Xiaoda Sun

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.

Inhibition of Condensation Frosting by Arrays of Hygroscopic Antifreeze Drops

The formation of frost and ice can have negative impacts on travel and a variety of industrial processes and is typically addressed by dispensing antifreeze substances such as salts and glycols. Despite the popularity of this anti-icing approach, some of the intricate underlying physical mechanisms are just being unraveled. For example, recent studies have shown that in addition to suppressing ice formation within its own volume, an individual salt saturated water microdroplet forms a region of inhibited condensation and condensation frosting (RIC) in its surrounding area. This occurs because salt saturated water, like most antifreeze substances, is hygroscopic and has water vapor pressure at its surface lower than water saturation pressure at the substrate. Here, we demonstrate that for macroscopic drops of propylene glycol and salt saturated water, the absolute RIC size can remain essentially unchanged for several hours. Utilizing this observation, we demonstrate that frost formation can be completely inhibited in-between microscopic and macroscopic arrays of propylene glycol and salt saturated water drops with spacing (S) smaller than twice the radius of the RIC (δ). Furthermore, by characterizing condensation frosting dynamics around various hygroscopic drop arrays, we demonstrate that they can delay complete frosting over of the samples 1.6 to 10 times longer than films of the liquids with equivalent volume. The significant delay in onset of ice nucleation achieved by dispensing propylene glycol in drops rather than in films is likely due to uniform dilution of the drops driven by thermocapillary flow. This transport mode is absent in the films, leading to faster dilution, and with that facilitated homogeneous nucleation, near the liquid-air interface.

Insensitive to touch: fabric-supported lubricant-swollen polymeric films for omniphobic personal protective gear.

The use of personal protective gear made from omniphobic materials that easily shed drops of all sizes could provide enhanced protection from direct exposure to most liquid-phase biological and chemical hazards and facilitate the postexposure decontamination of the gear. In recent literature, lubricated nanostructured fabrics are seen as attractive candidates for personal protective gear due to their omniphobic and self-healing characteristics. However, the ability of these lubricated fabrics to shed low surface tension liquids after physical contact with other objects in the surrounding, which is critical in demanding healthcare and military field operations, has not been investigated. In this work, we investigate the depletion of oil from lubricated fabrics in contact with highly absorbing porous media and the resulting changes in the wetting characteristics of the fabrics by representative low and high surface tension liquids. In particular, we quantify the loss of the lubricant and the dynamic contact angles of water and ethanol on lubricated fabrics upon repeated pressurized contact with highly absorbent cellulose-fiber wipes at different time intervals. We demonstrate that, in contrast to hydrophobic nanoparticle coated microfibers, fabrics encapsulated within a polymer that swells with the lubricant retain the majority of the oil and are capable of repelling high as well as low surface tension liquids even upon multiple contacts with the highly absorbing wipes. The fabric supported lubricant-swollen polymeric films introduced here, therefore, could provide durable and easy to decontaminate protection against hazardous biological and chemical liquids.

Suppression of Frost Nucleation Achieved Using the Nanoengineered Integral Humidity Sink Effect

Inhibition of frost formation is important for increasing efficiency of refrigeration systems and heat exchangers, as well as for preventing the rapid icing over of water-repellant coatings that are designed to prevent accumulation of rime and glaze. From a thermodynamic point of view, this task can be achieved by either increasing hydrophobicity of the surface or decreasing the concentration of water vapor above it. The first approach has been studied in depth, but so far has not yielded a robust solution to the problem of frost formation. In this work, we systematically explore how frost growth can be inhibited by controlling water vapor concentration using bilayer coatings with a porous exterior covering a hygroscopic liquid-infused layer. We lay the theoretical foundation and provide experimental validation of the mass transport mechanism that governs the integral humidity sink effect based on this coating platform as well as reveal intriguing sizing effects about this system. We show that the concentration profile above periodically spaced pores is governed by the sink and source concentrations and two geometrical parameters: the nondimensional pore size and the ratio of the pore spacing to the boundary layer thickness. We demonstrate that when the ratio of the pore spacing to the boundary layer thickness vanishes, as for the nanoporous bilayer coatings, the entire surface concentration becomes uniform and equal to the concentration set by the hygroscopic liquid. In other words, the surface concentration becomes completely independent of the nanopore size. We identified the threshold geometrical parameters for this condition and show that it can lead to a 65 K decrease in the nucleation onset surface temperature below the dew point. With this fundamental insight, we use bilayer coatings to nanoengineer the integral humidity sink effect to provide extreme antifrosting performance with up to a 2 h delay in nucleation onset at 263 K. The nanoporous bilayer coatings can be designed to combine optimal antifrosting functionality with a superhydrophobic water repelling exterior to provide coatings that can robustly prevent frost, rime, and glaze accumulation. By minimizing the required amount of antifreeze, this anti-icing method can have minimal operational cost and environmental impact.

