Surface stress-dependent adhesion to fluorinated polymers

Irradiation of thin commercial sheets of poly(tetrafluoroethylene) (PTFE) or a fluorinated ethylene-propylene copolymer (FEP) yield essentially the same results with mass spectroscopy or x-ray photoelectron spectroscopy (XPS). For both gas phase and surface (ca. 30 Å) products of irradiation, the same product distributions and exposure dependencies are observed. In addition, XPS of chemically etched (with sodium naphthalenide) PTFE and FEP shows the same extent of surface defluorination. In contrast to the more surface-sensitive XPS, Rutherford backscattering spectroscopy of etched PTFE shows that the defluorination extends to depths of ca. 3000 Å, while with etched FEP defluorination extends to only a few hundred Angstroms. Scanning electron microscopy shows the FEP surface to be smooth and featureless both before and after chemical etching, while etched PTFE is characterized by a crazed surface with a high density of unidirectional cracks oriented perpendicular to long macroscopic scratches existing in the virgin surface. Adhesion of Cu to this etched PTFE has previously been shown to be sufficiently strong that failure is a result of near-cohesive failure in the PTFE and not adhesive failure. Conversely, weak adhesion of Cu to etched FEP or to other smooth forms of fluorinated polymers, including polished PTFE, is observed. This correlation of strong adhesion with surface roughness and not with surface chemical changes is consistent with previous suggestions that a major component of adhesion to fluorinated polymers is mechanical interlocking. The differences between FEP and PTFE are discussed in terms of a model involving surface stresses expected from their different methods of manufacture: extrusion from the melt in the case of FEP and skiving from pressed cylinders in the case of PTFE. © 1994 John Wiley & Sons, Inc.     

Dynamic contact angle and spreading rate measurements for the characterization of the effect of dentin surface treatments

A new methodology capable of providing reliable and reproducible contact angle (theta) data has been employed to study the effect of clinical treatments grinding, acid etching, and deproteinization on medial dentin tissue. It is based on the application of the ADSA-CD algorithm to the determination of low-rate dynamic contact angles, obtained from slowly growing drops, and on contact angle measurement, as well as spreading behavior analysis, during the relaxation of the system (water on treated dentin) after initial drop growth. The theta data obtained were substantially more reproducible than those obtained with classical methods. A net effect of the treatment on theta was found, increasing dentin wettability: theta (polished) >theta (etched) >theta (deproteinized). The spreading rates correlate with the angles and are adequate for the dentin surface characterization. ANOVA and SNK tests show that for advancing contact angles the means corresponding to all treatments are significantly different. In the relaxing phase, mean angle and spreading rates on polished dentin differ significantly from those on etched and deproteinized dentin, but the latter do not differ significantly from each other.     

Rectangular bunched rutile TiO2 nanorod arrays grown on carbon fiber for dye-sensitized solar cells

Because of their special application in photovoltaics, the growth of one-dimensional single-crystalline TiO(2) nanostructures on a flexible substrate is receiving intensive attention. Here we present a study of rectangular bunched TiO(2) nanorod (NR) arrays grown on carbon fibers (CFs) from titanium by a "dissolve and grow" method. After a corrosion process in a strong acid solution, every single nanorod is etched into a number of small nanowires. Tube-shaped dye-sensitized solar cells are fabricated by using etched TiO(2) NRs-coated CFs as the photoanode. An absolute energy conversion efficiency of 1.28% has been demonstrated under 100 mW cm(-2) AM 1.5 illumination. This work demonstrates an innovative method for growing bunched TiO(2) NRs on flexible substrates that can be applied in flexible devices for energy harvesting and storage.     

Application of surface analysis in study on removal mechanism and abrasive selection during fused silica chemical mechanical polishing

