Adsorbed Surface Films Research Articles

Differences in nano-topography and tribochemistry of ZDDP tribofilms from variations in contact configuration with steel and DLC surfaces

In this work, we evaluated the effect of the counter-body material (the same or dissimilar) and contact configuration (moving or stationary body), at similar contact tribological conditions, on the tribochemical and nanotopography characteristics of adsorbed surface films. Zinc dialkyldithiophosphate (ZDDP), the best performing anti-wear additive, was used in self-mated steel/steel and DLC/DLC contacts, which were compared with mixed steel/DLC and DLC/steel contacts in 1-h and 6-h sliding tests. The macroscale (tribometer) and nanoscale (atomic force microscopy) friction, thickness, topography, and chemical (attenuated total reflection-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy) properties of the tribofilms were studied. The results revealed unexpectedly large differences in all the studied tribofilm parameters; this is because all the tribofilms are completely different; this includes the chemical composition, which is known to have a crucial effect on the nano- and macro-scale tribological properties. These results clearly demonstrate that the surface material, additives, and common contact operating parameters, that is, pressure, velocity, and temperature, crucially affect the ZDDP tribofilm as well as the position of the moving or stationary surface within the contact, and the material of the moving/stationary bodies.

Compatibility between Various Ionic Liquids and an Organic Friction Modifier as Lubricant Additives.

Tribological performance of a boundary lubrication contact is largely dominated by the friction modifier (FM) and antiwear (AW) additives in the lubricant. While oil-soluble ionic liquids (ILs) have recently demonstrated promising AW functionality, their compatibility with FMs is little known and even less understood for nonferrous alloys. Here, we report the latest results for several selected ILs when used together with an organic FM (OFM) in lubricating a steel-bronze contact. Depending on the IL chemistry, either synergistic or antagonistic effects were observed. The three aprotic ILs ([P8888][DEHP], [P66614][BTMPP], and [P66614][C17H35COO]) seemed to degrade the OFM's lubricating performance. In contrast, the protic IL [N888H][DEHP] exhibited a strong synergistic effect with the OFM, yielding an ultralow steady-state friction coefficient (0.02) and a lowwear rate (<10-8 mm3/(N m)), which significantly outperformed the IL or the OFM alone. Surface characterization found no chemically reacted tribofilm on the bronze worn surface. On the other hand, a unique physically adsorbed surface film as a result of interconnection between the IL and OFM molecules by hydrogen bonds is proposed on the basis of chemical analysis. Such an adsorption surface film is expected to be difficult to compress vertically but easy to shear horizontally, leading to low friction and wear.

Some aspects of the role of inhibitors in the corrosion of copper in tap water as observed by cyclic voltammetry

Cyclic voltammetric examination of the corrosion and inhibition of copper in hard and soft tap-waters in the presence of a commercial inhibitor containing benzotriazole (BTA) and triethanolamine (TEA), or its separate components, is presented. The anodic and cathodic reactions are both strongly inhibited, although the anodic reaction more so. BTA is by far the dominant inhibiting component. The inhibitor forms a polymerized reactive adsorbed surface film. Inhibition of the cathodic reaction (oxygen reduction) is not due to electron resistivity of the inhibitor, but rather, by heavily reduced surface coverage of adsorbed oxygen over a wide range of oxygen reduction overpotential.

Synovial fluid lubrication of artificial joints: protein film formation and composition

Despite design improvements, wear of artificial implants remains a serious health issue particularly for Metal-on-Metal (MoM) hips where the formation of metallic wear debris has been linked to adverse tissue response. Clearly it is important to understand the fundamental lubrication mechanisms which control the wear process. It is usually assumed that MoM hips operate in the ElastoHydrodynamic Lubrication (EHL) regime where film formation is governed by the bulk fluid viscosity; however there is little experimental evidence of this. The current paper critically examines synovial fluid lubrication mechanisms and the effect of synovial fluid chemistry. Two composition parameters were chosen; protein content and pH, both of which are known to change in diseased or post-operative synovial fluid. Film thickness and wear tests were carried out for a series of model synovial fluid solutions. Two distinct film formation mechanisms were identified; an adsorbed surface film and a high-viscosity gel. The entrainment of this gel controls film formation particularly at low speeds. However wear of the femoral head still occurs and this is thought to be due primarily to a tribo-corrosion mechanisms. The implications of this new lubrication mechanism and the effect of different synovial fluid chemistries are examined. One important conclusion is that patient synovial fluid chemistry plays an important role in determining implant wear and the likelihood of failure.

