Lubricant infused surfaces (LIS) are a recently developed and promising approach to fluid repellency for applications in biology, microfluidics, thermal management, lab-on-a-chip, and beyond. The design of LIS has been explored in past work in terms of surface energies, which need to be determined empirically for each interface in a given system. Here, we developed an approach that predicts a priori whether an arbitrary combination of solid and lubricant will repel a given impinging fluid. This model was validated with experiments performed in our work as well as in literature and was subsequently used to develop a new framework for LIS with distinct design guidelines. Furthermore, insights gained from the model led to the experimental demonstration of LIS using uncoated high-surface-energy solids, thereby eliminating the need for unreliable low-surface-energy coatings and resulting in LIS repelling the lowest surface tension impinging fluid (butane, γ ≈ 13 mN/m) reported to date.

Vapor condensation is routinely used as an effective means of transferring heat or separating fluids. Dropwise condensation, where discrete droplets form on the condenser surface, offers a potential improvement in heat transfer of up to an order of magnitude compared to filmwise condensation, where a liquid film covers the surface. Low surface tension fluid condensates such as hydrocarbons pose a unique challenge since typical hydrophobic condenser coatings used to promote dropwise condensation of water often do not repel fluids with lower surface tensions. Recent work has shown that lubricant infused surfaces (LIS) can promote droplet formation of hydrocarbons. In this work, we confirm the effectiveness of LIS in promoting dropwise condensation by providing experimental measurements of heat transfer performance during hydrocarbon condensation on a LIS, which enhances heat transfer by ≈450% compared to an uncoated surface. We also explored improvement through removal of noncondensable gases and highlighted a failure mechanism whereby shedding droplets depleted the lubricant over time. Enhanced condensation heat transfer for low surface tension fluids on LIS presents the opportunity for significant energy savings in natural gas processing as well as improvements in thermal management, heating and cooling, and power generation.

Responsive actuating surfaces have attracted significant attention as promising materials for liquid transport in microfluidics, cell manipulation in biological systems, and light tuning in optical applications via their dynamic regulation capability. Significant efforts have focused on fabricating static micro and nanostructured surfaces, [1,2,3,4,5] even with asymmetric features to realize passive functionalities such as directional wettability [6,7] and adhesion. [8,9] Only recent advances in utilizing materials that mechanically respond to thermal, [10,11,12,13] chemical [14,15,16,17] or magnetic [18,19,20,21,22,23] stimuli have enabled dynamic regulation.However the challenges with these surface designs are associated with the tuning range, [19,20] accuracy, [19,20,21,22,23] response time [10,11,12,13,15,16,17] and multi-functionality for advanced systems. Here we report dynamically tunable micropillar arrays with uniform, reversible, continuous and extreme tilt angles with precise control for real-time fluid and optical manipulation. Inspired by hair and motile cilia on animal skin and plant leaves for locomotion, [24] liquid transportation [25] and thermal-optical regulation, [26,27] our flexible uniform responsive microstructures (µFUR) consist of a passive thin elastic skin and active ferromagnetic microhair whose orientation is controlled by a magnetic field. We experimentally show uniform tilt angles ranging from 0° to 57°, and developed a model to accurately capture the tilting behavior. Furthermore, we demonstrate that the µFUR can control and change liquid spreading direction on demand, manipulate fluid drag, and tune optical transmittance over a large range. The versatile surface developed in this work enables 2 new opportunities for real-time fluid control, cell manipulation, drag reduction and optical tuning in a variety of important engineering systems, including applications that require manipulation of both fluid and optical functions.Dynamically tunable structured surfaces offer new manipulation capabilities in mechanical, fluidic, and optical systems. Examples from nature have inspired the design of such active systems: bacteria use flagella as propellers [24] and motile cilia in the lining of human respiratory airways move mucus and dirt out of the lungs. [25] These biological systems display well-defined structural patterns and controllable mechanical motion in response to different stimuli. Accordingly, researchers have investigated various approaches to fabricate tunable microstructures including temperature-sensitive liquid crystalline [13] and thermoplastic elastomers, [10] hydrogels that respond to thermal, chemical or optical stimuli, [11,12,14,15,16,17] and polymer-based magnetically actuated structures [18,19,20,21,22,23] over the past decade. However, the response of the thermally actuated elastomer is either irreversible [10] or slow [13] and the hydrogels require a liquid environment and have a long response time, thus limiting their applications.Magnetically actuated surfaces,...

Thin-film evaporation in wick structures for cooling high-performance electronic devices is attractive because it harnesses the latent heat of vaporization and does not require external pumping. However, optimizing the wick structures to increase the dry-out heat flux is challenging due to the complexities in modeling the liquid-vapor interface and the flow through the wick structures. In this work, we developed a model for thin-film evaporation from micropillar array wick structures and validated the model with experiments. The model numerically simulates liquid velocity, pressure, and meniscus curvature along the wicking direction by conservation of mass, momentum, and energy based on a finite volume approach. Specifically, the three-dimensional meniscus shape, which varies along the wicking direction with the local liquid pressure, is accurately captured by a force balance using the Young-Laplace equation. The dry-out condition is determined when the minimum contact angle on the pillar surface reaches the receding contact angle as the applied heat flux increases. With this model, we predict the dry-out heat flux on various micropillar structure geometries (diameter, pitch, and height) in the length scale range of 1-100 μm and discuss the optimal geometries to maximize the dry-out heat flux. We also performed detailed experiments to validate the model predictions, which show good agreement. This work provides insights into the role of surface structures in thin-film evaporation and offers important design guidelines for enhanced thermal management of high-performance electronic devices.

The generation of concentrated heat loads in advanced microprocessors, GaN electronics, and solar cells present significant thermal management challenges in defense, space and commercial applications. Liquid to vapor phase-change strategies are promising due to the high latent heat of vaporization of the working fluid. In particular, thin-film evaporation has received increased interest owing to advances in micro/nanofabrication and the potential to dissipate high heat fluxes by increasing the evaporative meniscus area. Yet, predictive tools to design various wicking structures are limited due to the complexity of the thermal-fluidic transport. In this work, we performed systematic experiments to characterize capillary-limited thin-film evaporation from silicon micropillar wicks in the absence of nucleate boiling. The insights gained from experiments were used to model the capillary pressure, permeability, and thermal resistance. Accordingly, we developed a semi-analytical model to determine the capillary-limited dryout heat flux and wall temperature with ±20% accuracy, compared to our experiments. The model provides a versatile platform to design and optimize micropillar wicks for next generation thermal management devices.International Journal of Heat and Mass Transfer 56 Nomenclature Area (m 2 ) Correction factor/height ratio (-)