Elucidating the Mechanism of Condensation-Mediated Degradation of Organofunctional Silane Self-Assembled Monolayer Coatings.

Dropwise condensation is favorable for numerous industrial and heat/mass transfer applications due to the enhanced heat transfer performance that results from efficient condensate removal. Organofunctional silane self-assembled monolayer (SAM) coatings are one of the most common ultrathin low surface energy materials used to promote dropwise condensation of water vapors because of their minimal thermal resistance and scalable synthesis process. These SAM coatings typically degrade (i.e., condensation transitions from the efficient dropwise mode to the inefficient filmwise mode) rapidly during water vapor condensation. More importantly, the condensation-mediated coating degradation/failure mechanism(s) remain unknown and/or unproven. In this work, we develop a mechanistic understanding of water vapor condensation-mediated organofunctional silane SAM coating degradation and validate our hypothesis through controlled coating synthesis procedures on silicon/silicon dioxide substrates. We further demonstrate that a pristine organofunctional silane SAM coating resulting from a water/moisture-free coating environment exhibits superior long-term robustness during water vapor condensation. Our molecular/nanoscale surface characterizations, pre- and post-condensation heat transfer testing, indicate that the presence of moisture in the coating environment leads to uncoated regions of the substrate that act as nucleation sites for coating degradation. By elucidating the reasons for formation of these degradation nuclei and demonstrating a method to suppress such defects, this study provides new insight into why low surface energy silane SAM coatings degrade during water vapor condensation. The proposed approach addresses a key bottleneck (i.e., coating failure) preventing the adoption of efficient dropwise condensation methods in industry, and it will facilitate enhanced phase-change heat transfer technologies in industrial applications.

Unified Modeling Framework for Thin-Film Evaporation from Micropillar Arrays Capturing Local Interfacial Effects.

Thin-film evaporation from micropillar array porous media has gained attention in a number of fields including energy conversion and thermal management of electronics. Performance in these applications is enhanced by leveraging the geometries of the micropillar arrays to both optimize flow through these arrays via capillary pumping and increase the curved liquid-vapor interface (meniscus) area for active phase-change heat transfer. In this work, we present a unified semianalytical modeling framework to predict the dry-out heat flux accurately for thin-film evaporation from micropillar arrays with the precise prediction of (i) the pressure profile along the wick achieved by discretizing the porous media domain and (ii) the local permeability that depends on the local meniscus shape. We validate the permeability model with 3D numerical simulations and verify the accuracy of the thin-film evaporation modeling framework with available experimental data from the literature. We emphasize the importance of predicting an accurate liquid-vapor interface shape for the prediction accuracy of both the permeability and the associated governing equations for liquid propagation and phase-change heat transfer through porous materials. This modeling framework is an accurate non-CFD-based methodology for predicting the dry-out heat flux during thin-film evaporation from micropillar arrays and will serve as a general framework for modeling steady liquid-vapor phase-change processes (evaporation and condensation) in porous media.

Spline Based Shape Prediction and Analysis of Uniformly Rotating Sessile and Pendant Droplets.

Prediction and analysis of the shapes of liquid-vapor interface of droplets under the influence of external forces is critical for various applications. In this regard, a geometric model that can capture the macroscopic shape of the liquid-vapor interface in tandem with the subtleties near the contact line, particularly in the regime where the droplet shape deviates significantly from the idealized spherical cap geometry, is desirable. Such deviations may occur when external forces such as gravity or centrifugal dominate over the surface tension force. Here we use vector parametrized cubic spline representation for axisymmetric fluid-fluid interfaces along with a novel thermodynamic free energy minimization based heuristic to determine the shape of liquid-vapor interface of droplets. We show that the current scheme can easily predict the shapes of sessile and pendant droplets under the action of centrifugal force over a broad range of surface contact angle values and droplet sizes encountered in practical applications. Finally, we show that the cubic spline based modeling approach makes it convenient to perform the inverse analysis as well, i.e., predict interfacial properties from the shape of a droplet under the action of various types of external forces including gravity and centrifugal. We believe that this versatile modeling approach can be extended to model droplet shapes under various other external forces including electric and acoustic. In addition, the simple shape analysis approach is also promising for the development of inexpensive interfacial analysis tools such as surface tensiometers.