Nucleate boiling performance on nano/microstructures with different wetting surfaces

ABSTRACT: A study of nucleate boiling phenomena on nano/microstructures is a very basic and useful study with a view to the potential application of modified surfaces as heating surfaces in a number of fields. We present a detailed study of boiling experiments on fabricated nano/microstructured surfaces used as heating surfaces under atmospheric conditions, employing identical nanostructures with two different wettabilities (silicon-oxidized and Teflon-coated). Consequently, enhancements of both boiling heat transfer (BHT) and critical heat flux (CHF) are demonstrated in the nano/microstructures, independent of their wettability. However, the increment of BHT and CHF on each of the different wetting surfaces depended on the wetting characteristics of heating surfaces. The effect of water penetration in the surface structures by capillary phenomena is suggested as a plausible mechanism for the enhanced CHF on the nano/microstructures regardless of the wettability of the surfaces in atmospheric condition. This is supported by comparing bubble shapes generated in actual boiling experiments and dynamic contact angles under atmospheric conditions on Teflon-coated nano/microstructured surfaces.     

Silicon nanowires prepared by electron beam evaporation in ultrahigh vacuum

ABSTRACT: One-dimensional silicon nanowires (SiNWs) were prepared by electron beam evaporation in ultrahigh vacuum (UHV). The SiNWs can be grown through either vapor-liquid-solid (VLS) or oxide-assisted growth (OAG) mechanism. In VLS growth, SiNWs can be formed on Si surface, not on SiO2 surfaces. Moreover, low deposition rate is helpful for producing lateral SiNWs by VLS. But in OAG process, SiNWs can be grown on SiO2 surfaces, not on Si surfaces. This work reveals the methods of producing large-scale SiNWs in UHV.     

Surface characterization of poly(styrene‐co‐ fluoroalkylfumarate): XPS and contact angle measurement study

We studied styrene and fluoroalkylfumarate (FAF) copolymers and their surfaces by means of contact angle measurement and X‐ray photoelectron spectroscopy (XPS). The surfaces of the copolymers were very hydrophobic (even with a small amount of FAF) because of the concentration of FAF segments at the surface. The hydrolyzed surfaces of the copolymers became slightly hydrophilic compared to the as cast. The XPS data suggested that the fluoroalkyl groups seemed to be primarily hydrolyzed. The surfaces with a large amount of FAF changed their characteristics to hydrophobic again under atmospheric conditions. This phenomenon was due to the inversion of the carboxyl and the fluoroalkyl groups. These hydrolyzed surfaces seemed to be useful for modifying polymer surfaces by attaching to other functional molecules. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1049–1054, 1999     

Immobilization of polyethylene oxide surfactants for non-fouling biomaterial surfaces using an argon glow discharge treatment

A non-fouling (protein-resistant) polymer surface is achieved by the covalent immobilization of polyethylene oxide (PEO) surfactants using an inert gas discharge treatment. Treated surfaces have been characterized using electron spectroscopy for chemical analysis (ESCA), static secondary ion mass spectrometry (SSIMS), water contact angle measurement, fibrinogen adsorption, and platelet adhesion. This paper is intended to review our recent work in using this simple surface modification process to obtain wettable polymer surfaces in general, and non-fouling biomaterial surfaces in particular.     

A Comparison of Chemical Activity at Ordered and Disordered Tetrahedrally Coordinated Semiconductor Surfaces

This paper discusses the surface order-dependent adsorptive activity at the surfaces of tetrahedrally coordinated semiconductors. It stresses the differences between elemental (e.g., Si) and compound materials (e.g., Gas, CdS, and ZnO) as well as the differences between ordered and disordered nonpolar compound semiconductor surfaces. Several examples will be presented that compare oxygen adsorption on ordered and disordered surfaces. It will be shown that adsorption on ordered nonpolar compound semiconductor surfaces is weak in comparison to adsorption on ordered elemental semiconductor surfaces, that among the compound semiconductors the ordered nonpolar surfaces of the II-VI materials (e.g.,) are much weaker adsorbents than the similar surfaces of the III-V materials (e.g., GaAs), and that adsorption on nonpolar compound semiconductor surfaces is much enhanced by surface disorder. It will be argued that these phenomena can collectively be attributed to bonding ionicity. Finally, an attempt will be made to establish a connection between the order/disorder phenomena described here and the collective electronic and active site models of surfaceactive catalysis.     

