Adsorbed Film Thickness Research Articles

New insight in characterization of red wine astringency using soft tribology method

AbstractThe variation of friction coefficient (μ) of model wine and model saliva mixtures with entrainment speed (VR) on simulated oral surfaces was evaluated by oral tribology. Combined with techniques of dynamic light scattering (DLS), atomic force microscopy (AFM), rheometer, and quartz crystal microbalance with dissipation monitoring (QCM‐D), the correlation between characteristics of the model wine and model saliva mixtures (particle size and morphology, suspension viscosity, adsorption film thickness, and viscoelasticity) and the oral lubrication loss was established. The results showed that the higher the concentration of tannin in the model wine, the larger the size of the complexes with the model saliva, and the higher the thickness, viscoelasticity, and roughness of the adsorption film formed, which resulted in the increase of friction coefficient in the boundary lubrication regime. Different from previous results, it is found that the maximum value of the friction coefficient (μmax) in the boundary regime has the best positive correlation with the astringency perception intensity of the model wine accordingly.

In-situ measurement of temporal changes in thickness of polymer adsorbed films from lubricant oil by vertical-objective-based ellipsometric microscopy

Understanding the formation mechanism and conformation of the adsorbed film formed by additives is critical to clarify the boundary lubrication characteristics. In this study, using the vertical-objective-based ellipsometric microscope (VEM), which can measure nm-gaps between solid surfaces, we proposed a new method for in-situ measuring the thickness of additive adsorbed film in the lubricant oil. In this method, the adsorbed film thickness was measured by measuring the gap between the solids, which was filled by the additive film. The formation process of polyalkylmethacrylate (PAMA) polymer additive adsorbed film was successfully observed. Moreover, we revealed that the PAMA adsorbed films adopted two different conformations depending on the molecular weight and concentration of the polymer: the flattened single-layer conformation and double-layer conformation.

Hydroxyl Influence on Adsorption and Lubrication of an Ultrathin Aqueous Triblock Copolymer Lubricant

This research aims to provide insights into the adsorption behaviors of two monomers of triblock copolymers (1,2-dimethoxyethane (1,2-DME) and 1,2-dimethoxypropane (1,2-DMP)) on a TiO2 surface in aqueous solution. A multiscale theoretical framework by means of the density functional theory (DFT), ab initio molecular dynamics (AIMD), and classical molecular dynamics (MD) simulations is established. The DFT calculation confirms that these molecules adsorb more energetically on a hydroxylated surface than pure oxide. There is a difference in adsorption behaviors between 1,2-DMP and 1,2-DME molecules due to the covalent bonding between carbons and oxygen of the hydroxylated TiO2 surface. The AIMD simulation reveals that the adsorption of both copolymers to the TiO2 surface is hindered by the presence of water with 1,2-DME exhibiting a weaker adsorption than 1,2-DMP. The presence of 1,2-DME on the TiO2 surface with water produced a smaller number of hydroxyl groups on the surface than 1,2-DMP. Moreover, the dissociative adsorption of water onto the rutile surface is the main cause for a chemical formation of terminating hydroxyl groups. The number of associated bonds is insignificant compared to the dissociated one since the dissociative adsorption is more favored than the associative one. MD simulation indicates that triblock copolymers adsorb stronger on the hydroxylated surface with a thinner adsorbed film thickness than that on the pure rutile. The presence of terminal hydroxyl groups on the rutile surface helps reducing the friction for aqueous 17R2 triblock copolymers, while it results in an increase of friction for normal copolymer L62.

Assessing the impact of disjoining pressure on thin-film evaporation with atomistic simulation and kinetic theory

Molecular dynamics (MD) simulations are applied to validate the Hertz–Knudsen–Schrage (HKS) relation for the evaporation mass flux in the presence of disjoining pressure. A non-equilibrium MD simulation system is designed to directly extract the evaporation mass flux for a Lennard-Jones fluid. The temperatures, pressures, properties, and disjoining pressure required to evaluate the HKS relation are obtained from the non-equilibrium MD simulation and complementary equilibrium MD simulations. The direct MD evaporation mass flux and that from the HKS relation agree within 10%. We define an evaporation Kapitza length that estimates where the liquid conduction and evaporation thermal resistances are equal. We propose that surface structures that promote film thicknesses comparable to or smaller than the evaporation Kapitza length, but larger than the adsorbed film thickness, will maximize the evaporation rate.