In this work, surface analysis technology is employed to investigate the removal mechanism and the selection of abrasive during fused silica chemical mechanical polishing (CMP). Morphology of abrasives is inspected by scanning electron microscope (SEM). The atomic force microscope (AFM) is used to determine the surface roughness (Rq) and undulating (PV) of the polished fused silica surface. The results show that abrasive morphology has a tremendous influence on removal rate (MRR) and PV but has little effect on the Rq. The AFM and infrared spectroscopy (IR) analysis show that a soft layer, called “silica gel membrane (SGM),” existed on the polished surface is the critical reason for the differences of MRR, Rq, and PV during CMP. For three kinds of micro-ceria abrasives, the abrasive with a rougher surface gets more opportunities to contact the surface of fused silica, yielding higher MRR. Regarding different kinds of nano-abrasives, there are more SGM induced by nano-ceria abrasive resulting from higher chemical reaction rate. The element contaminations on the polished fused silica have been assessed using X-ray photoelectron spectroscopy (XPS), and the results suggest that there are depths of 3.6 and 5.4-nm element contaminations on the polished surface of fused silica with nano-ceria and nano-alumina abrasives, respectively. While the surface polished by nano-silica is free of heterogeneous element contaminations. Based on study results, a novel polishing slurry is designed by modifying the chemical composition of nano-silica. Comparing with ceria-based slurry, the silica-based slurry has better removal efficiency, and surface quality in fused silica precision machining.     

Employing a cylindrical single crystal in gas-surface dynamics

We describe the use of a polished, hollow cylindrical nickel single crystal to study effects of step edges on adsorption and desorption of gas phase molecules. The crystal is held in an ultra-high vacuum apparatus by a crystal holder that provides axial rotation about a [100] direction, and a crystal temperature range of 89 to 1100 K. A microchannel plate-based low energy electron diffraction/retarding field Auger electron spectrometer (AES) apparatus identifies surface structures present on the outer surface of the cylinder, while a separate double pass cylindrical mirror analyzer AES verifies surface cleanliness. A supersonic molecular beam, skimmed by a rectangular slot, impinges molecules on a narrow longitudinal strip of the surface. Here, we use the King and Wells technique to demonstrate how surface structure influences the dissociation probability of deuterium at various kinetic energies. Finally, we introduce spatially-resolved temperature programmed desorption from areas exposed to the supersonic molecular beam to show how surface structures influence desorption features.     

Simulation of droplet formation and coalescence using lattice Boltzmann-based single-phase model

A lattice Boltzmann method-based single-phase free surface model is developed to study the interfacial dynamics of coalescence, droplet formation and detachment phenomena related to surface tension and wetting effects. Compared with the conventional multiphase models, the lattice Boltzmann-based single-phase model has a higher computational efficiency since it is not necessary to simulate the motion of the gas phase. A perturbation, which is given in the same fashion as the perturbation step in Gunstensen's color model, is added to the distribution functions of the interface cells for incorporating the surface tension into the single-phase model. The assignment of different mass gradients along the fluid-wall interface is used to model the wetting properties of the solid surface. Implementations of the model are demonstrated for simulating the processes of the droplet coalescence, the droplet formation and detachment from ceiling and from nozzles with different shapes and different wall wetting properties.     

Surface-enhanced Raman scattering (SERS) from different azobenzene self-assembled monolayers and sandwiches

Self-assembled monolayers (SAMs) of functionalized azobenzene thiols (RAzoCnSH, n=3-6 for R=H, abbreviated as AzoCnSH; and n=4 for R=CH(3)CONH, abbreviated as aaAzoC4SH) on different substrates RAzoCnSz.sbnd;z.sfnc;S (S represents substrates of vacuum-deposited gold (Au), silver foil (Ag), HNO(3) etched silver foil (EAg), and silver mirror (mAg)) have been studied by SERS in the near-infrared region. SERS of the SAMs on EAg and/or mAg exhibit SERS effects that vary with etching time and/or deposition time. The most appropriate time is 5 s for etching in 1:1 HNO(3) and 40 s for deposition in 0.1 M Ag(NH(3))(2)NO(3). Further, a layer of Ag mirror was conveniently deposited on the top of the SAMs on different substrates, yielding a more efficient SERS-active system possessing a "sandwiched" structure of mAgz.sfnc;RAzoCnS-z.sfnc;S. An appropriate surface roughness is required for the strongest SERS effect. Scanning electron microscopy (SEM) indicates that there exist a large number of projects around 100 nm on the surface showing the strongest SERS effect. When the surface roughness is decreased or increased, the SERS effect decreases sharply. The relationship between the SERS effect and the structural nature was investigated and showed that the enhancement factor decays exponentially with increasing in distances of the azobenzene group from the underlying substrate or the overlying silver mirror. This result reveals that the SERS effect may be the result of the electromagnetic coupling effect between two metal layers.     

