The Effect of Adhesion on the Rheological and Frictional Behavior of a Confined Lubricant Film

We investigated the rheological and frictional behavior of a model system of lubricated, atomically-smooth, solid surfaces at zero and negative external normal load. The measurements were performed with a surface forces apparatus modified for oscillatory shear. For low deflection amplitudes, and negative loads up to the point when the surfaces jumped apart, the confined liquid layer (0.7 ± 0.2 nm perfluorinated heptaglyme) showed a highly elastic behavior independent of load. In the sliding regime at large amplitudes, the behavior was mostly dissipative but also independent of normal load. The force necessary to separate the surfaces was not affected by any sliding conditions. However, the friction force showed a very pronounced decrease as a consequence of sliding at large amplitudes. Thus, for our system, friction and adhesion are decoupled. We propose a mechanism of in-plane rearrangements of the molecules and explain the shear-induced reduction of friction by the formation of shear-bands.     

The Effect of Time and Humidity on Particle Adhesion and Removal

Time and humidity greatly influence particle adhesion and removal in many particlesubstrate systems. The effect of time (aging) and humidity on the adhesion and removal of 22 μm PSL (Polystyrene Latex) particles on polished silicon wafers is investigated. The results show that the effect of time on the adhesion and removal of the 22 μm PSL particles on silicon substrates in high humidity environment is very significant. The removal efficiency of PSL particles significantly decreased after the samples were aged for more than one day in high humidity environment. The combined effect of the van der Waals force and the capillary force tend to accelerate the adhesion-induced deformation process. When capillary force occurs at the particle substrate interface, the removal efficiency decreases quickly by more than 50% within 24 hours. Without the capillary force, the adhesion-induced deformation is negligible within the first 24 hours.     

The Effect of Time and Humidity on Particle Adhesion and Removal

Time and humidity greatly influence particle adhesion and removal in many particlesubstrate systems. The effect of time (aging) and humidity on the adhesion and removal of 22 μm PSL (Polystyrene Latex) particles on polished silicon wafers is investigated. The results show that the effect of time on the adhesion and removal of the 22 μm PSL particles on silicon substrates in high humidity environment is very significant. The removal efficiency of PSL particles significantly decreased after the samples were aged for more than one day in high humidity environment. The combined effect of the van der Waals force and the capillary force tend to accelerate the adhesion-induced deformation process. When capillary force occurs at the particle substrate interface, the removal efficiency decreases quickly by more than 50% within 24 hours. Without the capillary force, the adhesion-induced deformation is negligible within the first 24 hours.     

Effect of particle characteristics on particle pickup velocity

Particle entrainment is investigated by measuring the velocity required to pick up particles from rest, also known as pickup velocity. Pickup velocity is a function of individual particle characteristics and interparticle forces. Although 5-200 μm particles are investigated, the work presented here focuses on the pickup of particles in a pile in the size range of 5-35 μm. These smaller particle sizes are more typical for pharmaceutical and biomedical applications, such as dry powder inhalers (DPIs). Pickup velocities varied from 3.9 to 16.9 m/s for the range of particle sizes investigated.There is a strong correlation between particle size and the dominating forces that determine the magnitude of the pickup velocity. Preliminary data investigating pickup velocity as a function of particle size indicate the existence of a minimum pickup velocity. For larger particle sizes, the mass of the particle demands a greater fluid velocity for entrainment, and for smaller particle sizes, greater fluid velocities are required to overcome particle-particle interactions. Pickup velocity remains relatively constant at very small particle diameters, specifically, less than 10 μm for glass spheres and 20 μm for nonspherical alumina powder. This can be attributed to the negligible changes in London-van der Waals forces due to a hypothesized decrease in interparticle spacing. At intermediate particle diameters, electrostatic forces are dominant.     

The Effect of Relative Humidity on Particle Adhesion and Removal

The removal of small particles is vital for contamination-free manufacturing. In humid environments liquid can condense between the particle and substrate and give rise to a very large capillary force, which increases the total force of adhesion. The removal and adhesion forces of polystyrene latex (PSL) particles and pigmented coating chips were measured on silicon, polyethylene terephthalate, metallized and polyester coating substrates as a function of humidity. The results indicate that the capillary force is significant at a relative humidity above 50% and dominates at a relative humidity above 70%. At relative humidity below 45%, the electrostatic force becomes significant. The adhesion forces varied depending on the particles and substrates used, but the trend of high adhesion at high and low relative humidity was observed for all PSL particles/substrate systems. The pigmented coating chips/substrate system however, exhibited high adhesion at high relative humidity and low adhesion at low relative humidity.     