A Comparison of Chemical Activity at Ordered and Disordered Tetrahedrally Coordinated Semiconductor Surfaces

This paper discusses the surface order-dependent adsorptive activity at the surfaces of tetrahedrally coordinated semiconductors. It stresses the differences between elemental (e.g., Si) and compound materials (e.g., Gas, CdS, and ZnO) as well as the differences between ordered and disordered nonpolar compound semiconductor surfaces. Several examples will be presented that compare oxygen adsorption on ordered and disordered surfaces. It will be shown that adsorption on ordered nonpolar compound semiconductor surfaces is weak in comparison to adsorption on ordered elemental semiconductor surfaces, that among the compound semiconductors the ordered nonpolar surfaces of the II-VI materials (e.g.,) are much weaker adsorbents than the similar surfaces of the III-V materials (e.g., GaAs), and that adsorption on nonpolar compound semiconductor surfaces is much enhanced by surface disorder. It will be argued that these phenomena can collectively be attributed to bonding ionicity. Finally, an attempt will be made to establish a connection between the order/disorder phenomena described here and the collective electronic and active site models of surfaceactive catalysis.     

Naturally Chiral Surfaces

Chiral surfaces are of growing importance as a result of their potential for use in enantioselective chemical processes. By far, the most widely used and commonly studied chiral surfaces are those prepared by templating of an achiral surface with chiral organic ligands. It is possible, however, to prepare naturally chiral surfaces by a number of means. This paper describes the various types of chiral surfaces. In addition, data are presented to suggest that naturally chiral surfaces of metals can be prepared by a process of imprinting in which chiral adsorbates induce reconstructions that create chiral kinks on metal surfaces.     

A glow discharge treatment to immobilize poly(ethylene oxide)/poly(propylene oxide) surfactants for wettable and non-fouling biomaterials

A non-fouling (protein resistant) polymer surface was achieved using an argon glow discharge treatment of a polyethylene surface which had been precoated with various poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) (PEO-PPO-PEO) tri-block copolymer surfactants. The surfactant is first deposited on the polymer surface via a solvent swelling and evaporation method. Then the coated surfactant is immobilized on the substrate surface by an inert gas discharge treatment. ESCA and water contact angle () measurements on treated and solvent washed surfaces show significant increases in both surface O/C ratios and surface water wettability (0 < 30°) compared to LDPE control surfaces, revealing the presence of PEO on the treated surfaces. A great reduction of fibrinogen adsorption on the modified surfaces is also observed for the highest PEO content surfactants. This simple surface modification process may have wide applicability to obtain wettable polymer surfaces in general, and non-fouling biomaterial surfaces in specific.     

Atomistic Modeling Study of Surface Segregation in Nd:YAG

This study investigates the composition of free surfaces of neodymium-doped yttrium–aluminum–garnet, using energy minimization techniques. Atomistic modeling shows that the dopant is concentrated in a zone within 5 Å of the surface, the enrichment factor being around 1300 when comparing surfaces with bulk concentrations. It is shown that the (111) surface can incorporate 1.8 times as much Nd as the (110) surface, the latter commonly found in YAG morphologies. Our results indicate that by using nanocrystalline ceramics and by modifying crystal growth to form (111) surfaces, the Nd content and thus the laser power may be significantly increased.     

Antibacterial surfaces obtained through dopamine and fluorination functionalizations

Self-polymerized dopamine was used to form a thin layer onto stainless steel (SS) and poly(ethylene terephthalate) (PET) sheets followed by covalent grafting of pentadecafluorooctanoyl chloride by esterification and amidation reactions. The surface functionalization was characterized at each step by contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The anti-adhesive properties of native surfaces, polydopamine-coated surfaces and hydrophobic fluorinated surfaces were tested against Gram-negative (Pseudomonas aeruginosa) and Gram-positive bacteria (Listeria monocytogenes and Staphylococcus aureus). The results reveal an inhibition of bacteria growth towards Gram-negative bacteria on fluorinated surfaces. This work proposes a novel method to easily fluorinate in two steps both metallic and organic surfaces using “universal” polydopamine coating as a key step.     

Generation of antifouling layers on stainless steel surfaces by plasma‐enhanced crosslinking of polyethylene glycol

Polyethylene glycol (PEG) structures were deposited onto stainless steel (SS) surfaces by spin coating and argon radio frequency (RF)‐plasma mediated crosslinking. Electron spectroscopy for chemical analysis (ESCA) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR) indicated the presence of  CH₂ CH₂ O structure and C C C linkage, as a result of the plasma crosslinking, on PEG‐modified SS surfaces. Scanning electron microscopy (SEM) indicated complete deposition, and water contact angle analysis revealed higher hydrophilicity on PEG‐modified surfaces compared to unmodified SS surfaces. Surface morphology and roughness analysis by atomic force microscopy (AFM) revealed smoother SS surfaces after PEG modification. The evaluation of antifouling ability of the PEG‐modified SS surfaces was carried out. Compared to the unmodified SS, PEG‐modified surfaces showed about 81–96% decrease in Listeria monocytogenes attachment and biofilm formation (p < 0.05). This cold plasma mediated PEG crosslinking provided a promising technique to reduce bacterial contamination on surfaces encountered in food‐processing environments. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 485–497, 2005     