An etching technique to reveal the supermolecular structure of crystalline polymers

Highly polished surfaces of nylon 6, nylon 66, polychlorotrifluoroethylene and polypropylene were etched by aromatic and chlorinated hydrocarbons at temperatures between 25 and 75°C to reveal surface morphology. Comparison with micrographs obtained with microtomed sections shows the absence of etching artifacts which are known to accompany the use of oxidizing etching agents.     

Chemical etching of zinc oxide for thin-film silicon solar cells

Chemical etching is widely applied to texture the surface of sputter-deposited zinc oxide for light scattering in thin-film silicon solar cells. Based on experimental findings from the literature and our own results we propose a model that explains the etching behavior of ZnO depending on the structural material properties and etching agent. All grain boundaries are prone to be etched to a certain threshold, that is defined by the deposition conditions and etching solution. Additionally, several approaches to modify the etching behavior through special preparation and etching steps are provided.     

Superlyophilic Interfaces Assisted Thermal Management

Thermal management has become a critical issue owing to the increasing need for various devices including heat dissipation and adsorption. Recently, the rapid growth of scientific reports is seen to improve thermal management efficiency by developing materials with high transfer coefficient and surface improvement to enhance heat transfer rate. Inspired by nature, constructing superlyophilic interfaces has been proved to be an effective way for thermal management and applied in industry and daily life. Herein, state-of-the-art developments of superlyophilic interfaces assisted thermal management are reported mainly from four perspectives around boiling, evaporation, radiation, and condensation. In particular, we discussed the unique role of superlyophilic interfaces during the heat transfer process, such as increasing bubble detachment rate, superspreading assisted efficient evaporation, directional liquid transfer in textiles during radiative cooling, and so forth. Finally, challenges of thermal management assisted by superlyophilic interfaces toward future applications are presented.     

Adsorption of nonionic surfactants on cellulose surfaces: adsorbed amounts and kinetics

Kinetic and equilibrium aspects of three different poly(ethylene oxide) alkylethers (C12E5, C12E7, C14E7) near a flat cellulose surface are studied. The equilibrium adsorption isotherms look very similar for these surfactants, each showing three different regions with increasing surfactant concentration. At low surfactant content both the headgroup and the tail contribute to the adsorption. At higher surface concentrations, lateral attraction becomes prominent and leads to the formation of aggregates on the surface. The general shape of the isotherms and the magnitude of the adsorption resemble mostly those for hydrophilic surfaces, but both the ethylene oxide and the aliphatic segments determine affinity for the surface. The adsorption and desorption kinetics are strongly dependent on surfactant composition. At bulk concentrations below the CMC, the initial adsorption rate is attachment-controlled. Above the CMC, the micellar diffusion coefficient and the micellar dissociation rate play a crucial role. For the most hydrophilic surfactant, C12E7, both parameters are relatively large. In this case, the initial adsorption rate increases with increasing surfactant concentration, also above the CMC. For C12E5 and C14E7 there is no micellar contribution to the initial adsorption rate. The initial desorption kinetics are governed by monomer detachment from the surface aggregates. The desorption rate constants scale with the CMC, indicating an analogy between the surface aggregates and those formed in solution.     

Chemical analysis of the submicron binder phase in hard metals

Summary Chemical Analysis of the Submicron Binder Phase in Hard Metals The chemical analysis of the cobalt binder phase in sintered hard metals poses severe problems if performed by means of physical methods of surface analysis, such as SIMS, EMPA or AES. Compared with their limit of lateral resolution the average mean free path of the binder phase between the carbide particles, especially in micrograin hard metals, is less than 500 nm and thus too small. An electrochemical technique has been developed by which the carbide phase can be selectively etched away from the surface of a polished section, leaving the cobalt binder phase chemically and morphologically unchanged. The carbide-free cobalt binder phase can now be separated from the bulk hard metal using a technique similar to the well-known extraction replica technique commonly applied in transmission electron microscopy. The extracted binder phase can then be analyzed by X-ray fluorescence or other methods.     