Cryogenic Performance of RF MEMS Switch Contacts

A series of experiments was performed to characterize RF microelectromechanical system switch performance under variable environmental conditions and cryogenic temperatures. Data were recorded in helium and nitrogen environments to lower stiction failure rates as well as to circumvent switch bouncing arising from low pressure at cryogenic temperatures. Contact resistance values were observed to be lower at cryogenic temperatures but still two orders of magnitude higher than the values predicted for the constriction resistance of gold asperity contacts, consistent with the presence of adsorbed films on the contacts. An asperity-heating model was applied, from which it was deduced that contact voltages can selectively disassociate adsorbed films from the contact surface while not softening the gold asperity contacts. The results are consistent with the reduced mobility of the adsorbed surface films at cryogenic temperatures.

Grating-Coupler Assisted Infrared Reflection Absorption Spectroscopy for the Characterization of Organic Thin Films

We demonstrate how grating-coupler assisted infrared reflection absorption spectroscopy can be used to simultaneously determine the chemical identity and relative thickness of organic thin films. With a grating substrate, a threshold anomaly associated with passing off of the -1 diffracted order occurs at grazing angles of incidence, resulting in a sharp absorbance in the infrared. The position of this peak is sensitive to the grating geometry as well as the dielectric environment near its surface. Thus, shifts in the peak position can be used to determine the relative thickness of adsorbed films or quantify molecular adsorption events. To illustrate the characteristics and sensitivity of this phenomenon, several samples were prepared and tested, including self-assembled alkanethiolate monolayers with 11-mercaptoundecanoic acid, 11-mercapto-1-undecanol, decanethiol, and a covalently linked layer of bovine serum albumin on a commercial, gold-coated grating. For these samples, the position of the threshold absorbance peak shifted to lower wavenumbers as film thickness increased, which is consistent with calculated shifts based upon an increasing refractive index at the interface. The sensitivity of this shift was measured to be 3.7 cm(-1) nm(-1). These results illustrate how a grating substrate can be exploited in a standard infrared reflectance measurement to provide additional information about the relative thickness of adsorbed surface films.

Axial symmetric rotation of a partially immersed body in a liquid with a surfactant layer

This paper gives a simple integral formula to evaluate the torque on a slowly rotating symmetric body partially immersed in a viscous liquid covered by an adsorbed surface film. Besides the results known earlier, new results have also been derived for small values of the surface shear viscosity parameter κ. It is seen that the effect of κ in all cases is to increase the torque.

Nano Scale Creep and the Role of Defects

AbstractThe modulating force method in nanoindentation gives a direct measure of contact stiffness, and being insensitive to drift, allows the accurate observation of creep in small indents to be carried out over long time periods. We present results for a range of metals at room temperature. Strain rate indices similar to those for macroscopic creep are found. Reverse creep occurs for step unloading greater than about half the starting load. For electropolished tungsten, we find quite different behaviour before and after the sudden pop-in. Afterwards, creep is as in other metals, but beforehand, it is essentially zero. The slight changes of stiffness observed at the very smallest loads are due to diffusion of adsorbed surface films into the contact zone. Our results show that the dislocations nucleated and multiplied at pop-in provide the mechanism of creep.

Interfacial Properties of the Aqueous Phases of Wheat Flour Doughs

Aqueous phases were isolated from wheat flour doughs by ultracentrifugation, using a selection of wheats differing in bread-making quality. The interfacial properties of the aqueous phases of doughs were measured using surface tensiometry and surface rheological measurements. It was found that the material in the aqueous fraction from the cultivar Brock produced the greatest reduction in the surface tension of water and those from the CWRS class of wheat produced the least. Avalon and Mercia cultivars produced intermediate values. The elastic modulus, Gδ, of the adsorbed surface films was highest for CWRS and least for Brock. Avalon and Mercia again produced intermediate values. Lipid and protein measurements of the aqueous phases of doughs and the parent flours showed CWRS flour to contain the highest level of lipid material, whereas its aqueous phase contained the least of the flours studied. Brock flour, which was found to contain the lowest amount of lipid material, had the highest level of lipid in its aqueous phase. Avalon and Mercia were found to contain intermediate values in the flour and their aqueous phases. The influence of endogenous surface-active materials of flour on the interfacial properties of the air-aqueous interface in doughs are discussed.

