Marlon L. Walker

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.

Antioxidant Cerium Oxide Nanoparticles in Biology and Medicine.

Previously, catalytic cerium oxide nanoparticles (CNPs, nanoceria, CeO2-x NPs) have been widely utilized for chemical mechanical planarization in the semiconductor industry and for reducing harmful emissions and improving fuel combustion efficiency in the automobile industry. Researchers are now harnessing the catalytic repertoire of CNPs to develop potential new treatment modalities for both oxidative- and nitrosative-stress induced disorders and diseases. In order to reach the point where our experimental understanding of the antioxidant activity of CNPs can be translated into useful therapeutics in the clinic, it is necessary to evaluate the most current evidence that supports CNP antioxidant activity in biological systems. Accordingly, the aims of this review are three-fold: (1) To describe the putative reaction mechanisms and physicochemical surface properties that enable CNPs to both scavenge reactive oxygen species (ROS) and to act as antioxidant enzyme-like mimetics in solution; (2) To provide an overview, with commentary, regarding the most robust design and synthesis pathways for preparing CNPs with catalytic antioxidant activity; (3) To provide the reader with the most up-to-date in vitro and in vivo experimental evidence supporting the ROS-scavenging potential of CNPs in biology and medicine.

Direct Imaging of Complex Nano- to Microscale Interfaces Involving Solid, Liquid, and Gas Phases

Surfaces with special wetting properties not only can efficiently repel or attract liquids such as water and oils but also can prevent formation of biofilms, ice, and clathrate hydrates. Predicting the wetting properties of these special surfaces requires detailed knowledge of the composition and geometry of the interfacial region between the droplet and the underlying substrate. In this work we introduce a 3D quantitative method for direct nanoscale visualization of such interfaces. Specifically, we demonstrate direct nano- to microscale imaging of complex fluidic interfaces using cryostabilization in combination with cryogenic focused ion beam milling and SEM imaging. We show that application of this method yields quantitative information about the interfacial geometry of water condensate on superhydrophilic, superhydrophobic, and lubricant-impregnated surfaces with previously unattainable nanoscale resolution. This type of information is crucial to a fundamental understanding as well as the design of surfaces with special wetting properties.

In Situ Ellipsometric Study of PEG ∕ Cl − Coadsorption on Cu, Ag, and Au

Spectroscopic ellipsometry was used to examine the adsorption of poly(ethylene glycol) (PEG) and Cl - on polycrystalline Cu, Ag, and Au electrodes in sulfuric acid. In halide-free sulfuric acid, PEG adsorption on Cu and Ag is minimal at potentials positive of the estimated potential of zero charge (pzc). PEG adsorption on Au was also negligible in halide-free environments positive of the pzc, but slight adsorption was evident near the pzc. In contrast, in the presence of adsorbed halide, PEG coadsorption is clearly evident at potentials near or positive of the pzc for all metal samples studied. The PEG/Cl - coadsorbed film does not provide a significant barrier to outer-sphere electron-transfer reactions [i.e., Ru 3 + / 2 + (NH 3 ) 6 ], although inner-sphere reactions associated with electrodeposition of copper are effectively inhibited. The implications of these observations with respect to the role of Cu + ions in the PEG-induced inhibition of Cu electrodeposition is discussed.

Dynamics of nanoparticle self-assembly into superhydrophobic liquid marbles during water condensation.

Nanoparticles adsorbed onto the surface of a drop can fully encapsulate the liquid, creating a robust and durable soft solid with superhydrophobic characteristics referred to as a liquid marble. Artificially created liquid marbles have been studied for about a decade but are already utilized in some hair and skin care products and have numerous other potential applications. These soft solids are usually formed in small quantity by depositing and rolling a drop of liquid on a layer of hydrophobic particles but can also be made in larger quantities in an industrial mixer. In this work, we demonstrate that microscale liquid marbles can also form through self-assembly during water condensation on a superhydrophobic surface covered with a loose layer of hydrophobic nanoparticles. Using in situ environmental scanning electron microscopy and optical microscopy, we study the dynamics of liquid marble formation and evaporation as well as their interaction with condensing water droplets. We demonstrate that the self-assembly of nanoparticle films into three-dimensional liquid marbles is driven by multiple coalescence events between partially covered droplets and is aided by surface flows causing rapid nanoparticle film redistribution. We also show that droplet and liquid marble coalescence can occur due to liquid-to-liquid contact or squeezing of the two objects into each other as a result of compressive forces from surrounding droplets and marbles. Irrelevant of the mechanism, coalescence of marbles and drops can cause their rapid movement across and rolling off the edge of the surface. We also demonstrate that the liquid marbles randomly moving across the surface can be captured and immobilized by hydrophilic surface patterns.

Potential Dependence of Competitive Adsorption of PEG, Cl − , and SPS/MPS on Cu An In Situ Ellipsometric Study

Potential-dependent adsorption of bis-(3-sulfopropyl) disulfide (SPS), 3-mercaptopropyl sulfonate (MPS), chloride (Cl - ), and polyethylene glycol (PEG) on evaporated Cu thin films was examined using in situ spectroscopic ellipsometry at -0.65 V MSE, -0.75 V MSE, and -0.85 V MSE under quiescent conditions. The adsorption for the thiol compound was similar at all potentials examined, but the adsorption of the disulfide increased at more negative potentials. The influence of Cl - on the substrate was greatly diminished and PEG concurrently less strongly adsorbed at more negative potentials, as measured by ellipsometric responses. Both adsorbed MPS and SPS inhibited the specific adsorption of PEG in the presence of Cl - . Likewise MPS and SPS added to the electrolyte displaced preadsorbed PEG/ Cl - layers, with the displacement by the disulfide compound strongly potential dependent. At all potentials studied, the disulfide was less effective than MPS at displacement of preadsorbed PEG/ Cl - .

