Sushant Anand

Mechanism of frost formation on lubricant-impregnated surfaces.

Frost formation is a major problem affecting a variety of industries including transportation, power generation, construction, and agriculture. Currently used active chemical, thermal, and mechanical techniques of ice removal are time-consuming and costly. The use of nanotextured coatings infused with perfluorinated oil has recently been proposed as a simple passive antifrosting and anti-icing method. However, we demonstrate that the process of freezing subcooled condensate and frost formation on such lubricant-impregnated surfaces is accompanied by the migration of the lubricant from the wetting ridge and from within the textured substrate to the surface of frozen droplets. For practical applications, this mechanism can comprise the self-healing and frost-repelling characteristics of lubricant impregnated-surfaces, regardless of the underlying substrates topography. Thus, further research is necessary to develop liquid-texture pairs that will provide a sustainable frost suppression method.

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.

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.

Fog-harvesting potential of lubricant-impregnated electrospun nanomats.

Hydrophobic PVDF-HFP nanowebs were fabricated by a facile electrospinning method and proposed for harvesting fog from the atmosphere. A strong adhesive force between the surface and a water droplet has been observed, which resists the water being shed from the surface. The water droplets on the inhomogeneous nanomats showed high contact angle hysteresis. The impregnation of nanomats with lubricants (total quartz oil and Krytox 1506) decreased the contact angle hysteresis and hence improved the roll off of water droplets on the nanomat surface. It was found that water droplets of 5 μL size (diameter = 2.1 mm) and larger roll down on an oil-impregnated surface, held vertically, compared to 38 μL (diameter = 4.2 mm) on a plain nanoweb. The contact angle hysteresis decreased from ~95 to ~23° with the Krytox 1506 impregnation.

How droplets nucleate and grow on liquids and liquid impregnated surfaces

Condensation on liquids has been studied extensively in context of breath figure templating, materials synthesis and enhancing heat transfer using liquid impregnated surfaces. However, the mechanics of nucleation and growth on liquids remains unclear, especially on liquids that spread on the condensate. By examining the energy barriers of nucleation, we provide a framework to choose liquids that can lead to enhanced nucleation. We show that due to limits of vapor sorption within a liquid, nucleation is most favoured at the liquid-air interface and demonstrate that on spreading liquids, droplet submergence within the liquid occurs thereafter. We provide a direct visualization of the thin liquid profile that cloaks the condensed droplet on a liquid impregnated surface and elucidate the vapour transport mechanism in the liquid films. Finally, we show that although the viscosity of the liquid does not affect droplet nucleation, it plays a crucial role in droplet growth.

Sub-Micrometer Dropwise Condensation under Superheated and Rarefied Vapor Condition

Phase change accompanying conversion of a saturated or superheated vapor in the presence of subcooled surfaces is one of the most common occurring phenomena in nature. The mode of phase change that follows such a transformation is dependent upon surface properties such as contact angle and thermodynamic conditions of the system. In present studies, an experimental approach is used to study the physics behind droplet growth on a partially wet surface. Superheated vapor at low pressures of 4-5 Torr was condensed on subcooled silicon surface with a static contact angle of 60° in the absence of noncondensable gases, and the condensation process was monitored using environmental scanning electron microscopy (ESEM) with sub-microscopic spatial resolution. The condensation process was analyzed in the form of size growth of isolated droplets before a coalescence event ended the regime of single droplet growth. Droplet growth obtained as a function of time reveals that the rate of growth decreases as the droplet increases in size. This behavior is indicative of an overall droplet growth law existing over larger time scales for which the current observations in their brief time intervals could be fitted. A theoretical model based on kinetic theory further support the experimental observations indicating a mechanism where growth occurs by interfacial mass transport directly on condensing droplet surface. Evidence was also found that establishes the presence of sub-microscopic droplets nucleating and growing between microscopic droplets for the partially wetting case.

Creating nanoscale emulsions using condensation

Nanoscale emulsions are essential components in numerous products, ranging from processed foods to novel drug delivery systems. Existing emulsification methods rely either on the breakup of larger droplets or solvent exchange/inversion. Here we report a simple, scalable method of creating nanoscale water-in-oil emulsions by condensing water vapor onto a subcooled oil-surfactant solution. Our technique enables a bottom-up approach to forming small-scale emulsions. Nanoscale water droplets nucleate at the oil/air interface and spontaneously disperse within the oil, due to the spreading dynamics of oil on water. Oil-soluble surfactants stabilize the resulting emulsions. We find that the oil-surfactant concentration controls the spreading behavior of oil on water, as well as the peak size, polydispersity, and stability of the resulting emulsions. Using condensation, we form emulsions with peak radii around 100 nm and polydispersities around 10%. This emulsion formation technique may open different routes to creating emulsions, colloidal systems, and emulsion-based materials.Emulsions—stabilized mixtures of immiscible liquids—are found in many products, ranging from pharmaceuticals to food. Here Guha et al. propose a simple emulsification method where water vapor is condensed onto oil with surfactant, producing a water-in-oil emulsion with droplets as small as 100 nm.

Synthesizing Pickering Nanoemulsions by Vapor Condensation

Nanoparticle-stabilized (Pickering) emulsions are widely used in applications such as cosmetics, drug delivery, membranes, and material synthesis. However, formulating Pickering nanoemulsions remains a significant challenge. Herein, we show that Pickering nanoemulsions can be obtained in a single step even at very low nanoparticle loadings (0.2 wt %) by condensing water vapor on a nanoparticle-infused subcooled oil that spreads on water. Droplet nuclei spontaneously submerge within the oil after nucleating at the oil-air interface, resulting in the suppression of droplet growth by diffusion, and subsequently coalesce to larger sizes until their growth is curtailed by nanoparticle adsorption. The average nanoemulsion size is governed by the competition between nanoparticle adsorption kinetics and droplet growth dynamics, which are in turn a function of nanoparticle size, concentration, and condensation time. Controlling such factors can lead to the formation of highly monodisperse nanoemulsions. Emulsion formation via condensation is a fast, scalable, energy-efficient process that can be adapted for a wide variety of emulsion-based applications in biology, chemistry, and materials science.

CHAPTER 10: Lubricant-Impregnated Surfaces

Lubricant-impregnated surfaces comprising a porous or textured solid and a liquid lubricant give rise to many novel properties. In this chapter, we review how to achieve a stable lubricant-impregnated surface and discuss its basic features including the wetting ridge and lubricant cloak that are relevant to most implementations. Next, applications of lubricant-impregnated surfaces are detailed including condensation, anti-icing, anti-fouling, fluid mobility, optics, and active surfaces. In each application, the design of a lubricant-impregnated surface has particularly relevant criteria, and both the achievements and current shortcomings of this technology are discussed.