Evaluation of physicochemical interaction between asphalt binder and mineral filler through interfacial adsorbed film thickness

The blend of asphalt binder and mineral filler forms asphalt mastic and the investigation of physicochemical interaction between them is significant to study the rheological properties of asphalt mixture. The interfacial physicochemical effect can be directly evaluated by the adsorbed film thickness between filler and binder. Therefore, the study proposed new methods based on Hashin model and K-B-G* parameter to calculate the adsorbed film thickness and the results were within 0.14 μm for different asphalt mastics. Additionally, the effects of asphalt binder and mineral filler characteristics on adsorbed film thickness were also investigated in this study. The results showed that asphalt binder with higher polar components content can form thicker adsorbed film with mineral filler particles. And the order of adsorbed film thickness of different mineral filler with asphalt binder from thick to thin was fly ash, hydrated lime and limestone. Furthermore, the study investigated the existence of filler critical concentration to provide proper ratio of mineral filler to asphalt binder for asphalt mixture design.

Influence of “tentacle structure” on the properties of jellyfish-like 3D dispersants based on tannic acid for preparing high-concentrated coal–water slurry

Three kinds of jellyfish-like three-dimensional dispersants with different “tentacle structures” were designed and synthesized via polymerization reaction of tannic acid (TA) with starch, allyl polyoxyethylene ether 1200 (APEG1200), and acrylic acid (AA), respectively. These dispersants were employed for preparing high-concentrated coal–water slurry (CWS). Their structures and surfactivity were characterized using FTIR, 1H NMR and surface tension. The effects of the TA-based dispersants on the performance of CWS were fundamentally investigated. The TA-based dispersant with starch as the “tentacle structure” (TA-St) exhibited the better viscosity-reducing and stabilizing capability for CWS than those with APEG1200 (TA-APEG) or AA (TAA). The apparent viscosity reduced to 317 mPa·s at 0.5 wt% TA-St, and the maximum coal content exceeded 71 wt%. Rheological experiments showed that the CWS prepared with the three TA-based dispersants behaved as pseudoplastic fluids, and the CWS with TA-St presented the smallest yield stress. After 3.5 days of storage, the separated water ratio of CWS with TA-St reached 4.15 wt%. Meanwhile, the TA-St dispersant showed a better effect in increasing the adsorption capacity and adsorption film thickness, wettability, and absolute value of zeta potential of the coal surface compared with TAA and TA-APEG. The saturated adsorption quantity of TA-St on the coal surface was 1.77 mg/g. The excellent performances of TA-St in dispersing and stabilizing coal particles in CWS were due to the great steric hindrance and electrostatic repulsion effect derived from the starch “tentacle structure”, which effectively reduced the aggregation among coal particles.

Understanding the temperature and loading frequency effects on physicochemical interaction ability between mineral filler and asphalt binder using molecular dynamic simulation and rheological experiments

Asphalt mastic consists of asphalt binder and mineral filler and the physicochemical interaction between two them plays significant role in the service performance of asphalt concrete, which can be directly evaluated by their interfacial adsorbed film thickness. This study investigated the effects of temperature, loading frequency and other factors on the adsorbed film thickness. The results showed that with the increase of temperature and the decrease of loading frequency, the adsorbed film thickness between asphalt binder and mineral filler became thicker. Additionally, the addition of SBS modifier into base asphalt led to the stronger interaction ability with mineral filler at high temperature (i.e., low frequency) region. Compared with inert mineral filler, the adsorbed film thickness between active filler and asphalt binder was thicker, which provided insight into asphalt mastic design. Furthermore, the investigation of physicochemical interaction mechanism between asphalt and filler paves the way to understand the effect of these factors on interaction ability, which was analyzed at the molecular and atomistic scale through molecular dynamic simulation in this study. The asphalt mastic molecular model was also used to simulate temperature how to influence the adsorbed film thickness. The findings showed that compared with non-polar molecules of asphalt binder, the adsorption force between mineral filler particles and polar molecules was stronger. Asphalt non-polar components moved away from the surface of filler particle with the temperature increasing, while more and more polar components were adsorbed by filler particle, which caused thicker adsorbed film thickness and thus stronger interaction ability between mineral filler and asphalt binder.