A comparative study on electrochemical micromachining of n-GaAs and p-Si by using confined etchant layer technique

The confined etchant layer technique has been applied to achieve effective three-dimensional (3D) micromachining on n-GaAs and p-Si. This technique operates via an indirect electrochemical process and is a maskless, low-cost technique for microfabrication of arbitrary 3D structures in a single step. Br(2) was electrogenerated at the mold surface and used as an efficient etchant for n-GaAs and p-Si; l-cystine was used as a scavenger, for both substrates. The resolution of the fabricated microstructure depended strongly on the composition of the electrolyte, and especially on the concentration ratio of l-cystine to Br(-). A well-defined, polished Pt microcylindrical electrode was employed to examine the deviation of the size of the etched spots from the real diameter of the microelectrode. The thickness of the confined etchant layer can be estimated, and thus the composition of the electrolyte can be optimized for better etching precision. The etched patterns were approximately negative copies of the mold, and the precision of duplication could reach the micrometer level for p-Si and the submicrometer level for n-GaAs. Although the same etchant (Br(2)) and scavenger (l-cystine) were used in the etching solutions for GaAs and Si, the etching process, or mechanism, is completely different in the two cases. Compared with the fast etching process on GaAs in an etching solution with a concentration ratio of 3:1 of l-cystine to Br(-), the concentration ratio needs to be 50:1 for etching of Si. For the micromachining of Si, the addition of a cationic surfactant (cetyltrimethylammonium chloride, CTACl) is necessary to reduce the surface tension of the substrate and hence reduce the influence of evolution of the byproduct H(2). The function of the surfactant CTACl in comparison with an anionic surfactant (sodium dodecyl sulfate) was studied in contact-angle experiments and micromachining experiments and then is discussed in detail.     

Interfacial layers from the protein HFBII hydrophobin: dynamic surface tension, dilatational elasticity and relaxation times

The pendant-drop method (with drop-shape analysis) and Langmuir trough are applied to investigate the characteristic relaxation times and elasticity of interfacial layers from the protein HFBII hydrophobin. Such layers undergo a transition from fluid to elastic solid films. The transition is detected as an increase in the error of the fit of the pendant-drop profile by means of the Laplace equation of capillarity. The relaxation of surface tension after interfacial expansion follows an exponential-decay law, which indicates adsorption kinetics under barrier control. The experimental data for the relaxation time suggest that the adsorption rate is determined by the balance of two opposing factors: (i) the barrier to detachment of protein molecules from bulk aggregates and (ii) the attraction of the detached molecules by the adsorption layer due to the hydrophobic surface force. The hydrophobic attraction can explain why a greater surface coverage leads to a faster adsorption. The relaxation of surface tension after interfacial compression follows a different, square-root law. Such behavior can be attributed to surface diffusion of adsorbed protein molecules that are condensing at the periphery of interfacial protein aggregates. The surface dilatational elasticity, E, is determined in experiments on quick expansion or compression of the interfacial protein layers. At lower surface pressures (<11 mN/m) the experiments on expansion, compression and oscillations give close values of E that are increasing with the rise of surface pressure. At higher surface pressures, E exhibits the opposite tendency and the data are scattered. The latter behavior can be explained with a two-dimensional condensation of adsorbed protein molecules at the higher surface pressures. The results could be important for the understanding and control of dynamic processes in foams and emulsions stabilized by hydrophobins, as well as for the modification of solid surfaces by adsorption of such proteins.     

Construction of Flexible-on-Rigid Hybrid-Phase Metal–Organic Frameworks for Controllable Multi-Drug Delivery

Multi-component MOFs contain multiple sets of unique and hierarchical pores, with different functions for different applications, distributed in their inter-linked domains. Herein, we report the construction of a class of precisely aligned flexible-on-rigid hybrid-phase MOFs with a unique rods-on-octahedron morphology. We demonstrated that hybrid-phase MOFs can be constructed based on two prerequisites: the partially matched topology at the interface of the two frameworks, and the structural flexibility of MOFs with acs topology, which can compensate for the differences in lattice parameters. Furthermore, we achieved domain selective loading of multiple guest molecules into the hybrid-phase MOF, as observed by scanning transmission electron microscopy–energy-dispersive X-ray spectrometry elemental mapping. Most importantly, we successfully applied the constructed hybrid-phase MOF to develop a dual-drug delivery system with controllable loading ratio and release kinetics.     

