and nanostructured surfaces which have been investigated for their potential to promote condensate film redistribution. In this study, the coupled effects of tube diameter, refrigerant properties, and surface structuring on flow condensation performance are systematically investigated. Experiments are conducted in tubes with internal diameters ranging from 1.5 mm to 4.7 mm using two low-GWP refrigerants, R515B and R1233zd(E), on etched aluminum and etched copper surfaces. A counter-flow tube-in-tube facility is employed, with particular attention to the experimental challenges associated with small diameter flow condensation measurements, including flow oscillations, streamwise saturation temperature gradients, and uncertainty control. The results show that heat transfer enhancement is achieved only within a narrow vapor quality range governed by the interaction between condensate film thickness and surface feature size. Averaged over the highest vapor qualities (0.7-1), enhancements of up to 45% are obtained on etched aluminum tubes with a diameter of 2.4 mm, while etched copper tubes exhibit enhancements up to 55% with a diameter of 3.1 mm. Film thickness and flow regime analyses indicate that surface structures become flooded at high vapor qualities (x > 0.8), suppressing capillary-driven film redistribution and eliminating enhancement occurring at lower vapor qualities. These findings establish critical thresholds for effective condensation enhancement in structured tubes.

Frost and ice formation on a solid surface is governed by nanoscale liquid‒solid intermolecular interactions that manifest as macroscopic surface wettability. This study uses rigorous experiments and modeling approaches to develop a framework for condensation frosting incipience on surfaces having different wettabilities. The model employs a comprehensive statistical framework, considers the effect of surface wettability, and captures the stochastic nature of both nucleation dynamics for condensation and droplet growth during the subsequent freezing process. By introducing a statistical description of the active nucleation sites and droplet size distribution, we provide a reliable model that can predict the overall surface coverage fraction by frozen droplets. Our model shows that superhydrophilic and hydrophilic surfaces have much higher coverage fractions than superhydrophobic surfaces and demonstrates the connection between coverage and frost density and adhesion strength under different wettabilities. The developed framework provides insight into surface design for applications where frost and ice formation are important.