Adam T. Paxson

Hydrophobicity of rare-earth oxide ceramics

Hydrophobic materials that are robust to harsh environments are needed in a broad range of applications. Although durable materials such as metals and ceramics, which are generally hydrophilic, can be rendered hydrophobic by polymeric modifiers, these deteriorate in harsh environments. Here we show that a class of ceramics comprising the entire lanthanide oxide series, ranging from ceria to lutecia, is intrinsically hydrophobic. We attribute their hydrophobicity to their unique electronic structure, which inhibits hydrogen bonding with interfacial water molecules. We also show with surface-energy measurements that polar interactions are minimized at these surfaces and with Fourier transform infrared/grazing-angle attenuated total reflection that interfacial water molecules are oriented in the hydrophobic hydration structure. Moreover, we demonstrate that these ceramic materials promote dropwise condensation, repel impinging water droplets, and sustain hydrophobicity even after exposure to harsh environments. Rare-earth oxide ceramics should find widespread applicability as robust hydrophobic surfaces.

Enhanced condensation on lubricant-impregnated nanotextured surfaces.

Nanotextured superhydrophobic surfaces have received significant attention due to their ability to easily shed liquid drops. However, water droplets have been shown to condense within the textures of superhydrophobic surfaces, impale the vapor pockets, and strongly pin to the surface. This results in poor droplet mobility and degrades condensation performance. In this paper, we show that pinning of condensate droplets can be drastically reduced by designing a hierarchical micro-nanoscale texture on a surface and impregnating it with an appropriate lubricant. The choice of lubricant must take into account the surface energies of all phases present. A lubricant will cloak the condensate and inhibit growth if the spreading coefficient is positive. If the lubricant does not fully wet the solid, we show how condensate-solid pinning can be reduced by proper implementation of nanotexture. On such a surface, condensate droplets as small as 100 μm become highly mobile and move continuously at speeds that are several orders of magnitude higher than those on identically textured superhydrophobic surfaces. This remarkable mobility produces a continuous sweeping effect that clears the surface for fresh nucleation and results in enhanced condensation.

Multimode multidrop serial coalescence effects during condensation on hierarchical superhydrophobic surfaces.

The prospect of enhancing the condensation rate by decreasing the maximum drop departure diameter significantly below the capillary length through spontaneous drop motion has generated significant interest in condensation on superhydrophobic surfaces (SHS). The mobile coalescence leading to spontaneous drop motion was initially reported to occur only on hierarchical SHS, consisting of both nanoscale and microscale topological features. However, subsequent studies have shown that mobile coalescence also occurs on solely nanostructured SHS. Thus, recent focus has been on understanding the condensation process on nanostructured surfaces rather than on hierarchical SHS. In this work, we investigate the impact of microscale topography of hierarchical SHS on the droplet coalescence dynamics and wetting states during the condensation process. We show that isolated mobile and immobile coalescence between two drops, almost exclusively focused on in previous studies, are rare. We identify several new droplet shedding modes, which are aided by tangential propulsion of mobile drops. These droplet shedding modes comprise of multiple droplets merging during serial coalescence events, which culminate in formation of a drop that either departs or remains anchored to the surface. We directly relate postmerging drop adhesion to formation of drops in nanoscale as well as microscale Wenzel and Cassie-Baxter wetting states. We identify the optimal microscale feature spacing of the hierarchical SHS, which promotes departure of the highest number of microdroplets. This optimal surface architecture consists of microscale features spaced close enough to enable transition of larger droplets into micro-Cassie state yet, at the same time, provides sufficient spacing in-between the features for occurrence of mobile coalescence.

Stable Dropwise Condensation for Enhancing Heat Transfer via the Initiated Chemical Vapor Deposition (iCVD) of Grafted Polymer Films

Ultra-thin copolymer films are deposited by initiated chemical deposition (iCVD) to investigate their performance under the condensation of water vapor. By forming a grafted interface between the coating and the substrate, the films exhibit stable dropwise condensation even when subjected to 100 °C steam. The applicability of the iCVD to complex substrate geometries is demonstrated on a copper condenser coil.

Dropwise Condensation of Low Surface Tension Fluids on Omniphobic Surfaces

Compared to the significant body of work devoted to surface engineering for promoting dropwise condensation heat transfer of steam, much less attention has been dedicated to fluids with lower interfacial tension. A vast array of low-surface tension fluids such as hydrocarbons, cryogens, and fluorinated refrigerants are used in a number of industrial applications, and the development of passive means for increasing their condensation heat transfer coefficients has potential for significant efficiency enhancements. Here we investigate condensation behavior of a variety of liquids with surface tensions in the range of 12 to 28 mN/m on three types of omniphobic surfaces: smooth oleophobic, re-entrant superomniphobic, and lubricant-impregnated surfaces. We demonstrate that although smooth oleophobic and lubricant-impregnated surfaces can promote dropwise condensation of the majority of these fluids, re-entrant omniphobic surfaces became flooded and reverted to filmwise condensation. We also demonstrate that on the lubricant-impregnated surfaces, the choice of lubricant and underlying surface texture play a crucial role in stabilizing the lubricant and reducing pinning of the condensate. With properly engineered surfaces to promote dropwise condensation of low-surface tension fluids, we demonstrate a four to eight-fold improvement in the heat transfer coefficient.

Superhydrophobic polymer surface via solvent-induced crystallization

We report on a rapid, single-step method to produce large-area superhydrophobic surfaces via acetone-induced phase transformation of polycarbonate. Crystallization of the polymer leads to the formation of a hierarchical structure composed of microporous spherulites covered with nano-fibrils, and resulted in superhydrophobic wetting behavior. A systematic study of the dependence of surface morphology on the acetone treatment time was conducted to optimize the treatment time and to elucidate the structure formation mechanism. The resulting surfaces exhibit high contact angles, low contact angle hysteresis, and complete dewetting during droplet impact. Theoretical analysis of the wetting and anti-wetting pressures shows that the nano-scale morphology is critical for achieving droplet impact resistance. This simple phase transformation approach could be more broadly applied to other solvent-polymer systems for fabricating large-area hierarchical surface textures.

Droplet Impingement and Wetting Hysteresis on Textured Hydrophobic Surfaces

We study the wetting energetics and wetting hysteresis of sessile and impacting water droplets on superhydrophobic surfaces as a function of surface texture and surface energy. Detailed experiments tracking contact line motion simultaneously with contact angle provides new insights on the wetting hysteresis, stick-slip behavior and dependence on contact line velocity. For sessile drops, we find three wetting regimes on these surfaces: equilibrium Cassie at small feature spacing, equilibrium Wenzel at large feature spacing, and an intermediate state at medium feature spacing. We observe minimum wetting hysteresis not on surfaces that exhibit Cassie wetting but rather on surfaces in the intermediate regime. We argue that droplets on these surfaces are metastable Cassie droplets whose internal Laplace pressure is insufficient to overcome the energy barrier required to homogeneously wet the surface. These metastable Cassie droplets show superior roll-off properties because the effective length of the contact line that is pinned to the surface is reduced. We develop a model that can predict the transition between the metastable Cassie and Wenzel regimes by comparing the Laplace pressure of the drop to the capillary pressure associated with the wetting-energy barrier of the textured surface. In the case of impacting droplets the water hammer and Bernoulli pressures must be compared with the capillary pressure. Experiments with impacting droplets show very good agreement with this simple pressure-balance model.Copyright