Effect of ammonia nitrogen and low-molecular-weight organics on the adsorption of additives on coal surface: A combination of experiments and molecular dynamics simulations

Experiments and molecular dynamics (MD) simulations were conducted to investigate the effect of ammonia nitrogen and low-molecular-weight organics on the adsorption of sodium lignosulfonate (SL) on Shenhua coal. Results of adsorption and adsorption film thickness experiments showed that ammonia nitrogen and low-molecular-weight organics have different effects on the adsorption of SL on coal surface, thus resulting in different adsorption configurations of SL. A ternary system that was composed of water–SL–Shenhua coal was established through MD simulation. The adsorption mechanism of SL on Shenhua coal in different solutions was explored, and the influencing factors were analyzed using a microscopic mechanism. Results showed that the adsorption conformations of SL molecules on coal surface in aqueous and low-molecular-weight organics solutions are single-layer and multi-point adsorption, whereas the adsorption conformations in ammonia nitrogen solutions are double-layer adsorption. The mechanism of the adsorption of SL molecule on Shenhua coal surface by ammonia nitrogen and low-molecular-weight organics was studied in detail. The calculation results of adsorption energy are consistent with the experimental results in thermodynamics. This result demonstrates the feasibility and accuracy of the MD simulation.

Properties of adsorbed supercritical methane film in nanopores

Adsorbed natural gas technology is an efficient technology for storing natural gas at low pressure and room temperature. This work investigates the properties of the adsorbed methane film in nanopores where methane is adsorbed by strong van der Waals forces in pores of few molecular diameter as a high-density fluid. BET surface area, porosity, and pore size distribution were measured using sub-critical nitrogen adsorption. The adsorbed film thickness, the film density, specific surface area, and methane average binding energy were extracted from a single supercritical methane adsorption isotherm using Langmuir and Ono-Kondo models. In addition, this method does not require a conversion between gravimetric excess adsorption and absolute adsorption. The adsorbed film thickness is between 4.2 and 4.4 Å and the density of the adsorbed film at maximum capacity is between 302 and 340 g/L. Specific surface areas obtained from supercritical isotherms are consistent with BET surface areas from subcritical nitrogen adsorption. The binding energies obtained from the two models are compared to the ones obtained from Clausius-Clapeyron method.

Atomic Force Microscopy Force Mapping Analysis of an Adsorbed Surfactant above and below the Critical Micelle Concentration.

Force curves collected using an atomic force microscope (AFM) in the presence of adsorbed surfactants are often used to draw conclusions about adsorbed film packing, rigidity, and thickness. However, some noteworthy features of such force curve characteristics have yet to be thoroughly investigated and explained. In this work, we collected force curves from tetradecyltrimethylammonium bromide films adsorbed on highly oriented pyrolytic graphite (HOPG), silica, and silica that had been hydrophobized by functionalization with dichlorodimethyl silane. Breakthrough events in the force curves from several different trials were compared to show that the breakthrough distance, often reported as the adsorbed film thickness, increased with concentration below the critical micelle concentration (CMC) but was approximately 3.5 nm on all surfaces between 2× and 10× CMC; an unexpected result because of the different surface chemistries for the three surfaces. We employed an AFM probe with a different force constant ( k) value as well as a colloidal probe and the breakthrough distance remained approximately 3.5 nm in all cases. Gradient mapping, a variant of force mapping, was also implemented on the three surfaces and resulted in a new technique for visualizing adsorbed surfactant in situ. The resulting maps showed patches of adsorbed surfactant below the CMC and revealed that with increasing concentration, the size of the patches increased resulting in full coverage near and above the CMC. These results are, to our knowledge, the first time force mapping has been used to spatially track patches of adsorbed surfactant. Finally, layers of surfactants on an AFM tip were investigated by collecting a force map on a single AFM tip using the tip of a separate AFM probe. A breakthrough event was observed between the tips, indicating that a layer of surfactant was present on at least one, if not both tips.

Micro-scratch behaviour of adsorbed film formed by aqueous copolymer lubricants with phosphate ester additive on Ti-coated surface

PurposeThis paper aims to assess the adsorption behaviour and the adhesion strength of lubricant films formed by polypropylene oxide-polyethylene oxide-polypropylene oxide (PPO-PEO-PPO) with phosphate ester additive on Ti-coated surface and to identify the influence of molecular architecture and phosphate ester additive.Design/methodology/approachThe thickness of the adsorbed PPO-PEO-PPO with phosphate ester lubricant films on Ti surfaces was measured by ellipsometry. The adhesion strength of the copolymer and the copolymer with phosphate ester lubricants was studied by the micro-scratch tests; the scratch tracks on the surfaces were observed by atomic force microscopy and scanning electronic microscopy.FindingsThe copolymer with a higher weight percentage of PPO not only formed a thicker film but also showed stronger adhesion and better lubrication performance. The added phosphate ester increased the film thickness and improved the tribological behaviour. The finding reveals that the adsorbed film thickness which depends on the PPO chain length and the presence of phosphate ester has a considerable effect on the scratch behaviour.Originality/valueThis paper fulfils the studies about adsorption behaviour and lubrication mechanism of this new lubricant which has not been adequately investigated on the metal surface.