The Effect of Relative Humidity on Particle Adhesion and Removal

The removal of small particles is vital for contamination-free manufacturing. In humid environments liquid can condense between the particle and substrate and give rise to a very large capillary force, which increases the total force of adhesion. The removal and adhesion forces of polystyrene latex (PSL) particles and pigmented coating chips were measured on silicon, polyethylene terephthalate, metallized and polyester coating substrates as a function of humidity. The results indicate that the capillary force is significant at a relative humidity above 50% and dominates at a relative humidity above 70%. At relative humidity below 45%, the electrostatic force becomes significant. The adhesion forces varied depending on the particles and substrates used, but the trend of high adhesion at high and low relative humidity was observed for all PSL particles/substrate systems. The pigmented coating chips/substrate system however, exhibited high adhesion at high relative humidity and low adhesion at low relative humidity.     

Effects of bulk and surface properties of materials on adhesion-induced deformations between submicrometer diameter particles and substrates

—Spherical particles of polyvinylidene fluoride (PVF₂), of 0.3 μm diameter, were deposited onto various substrates including polyester, a polyester-polydimethylsiloxane block copolymer (hereafter referred to as PSBC), and polished silicon. The adhesion force-induced deformations between the particles and substrates were then observed using scanning electron microscopy (SEM). It was found that the particles embedded most deeply into the soft PSBC. No embedding of the particles into the Si wafers was observed, although the particles, themselves, appeared to flatten. The particles were also observed to embed into the polyester, although to a lesser extent than they did into the PSBC. Moreover, when the particles contacted samples of polyester which had been plasma-treated in argon, the embedding decreased. Measured contact area diameters are compared to predictions of various models of adhesion. The effect of the thickness of a conducting (Au/Pd) coating on the appearance of the contact zone is also discussed.     

The effect of air on the packing structure of fine particles

A numerical study of the effect of air on the packing structure of fine particles has been performed by a combined continuum and discrete numerical model. The forces considered are gravity, contact force, drag force, and van der Waals forces. The results are analyzed in terms of particle rearrangement, local porosity, coordination number, radial distribution function, and the distribution of contact forces. The results indicate the degree to which drag and van der Waals forces promote mean porosity increases and mean coordination number decreases. Drag forces allow contacts of particles reaching a state of rest in a packing to be closer to the Coulomb failure criterion for shear displacement when van der Waals forces are small. Increasing van der Waals forces imposes contact conditions that are far away from the Coulomb failure criterion. Increased drag and van der Waals forces tends to lead to more heterogeneous structure. It is demonstrated that average normal contact force is related to the ratio of van der Waals forces to particle weight.     

Multicomponent phase equilibria of thin liquid films and their vapour

Subject of the paper are multicomponent phase equilibria between extremely thin liquid films and their vapour phase on curved or planar solid surfaces. The solid surface is either heated or cooled, so that the liquid films are evaporating or the vapour is condensing.The curvature of the film surface and long range molecular forces due to the van der Waals attraction acting over distances of about 0.2- between solid and liquid film can lead to a composition shift in liquid and vapour phase compared to the composition that would be observed at planar and not extremely thin liquid films in equilibrium with their vapour phase.The Kelvin equation for the interfacial pressure is derived, as well as the equations for the composition shift. As a numerical example shows the van der Waals forces considerably influence the pressures at the liquid-vapour interface. Responsible for the composition shift is a dimensionless thermodynamic quantity . For small values D_i→0, liquid-vapour phase equilibria become identical with those of planar surfaces. For D_i?0 the vapour phase contains less and for D_i?0 more of the lighter volatiles than planar interfaces.     

Direct Measurement of Energy Barriers on Rough and Heterogeneous Solid Surfaces

The “three-liquid” contact angle approach to the surface free energy components of solids was applied to poly (vinyl fluoride), rough and flattened, with and without flame treatment. Lifshitz-van der Waals (LW), γ ^LW _SL , and acid-base (AB), γ ^LW _SL , components were determined and used to calculate ?δG _SL (W ^adhesion _SL ) for the formation of interfaces of five liquids with polymer. The automated goniometer allowed the determination of the energy barriers, ?δ G? _SL as the advancing liquid moved from pinned configuration to a metastable one. The acid-base component of the barriers was much greater than the LW, and the magnitude of the barriers was only slightly reduced by flattening.     