Efficiency of lignosulfonates and humic-related substances as expanders in negative electrodes of the lead/acid system

Organic expanders represent essential additives to the negative active material of lead/acid batteries, since they prevent the negative electrode from compaction during life cycling. Focussing on stability and efficiency of expanders, industrial lignosulfonates and humic substances from peat bogs, in comparison, were fractionated, chemically altered and analysed. With respect to a correlation of functional groups and expander action, these organic samples, fractions, and derivatives were assessed based on a cyclovoltammetric determination of expander performance. The majority of the humic material showed poorer aptitude than the commercially available lignosulfonate expanders. Improved performance was found for precursors of humic acids with a low molecular weight. The obtained results indicate the bonding mechanisms by functional groups like cation exchange, chelating, or formation of π-ϵ complexes do not seem to be significant. A good expander aptitude of some humic derivatives is attributed to their colloidal nature. It is assumed that the coagulation of colloids with low molecular weight is hindered, as these colloids form adsorbed surface films in the negative active material. The films cause an overpotential for the reduction of PbSO 4 resulting in fine-grained structures of the lead particles.

Drop sizes of emulsions related to turbulent energy dissipation rates

Emulsion droplet sizes are compared with rates of turbulent energy dissipation in various types of practical equipment. These rates can be very high locally—up to 400 MW · kg −1 in valve homogenizers. The role of adosrbed surface films is discussed, and it is deduced that drop break-up involves an interface from which the adsorbed surface film is removed. The viscosity of the dispersed phase can be important in determining droplet size, and this is treated theoretically. Results are compared with systems of practical interest, including milk and bitumen emulsions. The calculations support the contention that turbulence is the basic mechanism of drop break-up.

SCALING LAWS FOR PARTICLE BREAKUP IN NOZZLE GENERATED SHOCKS

ABSTRACT Conditions for the onset of particle breakup in normal shock waves have been investigated. A normalized particle drag behind the shock has been determined in terms of gas stagnation conditions and particle diameter for a range of gas Mach numbers 1 ⩽ M1 ⩽ 5 by introducing appropriately defined particle Knudsen and Reynolds numbers into analytical expressions for the drag coefficient. Numerical computations of the particle drag, normalized with gas stagnation pressure and particle area, indicate a peak at a gas Mach number M1 ≳ 2.2, The magnitude of the peak was found to decrease with increasing particle diameter and reservoir gas density. Criteria for the onset of agglomerate breakup were defined in terms of a modified Weber number for the adhesion mechanisms due to Van der Waals forces, electrostatic attraction and adsorbed surface films. These results indicate that larger more closely packed agglomerates made up of smaller constituent particles have a greater tendency to resist breakup for a given set of stagnation conditions and shock Mach number.

Effects of adsorbed organic and primary fouling films on bryozoan settlement

Marine primary fouling films, which consist of molecular organic and microbial components, have been reported to facilitate colonization of immersed surfaces by marine fouling organisms. Larvae of the cosmopolitan fouling bryozoan Bugula neritina (Linnaeus) were offered various substrata for attachment and metamorphosis. The materials were offered (a) after detergent washing, (b) after sorption of dissolved organic molecular films, and (c) after formation of primary films consisting of both microbial and adsorbed organic material. Wettability of the substrata by sea water was determined by contact angle measurements for each substratum. On washed substrata, attachment was favored with contact angles greater than ≈45° (cos contact angle <0.7). Adsorbed surface films had no effect on the low settlement of larvae on glass and high settlement on plastics. Microbial primary films, however, made glass attractive and plastics unattractive. These settlement preference changes did not correlate with the changes in wettability observed on these substrata. Dispersion of larvae over the settlement surface was random except on wettable surfaces coated with bacterial films, where settlement was strongly clustered (contagious).