In situ observation of lubricant film formation in THR considering real conformity: The effect of model synovial fluid composition

Abstract The present paper explores the effect of synovial fluid composition on lubricant film formation in hip replacements. The measurements were realized utilizing pendulum hip joint simulator, while the film thickness was evaluated using optical interferometry. Contact couples consisted of metal and ceramic femoral heads articulating with glass acetabular cups. As the test lubricants, various model fluids were employed. Initially, static tests, aimed on the effect of material and load on adsorption, were conducted. It was found that adsorbed film thickness increases independently of the head material. Consequently, swinging flexion-extension experiments were realized, revealing that the film formation is substantially affected by composition of model fluid. The thickest film was observed when higher concentration of hyaluronic acid and phospholipids was applied.

<i>In Situ</i> Observation of Formation Behavior of Adsorption Film in the Narrow Space Using Surface Plasmon Resonance (SPR) Method

In this paper, an application of the surface plasmon resonance (SPR) method to real-time and in-situ measurements of changes in adsorption film thickness, e.g., fatty acid adsorbed films, is presented. As discussed in the previous study, the high sensitivity and overall simplicity of its optical system make the SPR method advantageous for monitoring lubricant film thickness. In order to demonstrate the high sensitivity of the SPR method, in-situ and real-time adsorption film growth processes of stearic acid (additives) in hexadecane (base oil) were monitored using the Otto configuration. Based on a comparison between experimental and model calculation results, changes in the adsorption film thickness during a series of different contact tests were measured.

Prediction of static contact angles on the basis of molecular forces and adsorption data.

At a three-phase contact line, a liquid bulk phase is in contact with and coexists with a very thin layer of adsorbed molecules. This adsorbed film in the immediate vicinity of a liquid wedge modifies the balance of forces between the liquid and solid phases such that, when included in the balance of forces, a quantitative relationship emerges between the adsorbed film thickness and the static contact angle. This relationship permits the prediction of static contact angles from molecular forces and equilibrium adsorption data by means of quantities that are physically meaningful and measurable. For n-alkanes on polytetrafluoroethylene, for which there are experimental data available on adsorption and contact angles, our computations show remarkable agreement with the data. The results obtained are an improvement on previously published calculations-particularly for alkanes with a low number of carbon atoms, for which adsorption is significant.

Adsorbed film structure and tribological performance of aqueous copolymer lubricants with phosphate ester additive on Ti coated surface

The adsorption of aqueous tri-block reverse copolymer polypropylene oxide–polyethylene oxide–polypropylene oxide, PPO–PEO–PPO with phosphate ester on Ti coated surfaces have been studied using a neutron reflectometer (NR). It is found that the adsorbed film consists of two layers. The hydrophobic PPO block forms the inner layer and acts as the anchor of the adsorbed film. Copolymer with higher weight percentage of PPO forms a thicker PPO layer. Phosphate ester affects the adsorbed film structure of PPO–PEO–PPO copolymer by forming a thicker mixed-layer of PPO and phosphate ester on the Ti surface. The tribological performance of the lubricants has also been studied using a ball-on-disc tribometer. The results show that copolymer molecular structure affects lubrication performance. Tri-block copolymer with a higher weight percentage of PPO shows a better performance. The addition of phosphate ester to tri-block copolymer solution not only significantly decreases the coefficient of friction but also increases anti-wear property. The finding indicates that adsorbed film thickness depends on the size of the PPO block and the presence of phosphate ester, and that adsorbed film thickness correlates with friction.

Effect of bovine serum albumin aqueous solution on steel slider surface under hydrodynamic lubrication

Lubrication and adsorption behaviours of bovine serum albumin (BSA) aqueous solution were studied using an optical slider-bearing tester. The lubricated contact consisted of a stationary steel slider and a moving glass disc. Interferometry was employed to measure the film thickness. Results showed that film thickness increased with increasing speed and decreased with increasing load. During the tests, a strip of corrosion on the steel slider surface grew from the inlet to the outlet and quick corrosion occurred at high sliding speeds. Corrosion was found to promote protein adsorption. Two types of adsorption, namely, continuous thin layer and discontinuous protein aggregate, were generated on the slider surface. A new approach based on the difference in optical film thickness formed with two different media was proposed for the measurement of adsorption film thickness. Moreover, results showed that high sliding speeds and low loads favored the growth of adsorbed film.