The adhesional electrification force between randomly rough surfaces with identical materials

Abstract

I present a general theory for the adhesional electrification and find a necessary force to separate two identical rough plates. This theory is a continuation of the previous research. In addition to the effect of the fluctuations of charge density, the roughness effects of two identical plates are considered. When both surfaces are rough but uncorrelated results an increase in the electroadhesion force in comparison to the case of only one rough surface. The roughness leads to an increase in the area of the rough surfaces as well as the magnitude of the positive and negative charges on the surfaces and this leads effectively to a larger charge induction. Therefore, the electroadhesion force increases between two rough plates by growing the magnitude of charges on the surfaces. When there is a cross correlation between two surfaces, the perturbed force is different from the case where there is no cross correlation between two surfaces for the constant charge density. As a result, if the cross correlation between two surfaces is positive (negative), the perturbed force is smaller (larger) than the case of two uncorrelated rough surfaces.     

Surface modification of biomaterials by photochemical immobilization and photograft polymerization to improve hemocompatibility

Thrombus formation and blood coagulation are serious problems associated with blood contacting products, such as catheters, vascular grafts, artificial hearts, and heart valves. Recent progresses and strategies to improve the hemocompatibility of biomaterials by surface modification using photochemical immobilization and photograft polymerization are reviewed in this paper. Three approaches to modify biomaterial surfaces for improving the hemocompatibility, i.e., bioinert surfaces, immobilization of anticoagulative substances and biomimetic surfaces, are introduced. The biomimetic amphiphilic phosphorylcholine and Arg-Gly-Asp (RGD) sequence are the most effective and most often employed biomolecules and peptide sequence for improving hemocompatibility of material surfaces. The RGD sequence can enhance adhesion and growth of endothelial cells (ECs) on material surfaces and increase the retention of ECs under flow shear stress conditions. This surface modification is a promising strategy for biomaterials especially for cardiovascular grafts and functional tissue engineered blood vessels.     

Surface modification of biomaterials by photochemical immobilization and photograft polymerization to improve hemocompatibility

Thrombus formation and blood coagulation are serious problems associated with blood contacting products, such as catheters, vascular grafts, artificial hearts, and heart valves. Recent progresses and strategies to improve the hemocompatibility of biomaterials by surface modification using photochemical immobilization and photograft polymerization are reviewed in this paper. Three approaches to modify biomaterial surfaces for improving the hemocompatibility, i.e., bioinert surfaces, immobilization of anticoagulative substances and biomimetic surfaces, are introduced. The biomimetic amphiphilic phosphorylcholine and Arg-Gly-Asp (RGD) sequence are the most effective and most often employed biomolecules and peptide sequence for improving hemocompatibility of material surfaces. The RGD sequence can enhance adhesion and growth of endothelial cells (ECs) on material surfaces and increase the retention of ECs under flow shear stress conditions. This surface modification is a promising strategy for biomaterials especially for cardiovascular grafts and functional tissue engineered blood vessels.     

Investigation of water absorption performance of polyester-woven fabrics coated with super absorbent polymer

Superabsorbent polymers (SAPs) constitute a special class of polymers widely used in various fields, especially in the hygiene and healthcare sectors. This study investigates the feasibility of achieving high water absorption capacity surfaces by coating powdered SAP using conventional coating methods onto textile surfaces. For this purpose, water-based coating pastes containing micronized SAP powder based on acrylamide/acrylic acid copolymer were coated onto polyester (PES)-woven fabric surfaces using a knife-over-roll coating technique. As the working parameters, the pH value of the coating paste, the coating thickness (the distance between the cloth and the knife), the drying/fixing temperature and time, the SAP concentration, and the water absorption capacity according to time were investigated. The results were evaluated by applying the coating thickness, the amount of coating material transferred to the fabric on the SAP-coated samples, water absorbing capacity, and centrifugal water retention tests. The obtained results have demonstrated that textile surfaces with high water absorption capacity (on average 200%–350%) can be achieved by coating hydrophobic fabric surfaces, such as PES, with SAP under suitable conditions.     