Direct Measurement of Energy Barriers on Rough and Heterogeneous Solid Surfaces

The “three-liquid” contact angle approach to the surface free energy components of solids was applied to poly (vinyl fluoride), rough and flattened, with and without flame treatment. Lifshitz-van der Waals (LW), γ^LW_SL, and acid-base (AB), γ^LW_SL, components were determined and used to calculate -δG_SL (W^adhesion_SL) for the formation of interfaces of five liquids with polymer. The automated goniometer allowed the determination of the energy barriers, -δ G*_SL as the advancing liquid moved from pinned configuration to a metastable one. The acid-base component of the barriers was much greater than the LW, and the magnitude of the barriers was only slightly reduced by flattening.     

Simple Views on Metal/Oxide Interfaces: Contributions of the Long-Range Interactions to the Adhesion Energy

The image and van der Waals contributions to the metal/oxide work of adhesion are compared through the extent to which they follow the known prevalent trends, i.e. the increase in work of adhesion (a) with narrowing oxide band gap and (b) with increasing conduction electron density of the metal. The van der Waals interaction is shown to follow both trends, while the image term is suggested to be significant only for dense metals in contact with very ionic oxides. The relative contribution of these long-range interactions to the overall metal/oxide work of adhesion is found to be maximized for systems involving metals with low electronic densities and oxides with wide band gaps. At variance, high metallic electronic densities and narrow oxide gaps likely favour short-range interactions arising from charge transfer.     

Simple Views on Metal/Oxide Interfaces: Contributions of the Long-Range Interactions to the Adhesion Energy

The image and van der Waals contributions to the metal/oxide work of adhesion are compared through the extent to which they follow the known prevalent trends, i.e. the increase in work of adhesion (a) with narrowing oxide band gap and (b) with increasing conduction electron density of the metal. The van der Waals interaction is shown to follow both trends, while the image term is suggested to be significant only for dense metals in contact with very ionic oxides. The relative contribution of these long-range interactions to the overall metal/oxide work of adhesion is found to be maximized for systems involving metals with low electronic densities and oxides with wide band gaps. At variance, high metallic electronic densities and narrow oxide gaps likely favour short-range interactions arising from charge transfer.     

A Novel Test for the Appraisal of Solid/Solid Interfacial Interactions

The Johnson, Kendall and Roberts (JKR) technique has been used with considerable success for assessing solid/solid interfacial interactions over the past 25 years or so. Nevertheless, the contact zone between the two spherical solids is often small and the energy of adhesion scales with the cube of the contact radius (at low load), thus potentially magnifying errors in adhesion assessment. The theoretical aspects of a novel technique are presented here, in which a hollow, slightly inflated, spherical membrane replaces a full sphere, and is placed in contact with a flat rigid solid. A judicious choice of experimental conditions should lead to increased contact radius and the energy of adhesion scales with its square (at low load), thus reducing possible errors. An added advantage is that the effective elasticity of the sphere depends on internal gas pressure. Thus surface and bulk effects are decoupled.     

A New Group Contribution Method based on Equation of State Parameters to Evaluate the Critical Properties of Simple and Complex Molecules

A new group contribution method to evaluate the critical properties (temperature, pressure and volume) is presented and applied to estimate the critical properties of biomolecules. Similar to other group contribution methods, the one proposed here divides the molecule into conveniently defined groups and evaluates the properties as the sum of the different contributions according to a specified model equation for each of the properties. The proposed method consists of a one‐step calculation that uses simple model equations and does not require additional data besides the knowledge of the structure of the molecule, except for isomers. For these substances the normal boiling temperature, the molecular mass and the number of atoms in the molecule are used to distinguish among isomers. The method is applicable to high molecular weight compounds, as most biomolecules and large molecules present in natural products.     

The vdW EoS hundred years later, yet younger than before. Application to the phase equilibria modeling of food-type systems for a green technology

The relevant applications that supercritical fluid extraction (SFE) has attained in the food industry in the last decades increased the interest in the development of thermodynamic models to represent the phase equilibria of systems comprising food-related substances and supercritical carbon dioxide (SCCO₂). The targets are twofold: (i) calculation of solid solubilities in SCCO₂ pure or with polar co-solvents (mainly ethanol in food applications), what directly affects the selectivity of one of the most applied processes, e.g. the semi-batch SFE of plant material, and (ii) calculation of vapor-liquid equilibrium compositions of liquid raw materials processed in countercurrent packed columns, what is straightforwardly related with the number of theoretical stages necessary to achieve the desired separation.Cubic equations of state derived from the equation proposed by van der Waals (vdW-type EoSs, e.g. Peng-Robinson and Soave-Redlich-Kwong EoS) are the most common thermodynamic models applied to achieve these objectives.In this work, the application of vdW-type EoSs to food-type substance + SCCO₂ is reviewed. More than 100 systems, comprising lipids, antioxidants, pigments, alkaloids, vitamins, etc. were investigated from 2003 up today. Even though phase equilibria modeling of SCCO₂ + food-related systems involves serious difficulties, the extensive and, in many cases, successful application of vdW-type EoSs demonstrates that though developed 100 years ago the van der Waals equation is lively and younger than before.     