Fluid structure in the immediate vicinity of an equilibrium three-phase contact line and assessment of disjoining pressure models using density functional theory

We examine the nanoscale behavior of an equilibrium three-phase contact line in the presence of long-ranged intermolecular forces by employing a statistical mechanics of fluids approach, namely, density functional theory (DFT) together with fundamental measure theory (FMT). This enables us to evaluate the predictive quality of effective Hamiltonian models in the vicinity of the contact line. In particular, we compare the results for mean field effective Hamiltonians with disjoining pressures defined through (i) the adsorption isotherm for a planar liquid film, and (ii) the normal force balance at the contact line. We find that the height profile obtained using (i) shows good agreement with the adsorption film thickness of the DFT-FMT equilibrium density profile in terms of maximal curvature and the behavior at large film heights. In contrast, we observe that while the height profile obtained by using (ii) satisfies basic sum rules, it shows little agreement with the adsorption film thickness of the DFT results. The results are verified for contact angles of 20°, 40°, and 60°.

The configuration of water on rough natural surfaces: Implications for understanding air‐water interfacial area, film thickness, and imaging resolution

Previous studies of air‐water interfacial areas in unsaturated porous media have often distinguished between interfacial area corresponding to water held by capillary forces between grains and area corresponding to water associated with solid surfaces. The focus of this work was on developing a better understanding of the nature of interfacial area associated with solid surfaces following drainage of porous media. Stereoscopic scanning electron microscopy was used to determine surface elevation maps for eight different surfaces of varying roughness. An algorithm was developed to calculate the true configuration of an air‐water interface in contact with the solid surface as a function of capillary pressure. The algorithm was used to calculate surface‐associated water configurations for capillary pressures ranging from 10 to 100 cm water. The results of the work show that, following drainage, the configuration of surface‐associated water is dominated by bridging of macroscopic surface roughness features over the range of capillary pressures studied, and nearly all of the surface‐associated water is capillary held. As such, the thicknesses of surface‐associated water were found to be orders‐of‐magnitude greater than might be expected at the same capillary pressures based on calculations of adsorbed film thickness. The fact that capillary forces in air‐water interfaces dominate surface‐associated water configuration means that interface shapes are largely unaffected by microscopic surface roughness, and interfaces are considerably smoother than the underlying solid. As such, calculations suggest that microscopic surface roughness likely has minimal impact on the accuracy of surface‐associated air‐water interfacial areas determined by limited‐resolution imaging methods such as computed microtomography.

Effect of Nanostructured Roughness on Evaporating Thin Films in Microchannels for Wenzel and Cassie–Baxter States

A model based on the augmented Young–Laplace equation and kinetic theory was developed to describe the nanostructured roughness effects on an extended evaporating meniscus in a microchannel for Wenzel and Cassie–Baxter states. The roughness geometries were analytically related to the disjoining pressure, slip length and thermal resistance across the roughness layer. The results show that the equivalent Hamaker constant and adsorbed film thickness increase with nanopillar height for Wenzel state. Thus, the spreading and wetting properties of the evaporating thin film increase with roughness for Wenzel state, leading to an elongated thin film and enhanced heat transfer rate compared to a flat hydrophilic surface. The equivalent Hamaker constant and disjoining pressure effect decrease with increasing nanopillar height for Cassie–Baxter state. The system wettability, thin film length and heat transfer rate increase with increasing slip length and with decreasing roughness for Cassie–Baxter state. A smaller roughness coexisting with a larger slip length on rough surfaces for Cassie–Baxter state results in a much higher heat transfer rate relative to a flat surface.

Characterization of adsorbed microlayer thickness on an oceanic glass plate sampler

The thickness of solution layers adsorbed onto rotating glass plates designed for use on an oceanic glass plate sampler was investigated in laboratory experiments using optical techniques. Using the Beer‐Lambert Law, light attenuation measurements were used to calculate the thickness of adsorbed solution layers on a rotating glass disk with and without salt and surfactants. The observations have shown that the adsorbed film thickness can vary between 80 and 40 µm for glass rotation speeds between 4 and 16 cm s−1, depending on salinity and surfactant concentrations. For example, the thickness of a film of water with 40 ppt of salt and 5 cm s−1 rotation speed was in the range of 80 µm. The adsorbed layer thickness increases with increasing salt concentration and with increasing concentrations of surface active substances. These results are comparable to results obtained by vertically dipping a glass plate and determining film thickness from the collected volume of water. Because the glass disk rotational speed significantly influences the thickness of the adsorbed solution layer, it is important that the speed is maintained at a value for which the adsorbed thickness has been calibrated. At typical oceanic salinities, the dependence of the adsorbed film thickness on rotation speed was limited. However, even small changes in surface active substances resulted in significant thickness changes.