Microscale Mechanism of Age Dependent Wetting Properties of Prickly Pear Cacti (Opuntia)

Cacti thrive in xeric environments through specialized water storage and collection tactics such as a shallow, widespread root system that maximizes rainwater absorption and spines adapted for fog droplet collection. However, in many cacti, the epidermis, not the spines, dominates the exterior surface area. Yet, little attention has been dedicated to studying interactions of the cactus epidermis with water drops. Surprisingly, the epidermis of plants in the genus Opuntia, also known as prickly pear cacti, has water-repelling characteristics. In this work, we report that surface properties of cladodes of 25 taxa of Opuntia grown in an arid Sonoran climate switch from water-repelling to superwetting under water impact over the span of a single season. We show that the old cladode surfaces are not superhydrophilic, but have nearly vanishing receding contact angle. We study water drop interactions with, as well as nano/microscale topology and chemistry of, the new and old cladodes of two Opuntia species and use this information to uncover the microscopic mechanism underlying this phenomenon. We demonstrate that composition of extracted wax and its contact angle do not change significantly with time. Instead, we show that the reported age dependent wetting behavior primarily stems from pinning of the receding contact line along multilayer surface microcracks in the epicuticular wax that expose the underlying highly hydrophilic layers.

Water permeation and corrosion resistance of single- and two-component hydrophobic polysiloxane barrier coatings

The degradation of corrosion preventative coatings contributes to the high cost and time requirements for maintaining structures in harsh environments. However, the development of new hydrophobic coatings holds the promise of extending the usable life of structures in marine environments. In this work, we quantify the barrier properties and corrosion resistance of two novel highly hydrophobic polysiloxane formulations and the legacy silicone alkyd topcoat used on the topside of Navy’s ships, all with haze gray pigmentation. Based on FTIR-ATR and EIS measurements of the pristine coatings, both the single- (1K) and the two-component (2K) polysiloxane provide significantly improved barrier characteristics (lower water diffusion coefficient and capacitance) than the silicone alkyd. These results were confirmed through a 3-month-long immersion corrosion test, which also showed that the 1K and 2K polysiloxane coatings had comparable degradation characteristic and remained highly hydrophobic.

Far-reaching geometrical artefacts due to thermal decomposition of polymeric coatings around focused ion beam milled pigment particles

Focused ion beam and scanning electron microscope (FIB‐SEM) instruments are extensively used to characterize nanoscale composition of composite materials, however, their application to analysis of organic corrosion barrier coatings has been limited. The primary concern that arises with use of FIB to mill organic materials is the possibility of severe thermal damage that occurs in close proximity to the ion beam impact. Recent research has shown that such localized artefacts can be mitigated for a number of polymers through cryogenic cooling of the sample as well as low current milling and intelligent ion beam control. Here we report unexpected nonlocalized artefacts that occur during FIB milling of composite organic coatings with pigment particles. Specifically, we show that FIB milling of pigmented polysiloxane coating can lead to formation of multiple microscopic voids within the substrate as far as 5 μm away from the ion beam impact. We use further experimentation and modelling to show that void formation occurs via ion beam heating of the pigment particles that leads to decomposition and vaporization of the surrounding polysiloxane. We also identify FIB milling conditions that mitigate this issue.