A study of the effect of surface pre-treatment on the adhesion of coatings

This paper deals with the study of effects of mechanical surface preparation on the adhesion of coating with high content of zinc dust. Five kinds of mechanically blasted surfaces were studied. The following were used as abrasives: steel shot, steel grit, brown corundum oxide and zirblast. The last surface type was modified by MBX Blaster technology (mechanical bristle blasting). The surfaces topography was quantified by a roughness profilometer. The shape and size of the incurred inequalities on the modified surfaces were studied using 3D microscope images of the surface. The purity of the surfaces after pre-treatment was evaluated by impurity glued on the tape and measuring the reflection of light from the surface. Fractal analysis was used to evaluate the diversity of inequalities on the prepared surfaces. Cross-sections were also taken of the prepared surfaces. The prepared surfaces were coated with zinc-filled coating. The adhesion of the coating to the substrate was evaluated by a pull-off test after curing the coating (as sprayed), as well as after exposure to severe corrosive environments. The best adhesion of the coating was found for the coating applied to the substrate which had been pre-treated with brown corundum and steel grit.     

Effects of large scale eddies and stagnation surfaces on microcrystallization

A Lagrangian marker particle (LMP) method is applied to measure the toroidal large-scale eddies (LSEs) and their enveloping stagnation surfaces in a 280 l bottom-sweeping model crystallizer. The trajectories of a 0.4 cm diameter LMP show that these stagnation surfaces inhibit transport. Analysis shows that the velocity component normal to stagnation surfaces vanish. Therefore, stagnation surfaces act as a semi-permeable barriers to particle transport. Microconductivity measurements show that the stagnation surfaces are leaky at the molecular scale. Thus particle transport through stagnation surfaces is size-dependent. The LMP measurements reveal the structure of the LSEs. This consists of (1) an upward-swirling flow adjacent to the tank perimeter extending from the bottom to the top of the tank, (2) a central, quiescent zone, and (3) a downward return flow between (1) and (2) through a system of nested, smaller diameter, secondary toroidal flows concentric with the impeller axis. A cylindrical stagnation surface surrounds the central quiescent zone. These results are corroborated by measurements of inhomogeneous concentration profiles in an industrial scale 2000 l batch crystallizer. This leads to an understanding of the effects of LSEs on silver halide microcrystal particle size distribution in the industrial scale crystallizer.     

Fabrication of Tuneable Thickness Silica Films on Solid Surfaces Using Amines and Proteins

We report an elegant and simple method to fabricate silica films with controlled thickness and roughness using protein coated solid surfaces as substrates. Bovine serum albumin (BSA) and lysozyme having different inherent charges have been used as model proteins (templates) to fabricate silica films. The formation of silica films was achieved by immobilization of BSA and lysozyme on amine (poly(allylamine) (PAH), poly(ethyleneimine) (PEI) and octadecyl amine (ODA)), coated surfaces, followed by treatment with silica precursors (tetramethoxysilane) under environmentally benign conditions of pH and temperature. BSA adsorbs strongly on hydrophilic surfaces (PAH and PEI coated) via electrostatic interaction, while lysozyme shows greater affinity towards hydrophobic surfaces (ODA coated) via largely hydrophobic interactions. The thickness (12–60 nm) and roughness of the films (1.30–3.75 nm) could be tuned by varying the amount of the adsorbed proteins on the amine-coated surfaces. This simple route to prepare silica films of controlled thickness could have potential application in membrane fabrication, biomedical devices, biosensors and next generation electronic components.     

Effects of Surface Relaxation on Enantiospecific Adsorption on Naturally Chiral Pt Surfaces

Many high Miller index metals surfaces are naturally chiral, offering opportunities for performing enantiospecific chemistry. Previous theoretical treatments of adsorption on these surfaces have used surfaces truncated from bulk crystals. This paper examines the effect of surface relaxation on enantiospecific adsorption on chiral Pt surfaces by using relaxed surface structures determined using density functional theory.     

Controlled Epitaxial Nucleation of Nickel Oxide on Microfabricated Magnesium Oxide Substrates in a CVD Process

It is shown that the nucleation of nickel oxide crystals grown by CVD on MgO depends on the surface structure of the substrates. On freshly cleaved {100} surfaces of MgO, nucleation occurs readily on cleavage steps. The crystallographic orientation of the surfaces of these steps is {110}, and their height ranges from 1 to 8 μm. In contrast, nucleation is sparse on highly polished {100} surfaces having a step height of less than 0.02 μm. Nucleation on polished {111} surfaces is also poor. However, highly polished {110} surfaces show profuse nucleation under similar growing conditions. This preference for nucleation on {110} surfaces and on the step height was used to obtain periodically spaced nuclei of NiO crystals by etching and microlithography of the (100) MgO surface which exposed the {110} surfaces at periodic locations. In this way the spacing of the nuclei could be controlled from 10 μm up to 100 μm.