The effect of van der Waals and charge induced forces on bed modulus of elasticity in ac/dc electrofluidized beds of fine powders—a unified theory

Perturbation theory is applied to an ac electrofluidzed bed of fine powder (glass and FCC) using electric field bubble control to infer the relation between interparticle forces (microscale) and the bulk bed modulus of elasticity (macroscale). Electrostatically induced and permanent van der Waals forces are modeled in a unified theory with bulk fluidized bed behavior. The extrapolation of the electric field to zero strength gives the permanent bed force and bulk bed modulus of elasticity as limiting cases. The resulting equations involve atomic as well as macros-scale parameters. The charge induced forces are identified through the bed modulus of elasticity as a function of the applied electric field strength. The semi-empirical approach is based on the principle that the conservation equations for the perturbed fluidized bed become unstable at the onset of bubbling giving characteristic eigenvalues for the bed modulus, a condition that is readily identifiable experimentally. Eigenvalues from the one-dimensional linearized conservation equations for the fluidized state are examined for growth, neutrality, or decay from the perturbation, which together with bed data are evaluated at bubbling conditions to give the bed modulus of elasticity. Both Richardson-Zaki and Carman-Kozeny bed expansion models of fluidization are examined. The former approach is found to give self-consistent results in which the bed modulus varies linearly with the electric field strength. The results are extended to dc beds as a limiting case of zero field frequency.The modulus of elasticity (a macroscopic bed property) is finally related to particle charge separation at the particle level through an interparticle force model applicable to ac-dc electric fields.     

A Novel Test for the Appraisal of Solid/Solid Interfacial Interactions

The Johnson, Kendall and Roberts (JKR) technique has been used with considerable success for assessing solid/solid interfacial interactions over the past 25 years or so. Nevertheless, the contact zone between the two spherical solids is often small and the energy of adhesion scales with the cube of the contact radius (at low load), thus potentially magnifying errors in adhesion assessment. The theoretical aspects of a novel technique are presented here, in which a hollow, slightly inflated, spherical membrane replaces a full sphere, and is placed in contact with a flat rigid solid. A judicious choice of experimental conditions should lead to increased contact radius and the energy of adhesion scales with its square (at low load), thus reducing possible errors. An added advantage is that the effective elasticity of the sphere depends on internal gas pressure. Thus surface and bulk effects are decoupled.     

Effect of meta‐dihydroxy benzene as a surface modifier on filler–filler and filler–polymer interaction of carbon black polymer composite

In the present context an attempt was made to use polyhydroxybenzene on rubber‐grade carbon black to suitably modify the surface morphology and hence to achieve improved thermomechanical and mechanical dynamic properties. FTIR studies clearly point out the presence of excess hydroxyl group in the modified black. The improvement in mechanical properties is attributed to higher crosslink density through hydrogen bonding and oxygen bridge participation during the covulcanization reaction. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2025–2033, 2002     

The influence of several silicates on the fretting behavior of UHMWPE composites

Five kinds of silicates (attapulgite, kaolin, montnorillonite, synthetic mica, and talc) reinforced ultra-high molecular weight polyethylene (UHMWPE) composites were prepared. The fretting performance of the composites was investigated using an oscillating reciprocating friction and wear tester in a ball-on-plate mode. The thermodynamic property and crystallinity of the composites were evaluated, and the worn morphology was also observed. Result shows that the heat-resistance and the crystallinity of the composites are generally improved. The friction coefficient and mass loss of the composites reduced than those of the neat UHMWPE. Thereinto, the attapulgite/UHMWPE composite shows the best fretting resistance performance. The heat resistance of composites increases due to the reinforcement of silicates. Meantime, the friction load can be distributed by the filler in the matrix. Moreover, the different fretting resistance performance of the composites is due to the different dispersion degree and the interfacial bonding force between the fillers and the UHMWPE matrix.