Use of inorganic salts during the etching process in the fabrication of chemically modified capillaries for open tubular electrochromatography

Capillaries for use in electrophoretic analyses are etched with ammonium bifluoride and in some cases a second inorganic salt is included in the process. The effects of the presence of these inorganic components in the surface matrix on the electromigration of heterocyclic aromatic amines and enkephalins are evaluated. Resolution, efficiency, and peak shape are used to compare the various columns. In one instance, the etched surface is then modified by the addition of an octadecyl moiety using a silanization/hydrosilation procedure. The surface properties of the various etched capillaries are also compared by electroosmotic flow measurements.     

Microstructure analysis of a carbon-carbon composite using argon ion etching.

The microstructure of carbon-carbon composites obtained by chemical vapor infiltration of a carbon fiber felt was comparatively studied by reflection light microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and laser scanning confocal microscopy (LSCM). Ar+ ion etching was used to reveal and distinguish structural units of the pyrolytic carbon matrix. Mechanically polished samples, polished and subsequently ion etched samples, and fractured samples were compared. The values of surface roughness and surface height after polishing or after polishing and subsequent etching determined by AFM and LSCM correlate well with the degree of texture of the matrix layers obtained by polarized light microscopy and selected area electron diffraction. The carbon matrix is composed of structural units or "cells," which contain a carbon fiber and a sequence of several differently textured layers around each fiber. Within high-textured layers columnar grains are well recognizable using polarized reflection light microscopy and confocal microscopy. The size of depressions within high-textured carbon layers found by AFM after ion etching correlates well with the size of differently tilted domains detected by both TEM and SEM.     

Molecular bond formation between surfaces: anchoring and shearing effects

Specific molecular bonds between apposing surfaces play a central role in many biological structures and functions. They display a widely varying anchoring to the cell surface, and they are subject to forces that affect their binding characteristics due to their hydrodynamic environments. Here, we examine both anchoring and shearing aspects using simplified model systems aimed at gaining insight into the formation of a 2D bond collection under stress using two different surface anchors. The highly specific streptavidin-biotin molecular bond was chosen as the model receptor-ligand pair, and grafted colloids were used as model surfaces. To explore the role of the surface anchor, we grafted biotin onto the particle surface following two different approaches: first, the grafting was performed directly on the particle amine functions; second, a 35-nm-long PEG spacer was used. Hybrid particle classes were brought into contact in a homogeneous shear (between 200 s(-)(1) and 1200 s(-)(1)) using a cone plate geometry. The bond association and dissociation kinetics were given by the time course assemblage of hybrid particles into doublets. We observed saturating kinetics profiles that we interpreted as a linkage-breakage equilibrium, which yielded the on and off rates. We found that the biotin-PEG spacer was needed in order to observe significant binding at any shear rate. We also showed that only the number of collisions per unit time, generated by the shear, affected the on rate of the binding. Neither the exerted forces nor the collision lifetime had any effect. The off rate decreased with shear, possibly because of the shortening of the force duration, which results from the increasing shear rate.     

Quantifying nanoparticle adhesion mediated by specific molecular interactions

Receptor-mediated targeting of nanometric contrast agents or drug carriers holds great potential for treating cardiovascular and vascular-associated diseases. However, predicting the ability of these vectors to adhere to diseased cells under dynamic conditions is complex due to the interplay of transport, hydrodynamic force, and multivalent bond formation dynamics. Therefore, we sought to determine the effects of adhesion molecule density and flow rate on adhesion of 210 nm particles, with the goal of identifying criteria to optimize binding efficiency and selectivity. Our system employed a physiologically relevant ligand, the vascular adhesion molecule ICAM-1, and an ICAM-1 specific antibody tethered to the nanoparticle using avidin-biotin chemistry. We measured binding and dissociation of these particles in a flow chamber as a function of antibody density, ligand density, and flow rate, and using a transport-reaction model we distilled overall kinetic rate constants for adhesion and detachment from the binding data. We demonstrate that both attachment and detachment of 210 nm particles can be correlated with receptor and ligand valency and are minimally affected by shear rate. Furthermore, we uncovered a time-dependent mechanism governing particle detachment, in which the rate of detachment decreases with contact time according to a power law. Finally, we use our results to illustrate how to engineer adhesion selectivity for specific molecular targeting applications. These results establish basic principles dictating nanoparticle adhesion and dissociation and can be used as a framework for the rational design of targeted nanoparticle therapeutics that possess optimum adhesive characteristics.