Competitive Adsorption of PEG, Cl − , and SPS/MPS on Cu: An In Situ Ellipsometric Study

The adsorption of Cu electrodeposition accelerating agents bis-(3-sodiumsulfopropyl disulfide) (SPS) and (3-mercaptopropyl) sulfonate (MPS) on evaporated Cu thin films was examined in situ using spectroscopic ellipsometry under quiescent conditions. Both the thiol MPS and disulfide SPS resulted in definitive changes in the optical response of the interface, indicative of irreversible chemisorption. The optical responses of the films formed from the two additives were different, however, suggesting a difference in the adsorbed states. Cl - can reversibly coadsorb with either MPS or SPS. Preadsorption of either MPS or SPS inhibits the specific adsorption of polyethylene glycol (PEG) in the presence of Cl - . Both SPS and MPS adsorption displace preadsorbed PEG layers. The kinetics of the PEG displacement reaction differ significantly between the thiol and disulfide, consistent with observations of their respective effect on metal deposition kinetics.

Use of an SF5+ polyatomic primary ion beam for ultrashallow depth profiling on an ion microscope secondary ion mass spectroscopy instrument

A magnetic sector secondary ion mass spectrometry (SIMS) instrument has been fitted with a modified hot filament duoplasmatron ion source for generation of SF5+ primary ion beams for SIMS depth profiling applications. The SF5+ primary ion beam has been evaluated by depth profiling of several low energy boron ion implants, boron delta-doped structures and a Ni/Cr metal multilayer depth profiling standard reference material. Using 3.0 keV impact SF5+ bombardment at a 52° impact angle with oxygen flooding gives a trailing edge decay length (1/e) for the boron implants and delta-doped layers of 1.3 nm. Under the same conditions, O2+ bombardment gives a trailing edge decay length (1/e) of 2.3 nm. The use of the SF5+ beam without oxygen flooding gives a substantial increase in decay length that is related to the formation of ripples as determined by atomic force microscopy. In the case of the Ni/Cr reference material, a significant reduction in sputter-induced topography is observed with SF5+ bombardment.

Anionic Triphenylmethane Dye Solutions for Low-Dose Food Irradiation Dosimetry

Abstract The radiolytic bleaching of aryl sulfonic-substituted para-diethyl-amino triphenylmethane dye solutions can be used for dosimetry in the absorbed dose range 10 to 400 Gy. The sulfonic anions provide solubility of these acid dyes in water. Two of these dyes are supplied as stable greenish-blue biological stains when dissolved in weakly-acidic aqueous solution, Light Green SF Yellowish and Fast Green FCF. They have, respectively, linear molar absorption coefficients of 7.14 × 103(at pH5.4) and 10.0 × 103 (at pH4.2) m2mol−1, when measured at the peaks of the primary absorption bands, 630 nm and 622 nm, respectively. The bleaching due to irradiation with gamma rays shows a linear function with a positive slope between the negative logarithm of the absorbance and the absorbed dose. The effect of pH on the response is studied, as well as the effects of light and temperature on pre- and post-irradiation stability. A mechanism, based mainly on radiolytic oxidation of the protonated phenolic or sulfonated phenyl group by .OH, with the abstraction of H-atom to water, is postulated for neutral to slightly acidic aerated aqueous solutions. The influence of alcohol on diminishing the negative yield is demonstrated. Alkaline aqueous solutions of these dyes (pH 10.2) have a shorter-wavelength absorption maximum than acidic aqueous solutions. The effect of irradiation is to cause acidification (to pH 7) due to displacement of OH groups and degradation of the dye molecule to lower molecular weight organic acids.

Alcohol solutions of triphenyl-tetrazolium chloride as high-dose radiochromic dosimeters

Abstract The radiolytic reduction of colorless tetrazolium salts in aqueous solution to the highly colored formazan dye is a well-known acid-forming radiation chemical reaction. Radiochromic thin films and three-dimensional hydrocolloid gels have been used for imaging and mapping absorbed dose distributions. The high solubility of 2,3,5-triphenyl-tetrazolium chloride (TTC) in alcohols provides a useful liquid dosimeter (45 mM TTC in aerated ethanol) and shows a linear response of absorbance increase (λ max = 480 nm) with dose over the range 1–16 kGy. The linear molar absorption coefficient (ɛ m ) for the formazan at the absorption peak is 1.5 × 10 3 m 2 mol −1 , and the radiation chemical yield for the above solution is G (formazan) = 0.014 μ mol J −1 . The irradiation temperature coefficient is about 0.8 percent per degree Celsius rise in temperature over the temperature range 0–30 °C but is much larger between 30° and 60 °C. The unirradiated and irradiated solutions are stable over at least five days storage at normal laboratory temperature in the dark, but when stored in daylight at elevated temperature, the unirradiated solution in sealed amber glass ampoules undergoes slow photolytic dye formation, and the irradiated solution experiences initial fading and subsequent reversal (photochromism) when exposed to direct sunlight.