Effect of lipids and other substances on the adsorption of dipalmitoyl phosphatidylcholine.

The effect of various lipids, proteins, and other substances on the adsorption rate of dipalmitoyl phosphatidylcholine (DPC) was investigated. Each substance was added to a sonicated suspension of DPC in saline, and the surface pressure of the adsorbed surface film was measured using a platinum plate suspended from an electrobalance. Pure DPC adsorbed very slowly, being complete only after 8 hr; its half-adsorption time was 91.8 +/- 8.3 min. Four additives markedly reduced the half-adsorption time of DPC: dipalmitoyl phosphatidylglycerol (1.1 +/- 0.2 min), and surfactant apoprotein (0.9 +/- 0.1 min). The half adsorption times of mixtures of DPC and the other additives ranged from 15.3 +/- 1.9 to 107.8 +/- 12.1 min.

The torque on a rotating disk in the surface of a liquid with an adsorbed film

In this paper we consider the problem of calculating the resistive torque on a disk rotating slowly with constant angular speed in the surface of a liquid with an adsorbed surface film. Using the method of complementary representations for generalised axially symmetric potential functions, the boundary-value problem for the azimuthal velocity component is reduced to the solution of a Fredholm integral equation of the second kind. This equation is solved numerically and asymptotically for all values of the ratio of the surface shear viscosity of the film to the viscosity of the substrate fluid, and values calculated for the substrate and film torques on the disk. The results are compared with previous work of Goodrich and his co-workers.

The electroreduction of hexavalent molybdenum in aqueous solution

Polarographic, voltammetric and rotating ring-disk studies have been used to study the electroreduction of hexavalent molybdenum in aqueous solutions of various pH. In all cases investigated, the formation of adsorbed surface films plays a predominant role in the overall electrochemical behavior. Correlations between the voltammetric and the rotating ring-disk studies on the one hand with the polarographic behavior on the other suggest that Mo(V) and Mo(III) species are the sole electrode products and that the appearance of more than two polarographic waves is most probably the result of surface film formation and does not thefefore reflect the formation of Mo(IV) or Mo(II) species as suggested previously.

The electroreduction of hexavalent molybdenum in aqueous solution

Summary Polarographic, voltammetric and rotating ring-disk studies have been used to study the electroreduction of hexavalent molybdenum in aqueous solutions of various pH. In all cases investigated, the formation of adsorbed surface films plays a predominant role in the overall electrochemical behavior. Correlations between the voltammetric and the rotating ring-disk studies on the one hand with the polarographic behavior on the other suggest that Mo(V) and Mo(III) species are the sole electrode products and that the appearance of more than two polarographic waves is most probably the result of surface film formation and does not thefefore reflect the formation of Mo(IV) or Mo(II) species as suggested previously.

An application of bubble shape theory to the determination of air transfer through adsorbed surface films

An application of bubble shape theory to the determination of air transfer through adsorbed surface films

The theory of absolute surface shear viscosity V. The effect of finite ring thickness

In the theory of Couette surface viscometers it is convenient to idealize a rotating ring thrust into a fluid interface supporting an adsorbed surface film as making a knife-edge contact of zero thickness. The validity of this assumption is examined by calculating from theory the flow patterns to be expected when the ring makes a contact of finite thickness with the interface. The zero thickness idealization is found to be valid for films of moderate to large surface viscosity, but it fails badly for films of low surface viscosity and places a limit upon the sensitivity of the ring method.

The theory of absolute surface shear viscosity II. The rotating disk problem

The numerical results of a rigorous mathematical analysis of fluid flow in the neighborhood of a rotating disk inserted into a liquid interface supporting an adsorbed surface film are presented. The theory (described in detail elsewhere) takes full account of the coupling of interfacial and substrate fluid flows, which is to say that it removes the deficiency present in previous theories of surface viscometry which assumed that these flows are independent. It is shown that in the low surface viscosity region the results are sensitively dependent upon the coupling of interfacial and substrate motion. Finally we demonstrate that the rotating disk viscometer is a poor experimental design for the determination of low surface viscosity.