FIB milling of polymer ceramic nanocomposites: far-reaching thermal artefacts and application to analysis of corrosion barrier coatings

FIB-SEMs are commonly used to characterize composite materials [1], however, their application to analysis of polymeric corrosion barrier coatings has been limited. This technique can be used to quantify 3D distributions of pigment particles and pores in pristine and corroded samples. Resistance of the coatings to penetration by water and dissolved ions is strongly affected by both of these geometrical features but their influence is typically quantified in effective terms using macroscopic measurements [2-3]. Chen et al.[2] recently quantified the 3D distribution of aluminum flakes dispersed in epoxy using serial block-face SEM imaging and x-ray tomography and used this information to theoretically predict barrier properties of the coating. As oppose to slicing using an ultramicrotome, FIB can be used to mill a smooth cross section of a pigmented paint without risk of artefacts arising from heterogeneous properties of the sample such as dislodging of particles by the microtome blade. One of the main concerns that arise when using a FIB-SEM to characterize polymeric materials is the possibility of ion beam heating induced damage [4-5]. Previously thermal damage stemming from a significant temperature rise at the point of ion beam impact has been described. This temperature rise is proportional to ratio of the beam power and thermal conductivity of the material and for polymers characterized by low thermal conductivity could reach over thousand of degrees Celsius [5]. The degree of sample heating can be decreased using cryogenic cooling of the sample, low ion beam current during milling, and intelligent navigation of the ion beam [4-7]. In this work FIB-SEM is used to study surface and internal morphology of polysiloxane corrosion barrier coatings with various concentrations of pigment nanoparticles prior to and at different stages of accelerated corrosion tests. These novel coatings are potential candidates for replacing silicone alkyds, which are currently used to paint the topside of naval surface vessels. The single and two component polysiloxane coatings are hydrophobic and have enhanced cleanability, gloss retention, hardness, and color-stability as compared to the silicone alkyds [8-9]. It is shown that room temperature FIB cross sectioning of pure polysiloxane can be done with a wide range of ion beam currents (1 to 20 nA) without causing any significant damage the exposed section (see Figure 1a and 1b). In contrast, FIB milling of the pigmented polysiloxane coating with ion beam current higher than 1 nA leads to an unexpected mode of severe polymer damage. Specifically, it is shown that FIB milling of such polymer-ceramic nanoparticle composites can lead to formation of multiple voids within the substrate as far as 5 μm away from the ion beam impact (see Figure 1c and 1d). Using systematic experimentation coupled with heat transfer modeling, it is shown that the primary void formation occurs due to decomposition and vaporization of the polymer around ion beam heated pigment nanoparticles (see schematic in Figure 1e). The primary void enlargement and formation of secondary voids likely occurs due to mechanical damage of the polymer induced by the entrapped vapors that could be pressurized up to ~100 MPa [10]. Similarly, secondary voids could form via cracking and seepage of the high-pressure vapors along weaker parts of the composite such as neighboring particle-polymer interfaces. This work demonstrates that processes occurring during FIB-milling of pure polymers and their composites with strongly 142 doi:10.1017/S1431927616001562 Microsc. Microanal. 22 (Suppl 3), 2016

Why is it difficult to wash aphids off from superhydrophobic kale

Many varieties of the cabbage family have leaves covered with superhydrophobic epicuticular wax, which provides them with self-cleaning characteristics. Since the wax also lowers insect adhesion, rinsing of the leaves with water should be an effective way of removing the insects. Conversely, we report that superhydrophobicity of tuscan kale increases resistance of aphids to hydrodynamic removal. The exterior surface of the insects is also superhydrophobic and acts as an extension of the leafs surface. As a result even at moderate impact velocities impinging water drops cannot penetrate under the pests. Consequently, liquid impact aids the insects adhesion by increasing the normal compressive forces they experience. We show that on a hydrophilic arugula leaf this mechanism is absent, and aphids can be easily washed off with water, as it is able to penetrate underneath them. As for removal of aphids from Tuscan kale, we show that lower surface tension liquids, such as oils and soapy water are more effective, because they are able to wet both the plant and insect surfaces. We also show that aerodynamic removal of aphids consisting of simply exposing the invaded leaf to an air flow is most effective.